The P0373 trouble code means the powertrain control module has detected an intermittent or erratic pulse pattern from the high-resolution timing reference Signal “A.”
This timing signal helps the PCM determine precise engine position and, on certain diesel applications, control fuel-injection timing. Unlike a completely dead sensor, an intermittent signal may work normally one minute and start dropping pulses the next. Naturally, electrical problems are considerate enough to behave perfectly whenever the scan tool comes out.
P0373 is commonly associated with electronically controlled diesel injection systems, but its exact diagnostic definition and component location can vary by manufacturer. Always check the service information for the specific year, make, model and engine.
P0373 Quick Reference
| Item | Details |
|---|---|
| Code | P0373 |
| Definition | Timing Reference High-Resolution Signal “A” Intermittent/Erratic Pulses |
| System | Ignition timing or fuel-injection timing reference |
| Common source | Optical sensor, crankshaft sensor or injection-pump timing sensor |
| Common causes | Loose connection, damaged wiring, contaminated optical sensor, failing sensor or damaged encoder disc |
| Common symptoms | Hard starting, stalling, hesitation, rough running or reduced power |
| Severity | Moderate to serious |
| Safe to drive? | Not recommended if the engine stalls, misfires or loses power |
| Typical diagnostic tool | Scan tool, digital multimeter and oscilloscope |
For additional definitions, visit the complete P-series OBD-II code list or the main OBD-II Doctor diagnostic center.
What Does the P0373 Code Mean?
P0373 is stored when the PCM receives an unstable high-resolution timing signal. The expected pulse train may appear correctly for a while and then develop random gaps, irregular spacing, voltage fluctuations or brief dropouts.
The PCM depends on accurate timing-reference information to identify engine position. Depending on the vehicle, this signal may come from:
- An optical sensor inside an electronically controlled diesel injection pump
- A high-resolution crankshaft position sensor
- A distributor-mounted optical pickup
- A separate timing-reference sensor
- A slotted encoder disc or reluctor wheel
- An ignition-control module that conditions the timing signal
The letter “A” identifies a particular timing-reference circuit. It does not automatically mean Bank 1 or a particular physical location. Manufacturer wiring diagrams should be used to identify the correct circuit before testing begins.
How the High-Resolution Timing Signal Works
Some timing systems use an optical sensor aimed at a rotating encoder disc. Slots in the disc repeatedly interrupt a light source as the engine or injection pump rotates. The sensor converts those transitions into electrical pulses.
Other systems use a Hall-effect or magnetic sensor reading a toothed reluctor wheel. Regardless of the hardware, the basic purpose is similar: provide the PCM with a rapid series of accurately spaced position pulses.
The PCM uses those pulses to calculate:
- Crankshaft or injection-pump position
- Engine speed
- Fuel-injection timing
- Injection-event synchronization
- Ignition timing
- Misfire or combustion irregularities
A stable waveform has consistent amplitude, clean switching edges and predictable spacing. With P0373, the waveform becomes unreliable temporarily. That distinction matters because a circuit can pass a resistance or voltage test while sitting in the driveway and still fail under heat, vibration or engine movement.
Common P0373 Symptoms
Symptoms depend on how frequently the timing signal becomes unstable and whether the PCM can substitute another position input.
Common symptoms include:
- Illuminated Check Engine Light
- Extended cranking
- Intermittent no-start condition
- Engine stalling
- Rough idle
- Hesitation during acceleration
- Sudden loss of power
- Uneven engine operation
- Poor throttle response
- Reduced-power or limp mode
- Increased fuel consumption
- Additional crankshaft, camshaft or engine-speed codes
A brief dropout may only turn on the Check Engine Light. A longer interruption can cause the PCM to lose synchronization and shut down fuel delivery or ignition.
If the engine-speed signal disappears completely, compare the diagnosis with P0322: Engine Speed Input Circuit No Signal. P0373 specifically identifies intermittent or erratic high-resolution Signal “A” pulses rather than a consistently absent general engine-speed input.
