P0373 Timing Reference High Resolution Signal "A" Intermittent

The P0373 diagnostic trouble code means that the engine control module (ECM/PCM) has detected an intermittent or unstable signal from the Timing Reference High Resolution Signal “A” circuit.

This is an engine timing-related fault. The ECM expects to receive a consistent series of high-resolution timing pulses as the engine rotates. With P0373, the signal is present at least some of the time, but it drops out, becomes erratic, or does not follow the expected pattern intermittently.

The code does not automatically mean that a timing sensor needs to be replaced. The sensor, wiring, connector, power supply, ground, trigger mechanism, electrical interference, and even mechanical timing can all produce this type of fault.

What Is the Timing Reference High Resolution Signal?

The ECM needs very precise information about engine rotation to control the engine correctly.

A high-resolution timing reference signal consists of a series of pulses generated as a timing target rotates. The ECM analyzes these pulses to determine engine position and speed with high precision.

This information can be used for:

• Ignition timing

• Fuel injection timing

• Engine speed calculation

• Crankshaft position

• Camshaft synchronization

• Variable valve timing

• Misfire detection

• Fuel and ignition control

The exact source of Signal “A” is manufacturer-specific. It may be associated with a crankshaft, camshaft, dedicated timing pickup, or another timing-reference system.

For this reason, P0373 should not automatically be interpreted as a particular crankshaft or camshaft sensor failure.

What Does “Intermittent” Mean?

The word “Intermittent” is the key to understanding P0373.

The ECM is not necessarily losing the signal permanently. Instead, the signal may behave normally and then temporarily become abnormal.

For example, the timing signal could:

• Disappear for a fraction of a second

• Produce an irregular pulse

• Drop out at a particular RPM

• Become noisy

• Change when the engine gets hot

• Fail when the wiring vibrates

• Return when engine speed changes

• Become unstable during acceleration

The engine may therefore operate normally most of the time while occasionally setting P0373.

This can make the problem particularly difficult to diagnose because a basic inspection may show nothing wrong when the vehicle is sitting in the workshop.

Symptoms of P0373

The symptoms depend on how frequently the timing signal becomes unstable and how important Signal A is to the engine-management system.

Possible symptoms include:

• Check Engine Light

• Difficult starting

• Extended cranking

• Intermittent no-start

• Engine stalling

• Rough idle

• Hesitation

• Poor acceleration

• Reduced engine power

• Unstable RPM

• Misfire

• Poor fuel economy

• Incorrect ignition timing

• Incorrect fuel injection timing

• Variable valve timing problems

• Limp-home mode

In mild cases, the driver may notice nothing other than the Check Engine Light.

In more severe cases, a momentary loss of timing information can cause a noticeable hesitation or even engine stalling.

Can P0373 Cause a No-Start?

Yes, it can.

If Signal A is essential for engine synchronization, an intermittent failure during cranking may prevent the ECM from establishing a reliable engine position.

The starter may turn the engine normally, but the engine may fail to start because the control module cannot obtain the timing information required for ignition or fuel injection.

A vehicle may also start normally when cold and then develop a no-start condition after the engine becomes hot if the timing sensor or wiring has a temperature-related failure.

Common Causes of P0373

Failing timing reference sensor

A sensor may begin failing internally while still functioning most of the time.

Heat-related sensor failures are particularly common with intermittent faults. The sensor may work correctly when cold and begin losing its signal after reaching operating temperature.

Damaged or broken wiring

A conductor can be partially broken inside its insulation. It may maintain electrical continuity until engine vibration or movement causes the connection to open temporarily.

Loose connector terminal

A connector terminal that does not grip the sensor pin correctly can produce momentary signal loss.

Corrosion or contamination

Moisture, corrosion, oil, or dirt inside a connector can increase resistance and disrupt the timing signal.

Poor sensor ground

For sensors that require an electrical ground, a poor ground connection can cause an unstable output.

Unstable sensor power supply

A Hall-effect sensor or other powered sensor may malfunction if its supply voltage is unstable.

Short circuit

The signal wire may intermittently contact ground, battery voltage, another circuit, or a neighboring signal wire.

Incorrect sensor gap

If the distance between the sensor and its timing target is incorrect, the signal may become weak or unstable.

Damaged reluctor or trigger wheel

A damaged, loose, bent, cracked, or contaminated timing target can cause intermittent irregularities in the pulse pattern.

Electrical interference

Ignition components, alternators, poor grounds, or incorrectly routed wiring can introduce noise into sensitive timing circuits.

Mechanical timing problem

A stretched timing chain, damaged timing component, incorrect timing installation, or movement of a timing target can affect the expected signal.

