P0377 Timing Reference High Resolution Signal "B" Too Few Pulses

The P0377 diagnostic trouble code means that the engine control module (ECM/PCM) has detected fewer pulses than expected from the Timing Reference High Resolution Signal “B” circuit.

This is a timing-related fault. The control module expects to receive a specific number and pattern of high-resolution timing pulses as the engine rotates. When fewer pulses are detected than expected, the ECM may no longer have the precise timing information it needs to accurately determine engine position.

P0377 does not automatically mean that the timing sensor is defective. A missing or reduced number of pulses can result from a sensor problem, damaged wiring, a poor connector, an incorrect sensor gap, a damaged reluctor or trigger wheel, electrical interference, or even a mechanical timing problem.

What Is the Timing Reference High Resolution Signal?

The engine control module needs extremely accurate information about engine rotation.

A high-resolution timing signal provides a series of electrical pulses as a crankshaft, camshaft, or another timing target rotates. By counting and measuring these pulses, the ECM can determine engine speed and position much more precisely than it could from a simple on/off indication that the engine is turning.

Depending on the vehicle, these timing signals may be used for:

• Ignition timing

• Fuel injection timing

• Crankshaft position

• Camshaft synchronization

• Engine speed calculation

• Variable valve timing

• Misfire detection

• Fuel and ignition control

The exact source of Signal “B” is manufacturer-specific. It should therefore not automatically be identified as a particular crankshaft or camshaft sensor without checking the vehicle's wiring diagram and service information.

What Does “Too Few Pulses” Mean?

The phrase “Too Few Pulses” describes what the ECM has detected in the timing signal.

Imagine that the control module expects a certain number of timing pulses during a specific period or engine rotation. If some of those pulses disappear, the ECM can determine that the signal does not match the expected pattern.

For example, the signal may look normal most of the time but occasionally contain:

• Missing pulses

• Weak pulses

• Distorted pulses

• Irregular pulse spacing

• Incorrect pulse frequency

• Signal dropouts

The result is that the ECM counts fewer pulses than it should.

This differs from P0379, where the ECM detects no pulses from the monitored signal. P0377 indicates that pulses are being detected, but there are not enough of them.

Why Does the Number of Pulses Matter?

The ECM uses the pulse pattern as a reference for engine position.

If some pulses are missing, the control module can temporarily lose precision when determining where the engine is in its rotation.

This can affect:

• Spark timing

• Injector timing

• Camshaft synchronization

• VVT operation

• Engine speed calculation

• Misfire detection

The severity depends on how many pulses are missing and whether another timing signal is available to provide backup information.

A small number of missing pulses may cause an intermittent warning light or slight drivability problem. A more serious signal problem can cause difficult starting, stalling, or a no-start condition.

Common Symptoms of P0377

The symptoms depend heavily on how frequently the signal loses pulses and how important Signal B is to the particular engine.

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

Some vehicles may continue to run almost normally because the ECM can use another timing signal as a reference.

Others may experience severe symptoms because the missing pulses prevent the ECM from maintaining accurate engine synchronization.

Can P0377 Cause a No-Start?

It can.

If the high-resolution Signal B is essential to the engine's synchronization strategy, losing enough timing pulses may prevent the ECM from establishing reliable engine position.

The starter may turn the engine normally, but the ECM may not be able to determine the precise timing needed to control ignition or injection.

However, a no-start is not guaranteed with P0377. Some vehicles use redundant timing signals or alternative synchronization strategies and may continue running despite the fault.

Most Common Causes of P0377

Faulty timing reference sensor

A sensor can become weak or unstable and fail to produce every expected pulse. Temperature, vibration, and engine speed can affect the failure.

Damaged wiring

A partially broken conductor or wire with excessive resistance can cause some pulses to disappear.

This is particularly important with intermittent faults because a harness may work normally when stationary but lose continuity when the engine vibrates.

Poor connector contact

Loose terminals, corrosion, oil contamination, moisture, or damaged connector pins can interrupt individual pulses.

