The P0388 OBD-II trouble code means the powertrain control module has detected voltage above the expected range in the Crankshaft Position Sensor “B” circuit.
On a Hall-effect crankshaft sensor, the signal may remain stuck at its high-voltage state instead of switching normally as the crankshaft rotates. Common causes include a signal wire shorted to voltage, an open sensor ground, a damaged connector or a sensor that can no longer pull the signal low.
P0388 does not automatically mean Crankshaft Position Sensor B has failed. The complete circuit—including the sensor’s power supply, ground, signal wire, reluctor wheel and PCM input—must be tested before replacing parts.
Sensor B also does not automatically mean Bank 2. It identifies the manufacturer-designated secondary crankshaft-position input. Apparently one universal sensor-labeling system would have made diagnosis much too convenient.
P0388 Quick Reference
| Item | Details |
|---|---|
| Trouble code | P0388 |
| Code definition | Crankshaft Position Sensor “B” Circuit High Input |
| System affected | Engine-position, ignition and fuel-injection control |
| Fault type | Sensor B signal voltage above the expected range |
| Common causes | Short to voltage, open ground, failed sensor, connector damage or PCM input fault |
| Common symptoms | Check Engine Light, hard starting, stalling, no-start condition, rough running or reduced power |
| Typical severity | Moderate to severe |
| Safe to drive? | Not recommended if starting or stalling symptoms are present |
| Repair difficulty | Intermediate to advanced electrical diagnosis |
What Does the P0388 Code Mean?
P0388 indicates that the PCM has detected an abnormally high voltage in the Crankshaft Position Sensor B circuit.
The PCM expects the Sensor B signal to change as the crankshaft reluctor wheel rotates. If the voltage remains above a calibrated threshold for too long, never switches low or exceeds the circuit’s normal operating range, the PCM may store P0388.
Depending on the vehicle and sensor design, the PCM may detect:
- A signal continuously stuck high
- Voltage above the expected signal range
- Missing low-side transitions
- A signal shorted to reference voltage
- A signal shorted to battery voltage
- An open sensor ground
- A failed internal sensor pull-down circuit
- An open signal circuit held high by an internal PCM pull-up
- A PCM input fault
P0388 describes the electrical condition detected by the PCM. It does not identify the failed component by itself.
What Is Crankshaft Position Sensor B?
Crankshaft Position Sensor B is a secondary engine-position input used on certain vehicles.
Manufacturers may use two crankshaft-position sensors for:
- Signal redundancy
- Improved crankshaft-angle resolution
- Faster synchronization during startup
- Accurate misfire monitoring
- Separate low-speed and high-speed detection
- Comparison between different reluctor patterns
- Fail-safe operation
Sensor B may be installed near the:
- Crankshaft pulley
- Front timing cover
- Engine block
- Flywheel
- Flexplate
- Transmission bellhousing
- Rear main-seal area
- Internal crankshaft reluctor wheel
The location must be confirmed through manufacturer service information.
Sensor B does not universally refer to Bank 2, the rear sensor or one particular side of the engine. The crankshaft is shared by both cylinder banks, and the letter designation identifies a circuit—not a bank.
How the Crankshaft-Position Circuit Works
A crankshaft-position sensor monitors a toothed, slotted or magnetized trigger wheel connected to the crankshaft, flywheel or flexplate.
As the crankshaft rotates, the sensor produces a sequence of electrical pulses. The PCM uses those pulses to calculate:
- Crankshaft angle
- Engine RPM
- Piston position
- Ignition timing
- Fuel-injection timing
- Engine synchronization
- Misfire activity
- Camshaft-to-crankshaft relationship
The trigger wheel normally contains a reference feature such as a missing tooth or wider gap. This allows the PCM to identify a known crankshaft position.
If Sensor B remains electrically high and stops producing usable low transitions, the PCM cannot read the pulse pattern correctly.
Hall-Effect and Magnetic CKP Sensors
Crankshaft-position sensors generally use a Hall-effect or magnetic design.
Hall-Effect Sensor
A Hall-effect sensor commonly uses three circuits:
- Power or reference voltage
- Sensor ground
- Signal output
The signal switches between specified high and low voltage levels as the reluctor or magnetic target passes the sensor.
A P0388 condition can occur when:
- The signal is shorted to sensor-supply voltage
- The signal is shorted to battery voltage
- The sensor ground is open
- The sensor cannot switch the signal low
- The signal wire is open and held high by a PCM pull-up
- The PCM input circuit is defective
Hall-effect systems are the most straightforward examples of a circuit-high fault because the expected square-wave signal can remain visibly stuck at its high state.