Common Causes of P0373
Possible causes include:
- Loose timing-sensor connector
- Spread, backed-out or weak connector terminals
- Corroded electrical contacts
- Water or oil inside the connector
- Broken wire beneath intact insulation
- Wiring routed too close to ignition or charging components
- Damaged shielding around the signal wire
- Intermittent short to ground
- Intermittent short to voltage
- Unstable sensor power supply
- Poor sensor or engine ground
- Failing optical, Hall-effect or magnetic timing sensor
- Contaminated optical sensor or encoder disc
- Cracked, warped or damaged encoder disc
- Excessive sensor air gap
- Loose reluctor wheel
- Excessive crankshaft end play
- Failing injection-pump electronics
- Low cranking voltage
- PCM connector damage
- Internal PCM failure
The sensor itself is only one possibility. Replacing it before inspecting the connector and waveform is parts-cannon diagnosis—occasionally entertaining, rarely efficient.
How Serious Is the P0373 Code?
P0373 should be treated as a moderate-to-serious fault.
A vehicle that runs normally with only an illuminated Check Engine Light may still be driven a short distance for diagnosis. However, an unstable timing-reference signal can deteriorate without warning.
Stop driving if the vehicle:
- Stalls in traffic
- Becomes difficult to restart
- Loses power unexpectedly
- Misfires heavily
- Enters reduced-power mode
- Produces abnormal diesel combustion noise
- Stores fuel-pressure or injection-timing faults with P0373
An intermittent timing signal can create a safety issue because the engine may shut off suddenly. It may also cause incorrect fuel-injection timing on affected diesel engines.
P0373 Compared With Related Codes
These codes describe different problems within the same timing-reference circuit:
| Code | Meaning | Diagnostic focus |
| P0370 | Signal “A” Malfunction | General circuit or signal failure |
| P0371 | Signal “A” Too Many Pulses | Extra transitions or excessive pulse count |
| P0372 | Signal “A” Too Few Pulses | Missing pulses or insufficient pulse count |
| P0373 | Signal “A” Intermittent/Erratic Pulses | Random dropouts or unstable pulse spacing |
| P0374 | Signal “A” No Pulses | Complete loss of high-resolution pulses |
P0373 should not be diagnosed as a generic crankshaft position sensor code. For a crankshaft Sensor “A” circuit failure, see the P0335 crankshaft position sensor guide.
It also differs from P0323: Engine Speed Input Circuit Intermittent. P0323 concerns the ignition or distributor engine-speed input. P0373 specifically concerns intermittent high-resolution timing-reference Signal “A” pulses.
How to Diagnose the P0373 Code
1. Scan All Stored Codes
Connect a capable scan tool and record:
- Stored codes
- Pending codes
- Freeze-frame data
- Engine speed
- Battery voltage
- Camshaft and crankshaft synchronization
- Injection timing data
- Fuel-system status
Do not immediately clear the codes. Freeze-frame information may reveal whether the fault occurred during cranking, acceleration, steady cruising or a hot restart.
Pay attention to related codes involving:
- Crankshaft position
- Camshaft position
- Engine-speed input
- Injection timing
- Sensor reference voltage
- Low system voltage
Multiple sensor codes can point toward a shared power supply, ground circuit or harness problem.
2. Identify the Correct Signal “A” Source
Consult the manufacturer’s wiring diagram and diagnostic information.
Determine:
- Which sensor generates Signal “A”
- Whether the sensor is optical, Hall-effect or magnetic
- Where the sensor is installed
- Which terminals provide power, ground and signal
- Whether the signal passes through another module
- The expected waveform and voltage range
This step matters because P0373 does not describe the same physical component on every vehicle.
3. Inspect the Wiring and Connector
Visually inspect the harness from the timing sensor to the PCM or relevant control module.
Look for:
- Loose connectors
- Broken locking tabs
- Bent or recessed pins
- Corrosion
- Oil contamination
- Water intrusion
- Chafed insulation
- Harness contact with hot engine parts
- Wiring stretched across brackets
- Previous repairs or poorly crimped splices
- Missing shielding
Gently tug each wire near the connector. A conductor may be broken internally even when its insulation looks normal.
Pay special attention to areas that move with engine torque. A harness that is barely connected may open only when the engine shifts under acceleration.
4. Perform a Wiggle Test
Monitor the timing signal, engine-speed data or synchronization status while moving the harness and connector by hand.
If the signal drops out when a particular section is moved, inspect that area closely for:
- Loose terminals
- Internal wire breaks
- Connector pin-tension problems
- Harness damage
- Poor previous repairs
Perform the test with the engine cold and again after it reaches operating temperature when safe. Heat expansion can expose faults that are invisible during a cold inspection.
