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P0377 Code: Timing Reference High-Resolution Signal “B” Too Few Pulses

P0377 means the PCM detected too few high-resolution timing-reference Signal “B” pulses. Learn the causes, diagnostic process and repair costs.

The P0377 code means the powertrain control module detected fewer high-resolution timing-reference Signal “B” pulses than expected during a measured engine cycle.

Signal “B” is still present, which separates P0377 from a complete no-signal condition. However, some expected pulses are missing or too weak for the PCM to recognize. Common causes include optical contamination, a damaged encoder disc, excessive sensor clearance, weak sensor output, wiring resistance or low cranking voltage.

P0377 is commonly associated with electronically controlled diesel injection systems, although its exact application varies by manufacturer. Always identify the correct Signal “B” source through vehicle-specific service information before replacing parts.

P0377 Quick Reference

ItemDetails
CodeP0377
DefinitionTiming Reference High-Resolution Signal “B” Too Few Pulses
SystemEngine-position or injection-timing reference
Primary faultSignal “B” pulse count below the expected value
Common causesMissing encoder transitions, weak sensor output, contamination or excessive air gap
Common symptomsHard starting, rough running, stalling, hesitation or reduced power
SeverityModerate to serious
Safe to drive?Not recommended when drivability symptoms are present
Best diagnostic toolScan tool and oscilloscope

For more definitions, visit the complete P-series OBD-II code list or browse the Pro Street OBD-II Doctor.

What Does the P0377 Code Mean?

P0377 is stored when the PCM counts fewer Signal “B” timing pulses than its programmed strategy expects.

The PCM knows how many pulses should occur during a defined crankshaft, camshaft or injection-pump rotation. If the detected count remains below that value, it determines that part of the high-resolution timing information is missing.

Depending on the vehicle, Signal “B” may come from:

The “B” identifies a particular signal circuit. It does not automatically mean Bank 2, Sensor 2, Camshaft Sensor “B” or Ignition Coil “B.”

How Signal “B” Generates Timing Pulses

Many systems associated with P0377 use an optical sensor and rotating encoder disc.

The disc contains precisely arranged slots. As it rotates, the slots allow or block light traveling toward the sensor. Each transition creates an electrical pulse.

Other systems use a Hall-effect or magnetic sensor that reads a toothed reluctor wheel. Each tooth or opening creates a voltage transition corresponding with rotational position.

The PCM may use Signal “B” to calculate or verify:

A healthy waveform contains the correct number of clean, evenly spaced pulses. A P0377 waveform contains gaps or weak transitions that the PCM fails to count.

What Causes Too Few Timing Pulses?

P0377 usually results from either missing physical transitions or a waveform too weak for the PCM to recognize.

Missing physical transitions

Physical encoder or reluctor damage may eliminate expected pulse events.

Possible causes include:

Weak electrical transitions

The sensor may produce a waveform, but parts of it may not cross the PCM’s recognition threshold.

Possible causes include:

The waveform exists, but some pulses arrive looking more like vague suggestions. The PCM prefers actual voltage transitions.

Common P0377 Symptoms

Common symptoms include:

A small pulse-count discrepancy may only illuminate the Check Engine Light. A large number of missing pulses can cause incorrect timing calculations or prevent synchronization.

Common Causes of P0377

Possible causes include:

How Serious Is P0377?

P0377 is a moderate-to-serious fault because the PCM is losing part of a position signal used for timing calculations.

Possible consequences include:

A vehicle with no noticeable symptoms may be driven a short distance for diagnosis. Stop driving if it stalls, loses power, runs harshly or becomes difficult to restart.

P0377 Compared With Related Codes

CodeMeaningDiagnostic focus
P0372Signal “A” Too Few PulsesInsufficient pulse count on Signal “A”
P0374Signal “A” No PulsesComplete loss of Signal “A”
P0375Signal “B” MalfunctionGeneral Signal “B” fault
P0376Signal “B” Too Many PulsesExcessive Signal “B” pulse count
P0377Signal “B” Too Few PulsesInsufficient Signal “B” pulse count
P0378Signal “B” Intermittent/ErraticUnstable or randomly dropping pulses
P0379Signal “B” No PulsesComplete loss of Signal “B”

P0377 should remain centered on an insufficient pulse count. It is not the general Signal “B” malfunction article and does not cover a completely dead circuit.