Magnetic or Variable-Reluctance Sensor
A magnetic sensor normally uses two wires and generates an alternating-current waveform without an external power supply.
Its output voltage increases with crankshaft speed. On some vehicles, excessive amplitude, incorrect circuit bias or a short to voltage may be interpreted as a high-input condition.
Testing depends heavily on the manufacturer’s circuit design. Do not apply Hall-effect voltage expectations to a magnetic pickup.
Symptoms of a P0388 Code
Possible symptoms include:
- Illuminated Check Engine Light
- Extended cranking
- Hard starting
- Engine cranks but does not start
- Intermittent engine stalling
- Rough idle
- Engine hesitation
- Reduced power
- Poor throttle response
- Misfire-like operation
- Unstable tachometer reading
- Incorrect engine RPM data
- Limp-home mode
- Engine that starts after the connector is moved
- No noticeable driveability symptoms
Some vehicles may continue operating using Crankshaft Position Sensor A. Others may lose ignition or injector synchronization when Sensor B becomes unusable.
If the starter turns the engine but it refuses to run, use the broader engine cranks but won’t start diagnostic guide to check spark, fuel pressure, injector command and engine-position data.
If the engine shuts off primarily after reaching operating temperature, review why an engine stalls when warm.
Common Causes of P0388
Potential causes include:
- Sensor B Signal Wire Shorted to VoltageDamaged insulation may allow the signal wire to contact a reference-voltage or battery-voltage circuit.
- Open Sensor GroundA Hall-effect sensor may remain at a high output state if it loses its ground connection.
- Failed Crankshaft Position Sensor BInternal electronics may prevent the sensor from switching its output low.
- Open Signal CircuitSome PCM inputs use an internal pull-up resistor. An open signal wire may therefore remain at a high voltage instead of falling to zero.
- Connector CorrosionCorrosion can create resistance, cross-connect adjacent terminals or interrupt the sensor ground.
- Bent or Spread Connector TerminalsPoor contact may leave the signal circuit electrically floating or prevent a proper ground connection.
- Water or Oil IntrusionFluid inside the connector may bridge the signal circuit to a powered terminal.
- Damaged Wiring HarnessWiring routed near the starter, alternator, exhaust or transmission can become pinched, melted or rubbed through.
- Incorrect Aftermarket WiringRemote-start equipment, engine-management systems or prior repairs can accidentally introduce voltage into the signal circuit.
- Incorrect Replacement SensorA sensor with the wrong electrical design may hold the output above the PCM’s expected range.
- Charging-System OvervoltageExcessive system voltage can influence sensor and PCM circuits, usually while causing additional codes.
- Poor PCM GroundA shifted PCM ground reference can make the sensor voltage appear higher than expected.
- Failed PCM Input CircuitAn internal PCM problem may hold the Sensor B circuit high or interpret the signal incorrectly.
- Reluctor or Air-Gap ProblemAlthough more commonly associated with range/performance faults, an incorrect trigger relationship can prevent certain Hall-effect sensors from switching properly.
How Serious Is the P0388 Code?
P0388 is a moderate-to-severe fault because the PCM depends on accurate crankshaft-position information to control the engine.
The vehicle may continue running if the PCM can rely on Sensor A or another engine-position input. However, P0388 may also cause:
- Unexpected stalling
- Extended cranking
- A complete no-start condition
- Reduced engine power
- Incorrect ignition timing
- Incorrect injection timing
- Misfire-monitoring problems
Diagnosis should be scheduled promptly. A vehicle that stalls unpredictably should not be driven in traffic.
P0385 vs. P0386 vs. P0387 vs. P0388
These codes identify different problems within the Crankshaft Position Sensor B system.
| Code | Definition | Primary diagnostic focus |
| P0385 | Sensor B Circuit Malfunction | General electrical failure or missing signal |
| P0386 | Sensor B Circuit Range/Performance | Implausible waveform, timing, frequency or amplitude |
| P0387 | Sensor B Circuit Low Input | Signal stuck low, short to ground, missing power or weak output |
| P0388 | Sensor B Circuit High Input | Signal stuck high, short to voltage, open ground or failed pull-down |
The exact code matters. A short-to-ground diagnosis for P0387 is the opposite electrical direction from a short-to-voltage diagnosis for P0388.
P0388 vs. P0338
P0338 is the corresponding high-input code for Crankshaft Position Sensor A.
| Code | Sensor | Fault |
| P0338 | Crankshaft Position Sensor A | Circuit high input |
| P0388 | Crankshaft Position Sensor B | Circuit high input |
Do not assume the sensors share the same connector pinout, part number or operating voltage.