Incorrect sensor installation

If the sensor is installed incorrectly or its air gap is outside the manufacturer's specification, the signal may become weak or inconsistent.

Damaged reluctor or trigger wheel

The sensor needs a properly shaped target to generate the expected pulse pattern.

A damaged, bent, loose, cracked, contaminated, or incorrectly positioned trigger wheel can result in missing pulses.

Electrical interference

Electrical noise can distort a timing signal and cause the ECM to misinterpret some pulses.

Sensor power or ground problem

On sensors that require an external power supply, unstable voltage or poor grounding can interfere with signal generation.

Mechanical timing problems

A stretched timing chain, damaged sprocket, incorrect timing installation, or other mechanical problem can affect the relationship between the sensor and its timing target.

Variable valve timing problems

On engines where the monitored signal is associated with camshaft timing, VVT problems can contribute to an unexpected timing pattern.

ECM/PCM problem

A control module fault is possible but should generally be considered only after the sensor, circuit, trigger mechanism, and mechanical timing have been tested.

P0377 Does Not Automatically Mean a Bad Sensor

This is one of the most important points when diagnosing the code.

The ECM is reporting that it counted fewer timing pulses than expected. It is not necessarily reporting that the sensor itself is defective.

For example, a perfectly good sensor can produce too few pulses if:

• Its power supply is unstable.

• Its ground connection is poor.

• The signal wire has an intermittent open.

• The connector terminal is loose.

• The trigger wheel is damaged.

• The sensor gap is incorrect.

Replacing the sensor without testing these possibilities can result in an unnecessary repair.

How to Diagnose P0377

A proper diagnosis should establish why the ECM is seeing fewer pulses.

1. Scan for additional codes

Begin by scanning all available modules.

Look for codes involving:

• Crankshaft position

• Camshaft position

• Engine synchronization

• Variable valve timing

• Ignition

• Fuel injection

• Communication

• Engine speed

Additional codes can help identify whether P0377 is an isolated electrical issue or part of a larger timing problem.

2. Examine freeze-frame data

Determine when the code was recorded.

Important information includes:

• Engine RPM

• Engine temperature

• Battery voltage

• Engine load

• Vehicle speed

• Starting or running condition

If the code appears only at high RPM, for example, the problem may behave differently from a fault that occurs immediately during cranking.

3. Identify Signal B

The term “B” is not universal.

Use the manufacturer's service information to determine exactly which timing circuit the vehicle identifies as Signal B.

Do not assume that it always refers to:

• A particular crankshaft sensor

• A particular camshaft sensor

• The exhaust camshaft

• The intake camshaft

The naming convention varies between manufacturers and engine designs.

4. Inspect the sensor and connector

Look carefully for:

• Corrosion

• Loose terminals

• Bent pins

• Damaged locking mechanisms

• Oil contamination

• Water intrusion

• Chafed wiring

• Broken insulation

• Harness damage

Pay particular attention to sections of wiring close to the engine where heat and vibration are significant.

5. Check power and ground

If the sensor requires a power supply, verify it according to the manufacturer's specifications.

Check the ground as well.

A circuit can show apparently normal voltage with no load but develop a significant voltage drop when the sensor is operating.

For this reason, voltage-drop testing can be more useful than simply measuring static voltage.

6. Check the actual timing waveform

An oscilloscope can be extremely valuable for P0377.

A scope allows the technician to see whether the timing signal contains:

• Missing pulses

• Weak pulses

• Irregular spacing

• Distorted waveforms

• Electrical noise

• Intermittent dropouts

• Changes related to engine speed

A scan tool may indicate that the engine is producing an RPM signal, but that does not necessarily prove that every high-resolution Signal B pulse is present.

7. Inspect the trigger mechanism

If the electrical circuit checks out, inspect the physical target that produces the signal.

Depending on the vehicle, this could be a:

• Reluctor wheel

• Trigger wheel

• Tone wheel

• Crankshaft target

• Camshaft target

• Timing gear

• Other manufacturer-specific timing component

A damaged or loose target can produce missing pulses even when the sensor itself is functioning correctly.

