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P0388 Code: Crankshaft Position Sensor “B” Circuit High Input

P0388 indicates excessive voltage in the Crankshaft Position Sensor B circuit. Diagnose its signal wire, sensor ground, connector and PCM input.

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

ItemDetails
Trouble codeP0388
Code definitionCrankshaft Position Sensor “B” Circuit High Input
System affectedEngine-position, ignition and fuel-injection control
Fault typeSensor B signal voltage above the expected range
Common causesShort to voltage, open ground, failed sensor, connector damage or PCM input fault
Common symptomsCheck Engine Light, hard starting, stalling, no-start condition, rough running or reduced power
Typical severityModerate to severe
Safe to drive?Not recommended if starting or stalling symptoms are present
Repair difficultyIntermediate 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:

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:

Sensor B may be installed near the:

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:

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:

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:

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:

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:

  1. Sensor B Signal Wire Shorted to VoltageDamaged insulation may allow the signal wire to contact a reference-voltage or battery-voltage circuit.
  2. Open Sensor GroundA Hall-effect sensor may remain at a high output state if it loses its ground connection.
  3. Failed Crankshaft Position Sensor BInternal electronics may prevent the sensor from switching its output low.
  4. 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.
  5. Connector CorrosionCorrosion can create resistance, cross-connect adjacent terminals or interrupt the sensor ground.
  6. Bent or Spread Connector TerminalsPoor contact may leave the signal circuit electrically floating or prevent a proper ground connection.
  7. Water or Oil IntrusionFluid inside the connector may bridge the signal circuit to a powered terminal.
  8. Damaged Wiring HarnessWiring routed near the starter, alternator, exhaust or transmission can become pinched, melted or rubbed through.
  9. Incorrect Aftermarket WiringRemote-start equipment, engine-management systems or prior repairs can accidentally introduce voltage into the signal circuit.
  10. Incorrect Replacement SensorA sensor with the wrong electrical design may hold the output above the PCM’s expected range.
  11. Charging-System OvervoltageExcessive system voltage can influence sensor and PCM circuits, usually while causing additional codes.
  12. Poor PCM GroundA shifted PCM ground reference can make the sensor voltage appear higher than expected.
  13. Failed PCM Input CircuitAn internal PCM problem may hold the Sensor B circuit high or interpret the signal incorrectly.
  14. 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:

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.

CodeDefinitionPrimary diagnostic focus
P0385Sensor B Circuit MalfunctionGeneral electrical failure or missing signal
P0386Sensor B Circuit Range/PerformanceImplausible waveform, timing, frequency or amplitude
P0387Sensor B Circuit Low InputSignal stuck low, short to ground, missing power or weak output
P0388Sensor B Circuit High InputSignal 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.

CodeSensorFault
P0338Crankshaft Position Sensor ACircuit high input
P0388Crankshaft Position Sensor BCircuit 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:

Do not clear the code before recording its freeze-frame conditions.

Determine whether P0388 occurred:

2. Check for Related Codes

Look for codes involving:

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:

Watch for:

4. Identify Sensor B Correctly

Consult factory service information and identify:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

PCM replacement may require programming, security-system initialization and crankshaft-position variation relearn.

Common P0388 Diagnostic Mistakes

Avoid these common errors:

Repairs That May Fix P0388

Possible repairs include:

Estimated P0388 Repair Costs

Actual costs depend on the vehicle and sensor accessibility.

RepairEstimated 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:

See what the Check Engine Light means for additional warning-light guidance.

How to Verify the Repair

After completing the repair:

  1. Reconnect all sensors and modules.
  2. Confirm proper connector engagement.
  3. Secure the harness away from heat and moving parts.
  4. Clear current and pending codes.
  5. Monitor Sensor B voltage during cranking.
  6. Verify that the signal switches between its specified high and low states.
  7. Compare Sensor A and Sensor B waveforms.
  8. Warm the engine fully.
  9. Repeat the harness wiggle test.
  10. Recreate the original freeze-frame conditions.
  11. Complete the required drive cycle.
  12. Perform a crankshaft-position relearn if specified.
  13. 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.

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