P0388 vs. P0389
P0388 identifies a signal that remains above the expected voltage range.
P0389 identifies an intermittent Sensor B circuit in which the signal drops out, becomes erratic or returns unpredictably.
A connector problem can produce either condition, but the waveform and code-setting criteria differ.
How to Diagnose the P0388 Code
Begin by identifying the Sensor B circuit design. A high reading means little without knowing whether the signal should switch between zero and five volts, zero and battery voltage or an entirely different range.
1. Confirm the Code
Connect a capable OBD-II scan tool and record:
- P0388
- Current, pending and history codes
- Freeze-frame information
- Engine RPM
- Battery voltage
- Coolant temperature
- Vehicle speed
- Engine load
- Crankshaft synchronization
- Camshaft synchronization
- Sensor A and Sensor B data, when supported
Do not clear the code before recording its freeze-frame conditions.
Determine whether P0388 occurred:
- During cranking
- Immediately after startup
- At idle
- Under acceleration
- At highway speed
- After the engine became hot
2. Check for Related Codes
Look for codes involving:
- Crankshaft Position Sensor A
- Crankshaft Position Sensor B
- Camshaft-position sensors
- Crankshaft-to-camshaft correlation
- Sensor reference voltage
- System overvoltage
- PCM power or ground
- Engine-speed input
- Module communication
Several sensor-high codes appearing together may indicate a shared ground or charging-system problem.
3. Verify the Symptoms
Crank or start the engine while monitoring:
- Engine RPM
- Sensor B status
- Crankshaft synchronization
- Camshaft synchronization
- Battery and charging voltage
Watch for:
- Zero or implausible RPM
- Sensor B stuck in one state
- Synchronization changing unexpectedly
- P0388 returning immediately
- Signal changes when the connector or harness is moved
- Failure that appears only after warm-up
4. Identify Sensor B Correctly
Consult factory service information and identify:
- Sensor B location
- Sensor type
- Connector pinout
- Supply voltage
- Ground circuit
- Signal circuit
- Expected signal bias
- Expected waveform
- PCM connector terminals
Do not assume Sensor B is the sensor on Bank 2. It may not even be located near a cylinder bank.
5. Inspect the Wiring and Connector
Switch the ignition off and inspect:
- Sensor B connector
- PCM connector where accessible
- Signal-wire insulation
- Sensor ground
- Sensor power circuit
- Harness routing near battery and starter cables
- Wiring near the exhaust and transmission
- Oil or water intrusion
- Corroded terminals
- Bent or pushed-out pins
- Damaged shielding
- Improper splices
- Aftermarket wiring
Look closely for a signal wire contacting a powered circuit. A harness can appear intact externally while containing damaged conductors beneath the insulation.
6. Check Battery and Charging Voltage
Test:
- Battery voltage with the engine off
- Minimum voltage during cranking
- Charging voltage at idle
- Charging voltage with electrical loads applied
- Engine and PCM ground voltage drop
An overcharging alternator can raise circuit voltage and may store multiple high-input codes. Repair a major charging-system fault before continuing.
7. Verify Hall-Effect Sensor Power
For a Hall-effect sensor, confirm the specified supply voltage.
The sensor may use:
- Five volts
- Eight volts
- Battery voltage
- Another regulated supply
Do not assume every three-wire sensor uses five volts.
A correct supply reading proves only that the power circuit is present. It does not confirm that the sensor ground or signal circuit is functioning.
8. Test Sensor Ground
Back-probe the sensor ground and perform a voltage-drop test while the circuit is operating.
An open or high-resistance ground may cause the signal to remain high.
Check:
- Ground terminal condition
- Ground-splice integrity
- Continuity to the correct ground point
- Voltage drop during cranking
- Changes when the harness is moved
A resistance check performed with the circuit unloaded may miss a poor connection.
9. Measure the Sensor B Signal
Use a high-impedance digital multimeter or oscilloscope.
A Hall-effect Sensor B signal should switch between its specified high and low voltage states while the engine rotates.
A P0388 condition may show:
- Signal continuously at five volts
- Signal continuously near battery voltage
- High state with no low transitions
- Signal exceeding the normal high-state specification
- Brief low transitions that do not reach the required threshold
- Voltage that remains high even with the sensor disconnected
A multimeter may average a switching signal and conceal brief transitions. An oscilloscope is the preferred tool.