5. Check Battery and Charging Voltage
Low voltage can reduce sensor output or disrupt PCM signal processing.
Test:
- Battery condition
- Cranking voltage
- Alternator output
- Battery-terminal voltage drop
- Engine-ground voltage drop
- PCM power and ground circuits
If cranking voltage falls below the manufacturer’s specification, correct the battery, cable or starter problem before condemning the timing sensor.
6. Test Sensor Power and Ground
Back-probe the sensor connector using the manufacturer’s specifications.
Verify:
- Correct supply voltage
- Stable ground
- Low voltage drop
- No intermittent power interruption
- No unwanted voltage on the ground circuit
A five-volt reference or sensor ground that fluctuates can create an erratic output even when the sensor is working correctly.
Avoid piercing wiring insulation unless the repair procedure allows it. Holes in insulation have a remarkable talent for becoming future corrosion problems.
7. Examine the Waveform With an Oscilloscope
An oscilloscope is the most useful tool for diagnosing P0373 because it shows faults that a multimeter averages out.
A healthy waveform should have:
- Consistent amplitude
- Clean rising and falling edges
- Uniform pulse spacing
- No unexpected gaps
- No random voltage spikes
- Stable operation as RPM changes
A P0373 waveform may show:
- Brief flat sections
- Random missing pulses
- Irregular pulse spacing
- Unstable amplitude
- Electrical noise
- Distorted switching edges
- Dropouts caused by heat or vibration
Capture the waveform during cranking, idle and controlled RPM changes. If possible, record it while reproducing the driving condition stored in the freeze-frame data.
8. Inspect the Encoder Disc or Reluctor Wheel
If the sensor waveform is erratic despite stable power, ground and wiring, inspect the component being read by the sensor.
Possible problems include:
- Cracked encoder slots
- Dirt or fuel contamination
- Metal debris
- Warped disc
- Damaged reluctor teeth
- Loose reluctor wheel
- Excessive shaft movement
- Incorrect sensor air gap
On optical systems, contamination can partially block light passing through the encoder slots. On magnetic systems, debris or excessive clearance can weaken the waveform.
9. Compare Related Sensor Signals
Use a dual-channel oscilloscope when possible to compare the high-resolution Signal “A” waveform with the crankshaft or camshaft signal.
The waveforms should maintain a consistent relationship. A changing correlation may indicate:
- Mechanical timing movement
- Loose encoder hardware
- Excessive shaft play
- Sensor mounting problems
- Injection-pump drive problems
If components were recently replaced, the PCM may also require a crankshaft position variation relearn. See the P0315 crankshaft position variation relearn guide.
10. Test the PCM Last
PCM failure is possible, but it is considerably less common than wiring, connector, sensor or encoder problems.
Before replacing or programming the PCM, verify:
- Sensor output reaches the PCM connector
- PCM powers and grounds remain stable
- Connector terminals have proper tension
- No technical service bulletin identifies a known software issue
- All required relearn procedures have been performed
Module replacement should be the conclusion of testing—not the opening act.
Common P0373 Diagnostic Mistakes
Replacing the Sensor Without Testing the Harness
An intermittent open circuit can produce the same symptoms as a failing sensor. Inspect terminal tension and monitor the circuit during a wiggle test before ordering parts.
Using Only a Multimeter
A multimeter may display an apparently normal average voltage while the signal repeatedly drops out. An oscilloscope reveals actual pulse quality.
Testing Only With the Engine Cold
Some sensors and connectors fail only after heat expansion. If the problem occurs after warm-up, testing a cold engine may produce a false pass.
Ignoring Battery Voltage
Low cranking voltage can weaken sensor signals and confuse the PCM. Test the starting and charging system before diving into expensive injection-pump components.
Confusing P0373 With P0372 or P0374
P0372 indicates an insufficient pulse count. P0374 indicates no detected pulses. P0373 indicates a signal that appears and disappears or becomes erratic.
Ignoring Mechanical Movement
A loose reluctor, damaged encoder disc or excessive shaft play can create an electrical-looking fault. The waveform may be ugly because the hardware generating it is moving incorrectly.