P0377 Versus P0376

P0376 means the PCM detects excessive Signal “B” pulses.

P0377 means the PCM detects fewer Signal “B” pulses than expected.

The diagnostic paths overlap, but their waveform conditions are opposite:

P0377 Versus P0378

P0377 identifies an insufficient pulse count during a monitored cycle. The signal may remain consistently weak or lose the same pulse group repeatedly.

P0378 identifies intermittent or erratic pulses that appear and disappear unpredictably, often because of vibration, heat or an unstable connection.

P0377 Versus P0379

P0377 means at least some pulses are detected.

P0379 means the PCM receives no Signal “B” pulses at all.

That distinction is especially clear on an oscilloscope: P0377 shows a waveform with gaps, while P0379 shows a flat or inactive signal.

How to Diagnose the P0377 Code

1. Scan All Modules

Connect a capable scan tool and record:

Do not clear the codes before recording the data.

The freeze-frame information can show whether the missing pulses occurred during cranking, idle, acceleration or a hot restart.

2. Identify the Signal “B” Source

Consult the correct service information and determine:

Testing the wrong “B” sensor is an impressively efficient way to learn nothing.

3. Check for Manufacturer Bulletins

Search for technical service information concerning:

A manufacturer-specific test plan should take priority over generic procedures.

4. Check Battery Condition and Cranking Speed

A weak battery or slow starter can reduce the amplitude and frequency of a position-sensor waveform.

Test:

Correct low voltage or slow cranking before replacing the timing sensor.

5. Inspect the Connector and Wiring

Inspect the Signal “B” circuit from the sensor to the PCM or associated control module.

Look for:

A corroded or loose terminal can reduce signal amplitude enough for some pulses to disappear from the PCM’s perspective.

6. Verify Sensor Power and Ground

Back-probe the sensor using the manufacturer’s specifications.

Confirm:

Test the circuits while cranking when possible. A supply voltage that looks normal with the engine off may collapse under starter load.

7. Check Signal-Circuit Integrity

Test the signal wire for:

A basic continuity test may not expose a circuit that fails under load. Use a loaded-circuit or voltage-drop test when the service procedure permits it.

8. Examine Signal “B” With an Oscilloscope

An oscilloscope is the best tool for identifying missing pulses.

A healthy waveform should display:

A P0377 waveform may display:

Count the pulses and compare the waveform with manufacturer specifications.

9. Compare the Sensor and PCM Waveforms

Measure Signal “B” at the sensor and again at the PCM.

Possible results include:

This comparison prevents replacing a good sensor when the signal is being lost inside the harness.

10. Compare Signals “A” and “B”

Use a dual-channel oscilloscope when possible.

Compare:

If Signal “A” remains normal while Signal “B” loses pulses, focus on the Signal “B” channel.

If both signals weaken during cranking, check battery voltage, shared power, grounds and mechanical speed.

11. Inspect the Optical System

On an optical timing system, inspect:

Contamination may block certain slots while leaving others visible. That produces exactly the kind of incomplete pulse train associated with P0377.

12. Inspect the Reluctor and Sensor Air Gap

On Hall-effect or magnetic systems, check:

An excessive air gap weakens the signal. Some transitions may remain visible while others fall below the PCM’s recognition threshold.

13. Inspect Mechanical Operation

Verify:

A damaged or slipping drive can prevent the sensor from producing the expected number of pulses.

14. Test Injection-Pump Electronics

On applicable diesel engines, inspect:

Do not replace the complete pump until the external circuit, voltage supply, ground and waveform have been tested.

15. Evaluate the PCM Last

Consider the PCM only after verifying:

PCM failure is possible, but sensors, wiring and physical pulse-generating components fail more frequently.

Common P0377 Diagnostic Mistakes

Replacing the Sensor Without Checking Cranking Voltage

Low cranking speed can reduce waveform strength and cause the PCM to miss pulses.