10. Check for a Short to Voltage
Disconnect the sensor and PCM according to the manufacturer’s procedure.
With both ends isolated, test the signal circuit for continuity to:
- Sensor-supply voltage
- Battery-positive circuits
- Ignition-voltage circuits
- Other powered sensor signals
- Alternator wiring
- Starter-control wiring
Move the harness while watching the meter.
Any connection between the isolated signal wire and a powered circuit indicates damaged wiring or an incorrect splice.
Never perform resistance testing on a powered circuit.
11. Check for an Open Sensor Ground
If the signal is high with the sensor connected, verify whether restoring a proper ground causes normal switching.
Do not ground the circuit blindly. Use the wiring diagram and an approved fused jumper only when the manufacturer’s diagnostic procedure calls for it.
Grounding the wrong PCM circuit can replace a straightforward sensor code with a considerably more expensive electronics lesson.
12. Isolate the Sensor
Observe the signal-circuit voltage with Sensor B connected and disconnected.
Possible results include:
- Voltage returns to normal when the sensor is disconnected: sensor may be internally shorted to supply
- Voltage remains excessively high with the sensor disconnected: wiring short or PCM pull-up/input issue
- Voltage changes when a temporary approved ground is applied: ground-circuit problem
- Voltage is normal at the sensor but high at the PCM: harness or connector fault
The expected disconnected voltage depends on whether the PCM uses an internal pull-up or pull-down resistor.
13. Compare Sensor A and Sensor B Waveforms
When both signals are available, display them simultaneously.
Compare:
- High-state voltage
- Low-state voltage
- Pulse spacing
- Reference gaps
- Signal timing
- Response to engine speed
- Stability during warm-up
A normal Sensor A waveform and a Sensor B waveform stuck high strongly support a Sensor B circuit problem.
If both signals show unusual voltage, investigate shared power, ground, PCM or charging-system faults.
14. Compare the Signal at the Sensor and PCM
Measure Sensor B at its connector and then at the PCM.
If the waveform is normal at the sensor but stuck high at the PCM, inspect:
- Signal-wire damage
- Connector-terminal contact
- Corroded splices
- Cross-shorts between circuits
- Improper shielding
- Previous wiring repairs
If the signal reaches the PCM correctly but scan data remains incorrect, verify PCM power, ground, software and input-circuit operation.
15. Inspect the Reluctor and Sensor Air Gap
Although P0388 is primarily an electrical high-input fault, inspect the trigger system if the circuit tests correctly.
Check for:
- Excessive sensor air gap
- Incorrect sensor installation
- Damaged reluctor teeth
- Shifted reluctor ring
- Metal debris
- Bent flexplate
- Incorrect replacement sensor
Some Hall-effect sensors may remain in one state if the trigger wheel is absent, improperly aligned or outside the sensing range.
16. Perform a Heat and Wiggle Test
If P0388 is intermittent:
- Monitor the waveform during warm-up
- Move the harness carefully
- Apply controlled heat according to service procedures
- Cool the sensor and repeat the test
- Inspect connector tension as the circuit operates
A sensor may fail internally when hot, while a damaged wire may contact a powered conductor only when vibration moves the harness.
17. Check the PCM Last
Consider PCM failure only after confirming:
- Correct Sensor B identification
- Correct supply voltage
- Reliable sensor ground
- No short to voltage
- Proper signal continuity
- Correct sensor installation
- Good reluctor condition
- Correct charging-system voltage
- Proper PCM power and ground
- No applicable software update or technical service bulletin
PCM replacement may require programming, security-system initialization and crankshaft-position variation relearn.
Common P0388 Diagnostic Mistakes
Avoid these common errors:
- Assuming Sensor B means Bank 2
- Replacing Sensor A instead of Sensor B
- Replacing Sensor B without checking its ground
- Ignoring a signal shorted to reference voltage
- Assuming an open signal wire must read zero volts
- Ignoring an internal PCM pull-up circuit
- Using a conventional test light on the signal wire
- Grounding the signal circuit without a wiring diagram
- Overlooking alternator overvoltage
- Ignoring aftermarket wiring
- Damaging connector terminals with oversized probes
- Replacing the PCM before isolating the harness
- Treating P0388 like the low-input P0387 code
Repairs That May Fix P0388
Possible repairs include:
- Replacing Crankshaft Position Sensor B
- Repairing a signal wire shorted to reference voltage
- Repairing a signal wire shorted to battery voltage
- Repairing an open sensor ground
- Repairing a high-resistance ground
- Cleaning or replacing corroded terminals
- Replacing a damaged sensor connector
- Repairing an open signal circuit
- Correcting improper aftermarket wiring
- Installing the correct replacement sensor
- Repairing charging-system overvoltage
- Correcting sensor installation or air gap
- Repairing a damaged reluctor wheel
- Updating PCM software
- Replacing and programming the PCM after confirmed failure
Estimated P0388 Repair Costs
Actual costs depend on the vehicle and sensor accessibility.