Variable valve timing problem

If Signal A is associated with camshaft timing, VVT operation or oil-pressure problems can contribute to an abnormal timing relationship.

ECM/PCM problem

A control-module fault is possible, but it should normally be investigated only after the sensor, circuit, signal, trigger mechanism, and mechanical timing have been tested.

Why Intermittent Wiring Problems Are Difficult to Find

One of the biggest challenges with P0373 is that the vehicle may appear completely normal during a workshop inspection.

A technician might measure continuity through a wire and find that it is fine.

But the wire could have an internal break that opens only when:

• The engine vibrates

• The harness moves

• The engine rocks under acceleration

• The temperature rises

• The wiring expands

• The connector moves

• RPM increases

This is why simply checking continuity with the engine switched off may not be enough.

A wiggle test, voltage-drop test, and waveform monitoring under the conditions that trigger the fault can be much more revealing.

P0373 Does Not Automatically Mean a Bad Sensor

This is particularly important with an intermittent timing code.

The ECM is reporting that the signal is not consistently behaving as expected.

It is not saying:

“The sensor itself is definitely defective.”

A good sensor can produce P0373 when its connector has a loose terminal or its signal wire intermittently opens.

Likewise, a perfectly good sensor can produce an abnormal signal if its trigger wheel is damaged or incorrectly positioned.

Replacing the sensor without checking the rest of the circuit may therefore fail to solve the problem.

How to Diagnose P0373

A structured diagnostic approach is important.

1. Scan for all codes

Start by checking all stored and pending DTCs.

Pay particular attention to codes involving:

• Crankshaft position

• Camshaft position

• Engine synchronization

• VVT

• Misfire

• Engine speed

• Ignition

• Fuel injection

• Communication

If several timing-related codes appear together, the underlying problem may be broader than Signal A alone.

2. Check freeze-frame data

Look at the conditions when P0373 was stored.

Useful information includes:

• Engine RPM

• Engine temperature

• Battery voltage

• Engine load

• Vehicle speed

• Starting or running condition

If the fault repeatedly occurs at a particular RPM or temperature, that can provide a valuable diagnostic clue.

3. Identify Signal A

The exact meaning of Signal “A” varies by manufacturer.

Use the vehicle's wiring diagram and service information to determine:

• Which sensor produces Signal A

• Sensor technology

• Connector pinout

• Power supply

• Ground

• Signal wire

• Timing target

• Expected waveform

Do not assume that Signal A always means a particular crankshaft or camshaft sensor.

4. Inspect the sensor

Check for:

• Physical damage

• Oil contamination

• Metal debris

• Incorrect installation

• Loose mounting

• Incorrect sensor position

• Damage near the sensing tip

If the sensor uses a specified air gap, verify that the gap is within the manufacturer's specification.

5. Inspect the connector

Look for:

• Corroded terminals

• Loose pins

• Bent pins

• Water intrusion

• Oil contamination

• Damaged connector locks

• Terminals pushed backward inside the connector

An apparently connected plug does not guarantee that every terminal is making reliable electrical contact.

6. Inspect the wiring

Follow the harness from the sensor toward the ECM.

Look for:

• Chafing

• Broken insulation

• Heat damage

• Pinched wires

• Previous repairs

• Poor aftermarket splices

• Contact with moving components

• Contact with exhaust components

Where appropriate, gently manipulate the harness while monitoring the signal. This can help expose an intermittent open or short.

Monitor the Signal While the Fault Occurs

For P0373, observing the actual signal can be much more informative than simply reading the stored code.

An oscilloscope can reveal:

• Momentary signal loss

• Missing pulses

• Irregular pulse spacing

• Weak signal amplitude

• Electrical noise

• Distorted waveform

• Changes at different RPM

• Temperature-related signal degradation

A scan tool may show a normal engine RPM value even though individual high-resolution pulses are being lost.

That is why waveform analysis can be especially valuable for intermittent timing faults.

Check Power and Ground Under Load

If Signal A comes from a powered sensor, its power and ground should be checked under operating conditions.

A circuit can show an apparently correct voltage when the engine is off and still have a problem while operating.

Voltage-drop testing can identify:

• Poor grounds

• Corroded connections

• High-resistance wiring

• Weak power connections

The correct voltage values depend on the sensor and vehicle, so manufacturer specifications should be followed rather than applying a universal value.

Check the Trigger or Reluctor

If the electrical circuit appears healthy, inspect the physical timing target.

Depending on the vehicle, it may be a:

• Reluctor wheel

• Trigger wheel

• Tone wheel

• Crankshaft target

• Camshaft target

• Timing gear

• Other manufacturer-specific timing component

Look for missing or damaged teeth, looseness, cracks, excessive contamination, or incorrect positioning.