Hall-Effect vs Variable-Reluctance Sensors

Different timing sensors produce signals in different ways.

A Hall-effect sensor normally uses an electrical supply and produces a digital switching signal as the target passes.

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

Because the technology differs, the correct testing method also differs.

There is no universal resistance or voltage value that can be applied to every P0377 vehicle. The manufacturer's specifications should always be used.

Can a Timing Chain Cause P0377?

Yes, depending on the engine and the source of Signal B.

A mechanical timing problem can alter the relationship between the timing target and sensor.

Possible issues include:

• Stretched timing chain

• Incorrectly installed timing chain

• Damaged timing sprocket

• Loose timing component

• Incorrect camshaft timing

• Damaged reluctor

• Variable valve timing malfunction

If P0377 appears together with camshaft/crankshaft correlation codes, abnormal timing-chain noise, difficult starting, or sudden engine stalling, mechanical timing should be investigated rather than simply replacing the sensor.

Can Low Battery Voltage Trigger P0377?

Low system voltage can contribute to timing signal problems on some vehicles.

During cranking, battery voltage can fall significantly. If the sensor or associated electronics cannot operate correctly at the available voltage, the timing signal may become weak or incomplete.

Nevertheless, low battery voltage should not be assumed to be the cause merely because P0377 is present.

Battery condition, starter operation, cranking voltage, grounds, and charging-system performance should be tested when voltage-related problems are suspected.

P0377 vs P0378 vs P0379

These codes are closely related but describe different signal conditions.

P0377 – Too Few Pulses

The ECM detects a timing signal but counts fewer pulses than expected.

P0378 – Intermittent/Erratic Pulses

The timing signal is present but becomes unstable, irregular, or intermittent.

P0379 – No Pulses

The ECM does not detect the expected timing pulses at all.

This distinction can help guide diagnosis, but the exact testing procedure still depends on the vehicle.

Can P0377 Be Intermittent?

Yes.

Although the official description is “Too Few Pulses,” the underlying cause can be intermittent.

For example, a damaged wire may maintain continuity while the vehicle is stationary but lose contact when:

• The engine moves under load

• The harness vibrates

• Engine temperature increases

• RPM increases

• The wiring is flexed

A sensor can also produce enough pulses at idle but begin missing pulses at higher RPM.

That is why reproducing the fault under the conditions shown in the freeze-frame data can be particularly useful.

Is It Safe to Drive With P0377?

If the engine starts easily, runs normally, and the Check Engine Light remains steady, a short trip to a repair facility may be possible.

However, P0377 should not be ignored.

A timing signal that is already losing pulses can potentially deteriorate further. If the signal becomes unreliable enough, the engine may stall or fail to restart.

Stop driving and investigate the vehicle promptly if there is:

• Sudden engine stalling

• Severe misfire

• Major loss of power

• Difficult starting

• Repeated no-start

• Strong timing-chain noise

• Significant engine vibration

A More Effective Diagnostic Strategy

A practical diagnostic sequence is:

P0377 → scan all codes → review freeze-frame → identify Signal B → inspect sensor and connector → check wiring → verify power and ground → monitor the signal → use an oscilloscope if necessary → inspect the trigger mechanism → compare timing/synchronization signals → check mechanical timing → repair → clear codes → road test → confirm that the expected pulse pattern has returned.

This sequence is preferable to replacing the timing sensor immediately.

Final Thoughts

P0377 – Timing Reference High Resolution Signal “B” Too Few Pulses indicates that the ECM/PCM is receiving a timing reference signal, but the number of detected pulses is lower than expected.

The cause can be a weak or failing sensor, but it can just as easily involve the wiring, connector, power supply, ground, sensor installation, trigger wheel, electrical interference, or mechanical timing.

The most useful diagnostic evidence comes from determining exactly what Signal B represents on the specific vehicle and then checking whether the expected pulse pattern is actually being produced. An oscilloscope can be particularly valuable because it allows the technician to see missing or distorted pulses that a basic scan-tool reading may not reveal.