| Repair | Estimated cost |
| Diagnostic inspection | $100–$250 |
| Connector or minor wiring repair | $100–$350 |
| Crankshaft Position Sensor B replacement | $180–$650 |
| Ground-circuit repair | $100–$400 |
| Charging-system repair | $200–$1,100 |
| Major harness repair | $300–$1,000 |
| Reluctor-wheel or flexplate repair | $700–$2,500 |
| PCM software update | $100–$300 |
| PCM replacement and programming | $800–$2,500 |
An accessible sensor is usually inexpensive to replace. A signal wire buried in a wrapped engine harness may require considerably more diagnostic labor, because electricity is invisible and apparently enjoys hiding.
Can You Drive With P0388?
Driving with P0388 is not recommended if the engine stalls, loses power or becomes difficult to start.
The vehicle may continue operating if the PCM can use another engine-position input, but the fault may worsen without warning.
Stop driving if:
- The engine stalls
- Starting becomes unreliable
- The tachometer drops suddenly
- The engine enters reduced-power mode
- The Check Engine Light flashes
- Multiple timing or misfire codes appear
- Wiring becomes hot or produces an electrical odor
See what the Check Engine Light means for additional warning-light guidance.
How to Verify the Repair
After completing the repair:
- Reconnect all sensors and modules.
- Confirm proper connector engagement.
- Secure the harness away from heat and moving parts.
- Clear current and pending codes.
- Monitor Sensor B voltage during cranking.
- Verify that the signal switches between its specified high and low states.
- Compare Sensor A and Sensor B waveforms.
- Warm the engine fully.
- Repeat the harness wiggle test.
- Recreate the original freeze-frame conditions.
- Complete the required drive cycle.
- Perform a crankshaft-position relearn if specified.
- Check for pending codes.
The repair is confirmed when Sensor B switches normally, synchronization remains stable and P0388 does not return.
Frequently Asked Questions
What does the P0388 code mean?
P0388 means the PCM has detected voltage above the expected range in the Crankshaft Position Sensor B circuit.
Does P0388 mean Crankshaft Position Sensor B is bad?
Not necessarily. The sensor may have failed, but a short to voltage, open ground, connector problem, charging fault or PCM input issue can cause the same code.
Is Crankshaft Position Sensor B located on Bank 2?
Not necessarily. Sensor B identifies a secondary crankshaft-position input and does not universally correspond to Bank 2.
What is the difference between P0387 and P0388?
P0387 indicates Sensor B voltage is too low. P0388 indicates Sensor B voltage is too high.
What is the difference between P0385 and P0388?
P0385 identifies a general Sensor B circuit malfunction. P0388 specifically identifies a circuit-high condition.
Can P0388 cause a no-start condition?
Yes. The engine may not start if the PCM cannot determine crankshaft position accurately enough to control spark or fuel injection.
Can an open ground cause P0388?
Yes. A Hall-effect sensor may remain at its high signal state if its ground circuit is open or has excessive resistance.
Can an open signal wire cause P0388?
Yes. If the PCM uses an internal pull-up resistor, an open signal circuit may remain at a high voltage.
Can alternator overvoltage cause P0388?
It is possible. Charging-system overvoltage can affect sensor circuits, although it will often produce other high-voltage codes.
How do you diagnose P0388?
Identify Sensor B, verify supply voltage and ground, inspect the waveform, isolate the signal circuit and test it for a short to voltage.
Final Thoughts
P0388 means the Crankshaft Position Sensor B circuit is electrically higher than the PCM expects. The most likely diagnostic directions include a signal shorted to voltage, an open sensor ground, a failed sensor or an open circuit held high by the PCM’s internal pull-up.
Start with the wiring diagram. Confirm Sensor B’s identity, inspect the connector and then measure its power, ground and signal with the correct equipment. An oscilloscope will reveal whether the waveform switches normally or remains stuck high.
That process separates a sensor failure from a wiring or PCM problem without treating every high-input code as permission to replace the first component mentioned in its definition.
Visit the OBD-II Doctor for additional diagnostic guides or browse the complete P-series OBD-II code reference to identify related powertrain faults.