An intermittent mechanical movement of the trigger can create a signal problem even when the sensor and wiring are good.

Could Mechanical Timing Cause P0373?

Yes.

Mechanical timing problems can alter the relationship between the engine's rotating components and the timing-reference system.

Potential causes include:

• Stretched timing chain

• Incorrectly installed timing chain

• Damaged timing sprocket

• Loose timing component

• Incorrect camshaft timing

• Damaged reluctor

• Variable valve timing malfunction

Mechanical timing should receive particular attention when P0373 occurs alongside:

• Cam/crank correlation codes

• Difficult starting

• Backfiring

• Abnormal timing-chain noise

• Sudden stalling

• Multiple synchronization faults

Hall-Effect and Variable-Reluctance Sensors

Timing sensors do not all operate using the same technology.

A Hall-effect sensor generally requires an electrical supply and produces a digital switching signal.

A variable-reluctance sensor generates an alternating signal as the magnetic field changes when the timing target passes the sensor.

These sensors require different testing procedures.

There is therefore no universal resistance or voltage specification for P0373. The correct values and waveform must be obtained from the manufacturer's service information.

Can Low Battery Voltage Cause P0373?

Low system voltage can contribute to timing-related faults in some vehicles.

During cranking, battery voltage can drop substantially. If the sensor or associated electronics cannot operate correctly at the available voltage, the timing signal may become unreliable.

However, low battery voltage should not automatically be considered the cause.

If voltage is suspected, check:

• Battery condition

• Cranking voltage

• Starter operation

• Battery terminals

• Engine and chassis grounds

• Charging-system performance

P0373 vs Related Timing Reference Codes

The nearby codes describe different timing-signal conditions.

P0373 – Timing Reference High Resolution Signal “A” Intermittent

The Signal A timing reference becomes intermittent or erratic.

P0372 – Timing Reference High Resolution Signal “A” Too Few Pulses

The ECM detects fewer pulses than expected.

P0371 – Timing Reference High Resolution Signal “A” Too Many Pulses

The ECM detects more pulses than expected.

P0370 – Timing Reference High Resolution Signal “A” Malfunction

A general malfunction has been detected in Signal A.

P0374 – Timing Reference High Resolution Signal “A” No Pulses

The expected Signal A pulses are not detected.

The distinction is useful because P0373 specifically points toward an unstable or intermittent signal, while P0374 indicates a complete absence of pulses.

Why Temperature Can Be an Important Clue

A timing sensor that fails only after the engine warms up can be particularly difficult to diagnose.

For example, the vehicle may:

• Start normally in the morning

• Run perfectly for several minutes

• Begin hesitating after reaching operating temperature

• Stall when hot

• Restart after cooling down

This pattern can point toward a temperature-sensitive sensor or electrical connection, although it does not prove the sensor is the cause.

Freeze-frame information and waveform testing while the fault is occurring can help confirm the relationship.

Is It Safe to Drive With P0373?

If the vehicle starts normally, runs smoothly, and the Check Engine Light remains steady, a short drive to a repair facility may be possible.

However, an intermittent timing signal should not be ignored.

The fault could become more frequent and eventually cause:

• Engine stalling

• Loss of power

• Difficult starting

• Complete no-start

Driving should be avoided if the vehicle is already stalling, experiencing severe misfires, or suddenly losing power.

A Practical Diagnostic Path

For P0373, a useful diagnostic sequence is:

P0373 → scan all codes → review freeze-frame → identify Signal A → inspect sensor → inspect connector → inspect wiring → verify power and ground → monitor the signal → perform wiggle/heat/vibration testing where appropriate → inspect trigger mechanism → check synchronization → investigate VVT and mechanical timing if indicated → repair → clear codes → road test under the original fault conditions → verify the signal remains stable.

This approach is particularly important because intermittent faults can easily lead to unnecessary sensor or ECM replacement.

Final Thoughts

P0373 – Timing Reference High Resolution Signal “A” Intermittent means the ECM/PCM is detecting an unstable or intermittently incorrect high-resolution timing reference signal.

The cause may be a deteriorating sensor, but it could also be a loose connector, damaged wiring, poor power or ground, electrical interference, an incorrect sensor gap, a damaged trigger wheel, VVT-related behavior, or a mechanical timing problem.

Because the fault is intermittent, diagnosing the vehicle while the problem is actually occurring is often the key. Freeze-frame data, harness movement tests, live data, and especially oscilloscope waveform analysis can reveal problems that a simple resistance or continuity test may completely miss.