Treating P0377 Like P0379

P0377 has some pulses. P0379 has none. That difference changes the most likely causes and testing strategy.

Using Only a Multimeter

A multimeter may display an acceptable average voltage while completely hiding missing individual pulses.

Ignoring the Encoder or Reluctor

A perfectly functional sensor cannot generate a complete waveform from a damaged or contaminated encoder.

Checking Continuity Without Circuit Load

A partially broken wire may pass a low-current continuity test while reducing signal amplitude during operation.

Replacing the Injection Pump Too Early

External power, ground, connector and wiring faults can imitate an internal pump problem. Expensive parts deserve actual evidence.

Common P0377 Repairs

Possible repairs include:

If repairs require a crankshaft-position relearn, see the P0315 crankshaft position variation guide.

Estimated P0377 Repair Costs

RepairEstimated cost
Diagnostic testing$100–$250
Connector or wiring repair$100–$450
Battery or ground repair$100–$500
Starter replacement$400–$1,200
Timing-reference sensor replacement$180–$700
Encoder or reluctor repair$350–$1,300
Injection-pump electronics repair$500–$1,800
Injection-pump replacement$1,000–$4,000+
PCM software update$100–$300
PCM replacement and programming$700–$2,000+

Actual repair costs depend on vehicle design, component accessibility and local labor rates.

Can You Drive With P0377?

A vehicle that starts normally and shows no drivability symptoms may be driven a short distance for diagnosis.

Stop driving if it:

An incomplete timing-reference waveform can deteriorate into a complete signal loss or no-start condition.

How to Verify the Repair

After completing the repair:

  1. Clear stored and pending codes.
  2. Confirm the battery is fully charged.
  3. Monitor cranking voltage.
  4. Observe Signal “B” with an oscilloscope.
  5. Confirm the correct pulse count.
  6. Compare Signal “B” at the sensor and PCM.
  7. Compare Signals “A” and “B.”
  8. Allow the engine to reach operating temperature.
  9. Reproduce the original freeze-frame conditions.
  10. Rescan for pending codes.
  11. Complete any required relearn procedure.

A successful repair produces the correct number of stable pulses throughout the engine-speed and temperature range.

P0377 Frequently Asked Questions

What does the P0377 code mean?

P0377 means the PCM detected fewer high-resolution timing-reference Signal “B” pulses than expected.

What commonly causes P0377?

Common causes include optical contamination, blocked encoder slots, a damaged reluctor, excessive sensor air gap, weak sensor output, wiring resistance or low cranking voltage.

Can a weak battery cause P0377?

Yes. Low battery voltage and slow cranking speed can weaken the sensor waveform enough for the PCM to miss pulses.

Is Signal “B” the same as Bank 2?

No. Signal “B” is a timing-reference circuit designation and does not universally identify Bank 2.

Can P0377 cause a no-start?

Yes. If too much timing information is missing, the PCM may be unable to synchronize ignition or fuel injection.

What is the difference between P0376 and P0377?

P0376 means Signal “B” has too many pulses. P0377 means it has too few pulses.

What is the difference between P0377 and P0379?

P0377 means some pulses are detected but the count is insufficient. P0379 means no Signal “B” pulses are detected.

Do you need an oscilloscope to diagnose P0377?

An oscilloscope is strongly recommended because it shows missing pulses, weak waveform sections and amplitude changes that a multimeter cannot display.

Final Thoughts

P0377 means timing-reference Signal “B” is active, but its pulse count is lower than expected. The PCM is receiving part of the timing information—not enough to trust, but just enough to complicate the diagnosis.

Check battery voltage, cranking speed, connector condition, sensor power and ground before replacing parts. Then compare the waveform at the sensor and PCM while inspecting the encoder, reluctor and sensor air gap.

The missing pulses went somewhere. Accurate testing finds out whether they disappeared mechanically, electrically or somewhere between the sensor and PCM.

For more diagnostic coverage, visit the Pro Street OBD-II Doctor and the complete P-series OBD-II code index.

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