Common P0373 Repairs
The correct repair depends on test results, but common repairs include:
- Cleaning and securing the sensor connector
- Repairing damaged wiring
- Replacing loose connector terminals
- Removing corrosion or moisture
- Restoring signal-wire shielding
- Repairing sensor power or ground circuits
- Replacing the timing-reference sensor
- Cleaning an optical timing assembly
- Correcting the sensor air gap
- Replacing a damaged encoder disc or reluctor wheel
- Repairing injection-pump electronics
- Repairing battery cables or engine grounds
- Replacing a weak battery
- Updating PCM software
- Replacing the PCM when confirmed defective
- Performing the required timing or crankshaft relearn
Do not replace the entire injection pump simply because the sensor is located inside it. Confirm whether the sensor or optical assembly can be serviced separately.
Estimated P0373 Repair Costs
Actual prices vary by vehicle, labor rate and component location.
| Repair | Estimated cost |
| Diagnostic inspection | $100–$220 |
| Connector or minor wiring repair | $100–$350 |
| Battery or ground repair | $100–$450 |
| Timing-reference sensor replacement | $180–$650 |
| Crankshaft position sensor replacement | $180–$500 |
| Encoder or reluctor repair | $300–$1,200 |
| Injection-pump repair or replacement | $900–$3,500+ |
| PCM software update | $100–$250 |
| PCM replacement and programming | $700–$2,000+ |
These figures are general estimates. On some diesel engines, accessing the timing sensor requires substantial disassembly, making labor the expensive part of the repair.
Can You Drive With a P0373 Code?
A short trip may be possible if the engine starts easily, runs smoothly and shows no loss of power. However, continuing to drive is risky because an intermittent signal can disappear completely.
Avoid driving if the engine stalls, hesitates heavily, becomes difficult to restart or loses power. Have the vehicle inspected before a minor dropout becomes a roadside demonstration of poor timing.
How to Verify the Repair
After completing the repair:
- Clear the stored and pending codes.
- Start the engine cold.
- Monitor Signal “A,” RPM and synchronization data.
- Allow the engine to reach operating temperature.
- Perform a harness wiggle test.
- Road-test the vehicle under the original freeze-frame conditions.
- Rescan for pending codes.
- Confirm that the waveform remains stable.
- Complete any required relearn procedure.
A cleared Check Engine Light is not proof of a successful repair. The signal must remain stable under heat, vibration and changing engine speed.
P0373 Frequently Asked Questions
What does P0373 mean?
P0373 means the PCM detected intermittent or erratic pulses from timing-reference high-resolution Signal “A.”
What causes the P0373 code?
Common causes include damaged wiring, loose connector terminals, unstable sensor power, a failing timing sensor, optical contamination, an incorrect sensor air gap or a damaged encoder disc.
Is P0373 a crankshaft position sensor code?
Not necessarily. Signal “A” may originate from a crankshaft sensor, optical sensor, distributor or diesel injection-pump timing sensor, depending on the vehicle.
Can a weak battery cause P0373?
Yes. Low cranking voltage can weaken sensor output or interrupt PCM signal processing. Battery and voltage-drop testing should be part of the diagnosis.
Can P0373 cause a no-start condition?
Yes. If the PCM cannot reliably determine engine position, it may delay or disable ignition or fuel injection.
How is P0373 different from P0370?
P0370 is a general high-resolution Signal “A” malfunction. P0373 specifically indicates that the pulse signal is intermittent or erratic.
How is P0373 different from P0323?
P0323 concerns an intermittent ignition or distributor engine-speed input circuit. P0373 concerns intermittent high-resolution timing-reference Signal “A” pulses.
Do I need an oscilloscope to diagnose P0373?
An oscilloscope is strongly recommended. A multimeter may miss brief dropouts, noise and irregular pulse spacing.
Final Thoughts
The P0373 code identifies an unstable high-resolution timing-reference signal. The most productive diagnosis is to reproduce the failure while monitoring the actual waveform.
Begin with the battery, grounds, connector and harness. Then verify sensor output, encoder condition and signal integrity before replacing major components. Intermittent electrical faults require evidence, patience and occasionally some creative harness wiggling—not a shopping cart full of sensors.
For more diagnostic information, browse the Pro Street OBD-II Doctor and the complete P-series OBD-II code index.
