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
| Item | Details |
|---|---|
| Code | P0377 |
| Definition | Timing Reference High-Resolution Signal “B” Too Few Pulses |
| System | Engine-position or injection-timing reference |
| Primary fault | Signal “B” pulse count below the expected value |
| Common causes | Missing encoder transitions, weak sensor output, contamination or excessive air gap |
| Common symptoms | Hard starting, rough running, stalling, hesitation or reduced power |
| Severity | Moderate to serious |
| Safe to drive? | Not recommended when drivability symptoms are present |
| Best diagnostic tool | Scan 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:
- An optical sensor inside a diesel injection pump
- A secondary timing-reference sensor
- A crankshaft position sensor
- A distributor-mounted optical pickup
- A Hall-effect sensor
- A magnetic position sensor
- A slotted encoder disc
- A toothed reluctor wheel
- A pump-mounted electronic module
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:
- Engine position
- Injection-pump position
- Engine speed
- Fuel-injection timing
- Injection-event synchronization
- Ignition timing
- Correlation with Signal “A”
- Combustion-event consistency
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:
- Blocked encoder slots
- Contaminated optical path
- Damaged encoder sections
- Missing reluctor teeth
- Metal debris on a magnetic sensor
- Warped encoder disc
- Loose reluctor wheel
- Excessive shaft movement
Weak electrical transitions
The sensor may produce a waveform, but parts of it may not cross the PCM’s recognition threshold.
Possible causes include:
- Excessive sensor air gap
- Weak sensor output
- Low sensor supply voltage
- Poor sensor ground
- High circuit resistance
- Corroded terminals
- Low cranking speed
- Low battery voltage
- Damaged wiring
- Improper sensor alignment
The waveform exists, but some pulses arrive looking more like vague suggestions. The PCM prefers actual voltage transitions.
Common P0377 Symptoms
Common symptoms include:
- Illuminated Check Engine Light
- Extended cranking
- Hard starting
- Intermittent no-start
- Rough idle
- Uneven engine operation
- Hesitation
- Poor acceleration
- Reduced engine power
- Limp mode
- Engine stalling
- Increased fuel consumption
- Abnormal diesel combustion noise
- Increased exhaust smoke
- Additional timing or synchronization codes
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:
- Failing timing-reference Sensor “B”
- Weak optical sensor output
- Contaminated optical sensor
- Blocked encoder slots
- Dirty encoder disc
- Cracked or damaged encoder disc
- Warped encoder disc
- Damaged reluctor wheel
- Missing reluctor teeth
- Metal debris on the sensor
- Excessive sensor air gap
- Incorrect sensor installation
- Loose sensor mounting
- Excessive shaft or bearing movement
- Loose electrical connector
- Corroded terminals
- Backed-out connector pins
- Open or high-resistance signal wire
- Poor sensor ground
- Low sensor supply voltage
- Low battery voltage
- Slow cranking speed
- Injection-pump electronics failure
- PCM connector damage
- Incorrect PCM calibration
- Internal PCM failure
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:
- Incorrect fuel-injection timing
- Hard starting
- Rough engine operation
- Reduced power
- Engine stalling
- Complete no-start
- Limp-mode operation
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
| Code | Meaning | Diagnostic focus |
| P0372 | Signal “A” Too Few Pulses | Insufficient pulse count on Signal “A” |
| P0374 | Signal “A” No Pulses | Complete loss of Signal “A” |
| P0375 | Signal “B” Malfunction | General Signal “B” fault |
| P0376 | Signal “B” Too Many Pulses | Excessive Signal “B” pulse count |
| P0377 | Signal “B” Too Few Pulses | Insufficient Signal “B” pulse count |
| P0378 | Signal “B” Intermittent/Erratic | Unstable or randomly dropping pulses |
| P0379 | Signal “B” No Pulses | Complete 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:
- P0376 may involve duplicate transitions, noise or extra encoder edges.
- P0377 may involve blocked slots, weak transitions or missing encoder sections.
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:
- Stored codes
- Pending codes
- Freeze-frame data
- Engine RPM
- Battery voltage
- Crankshaft and camshaft synchronization
- Injection timing
- Signal “A” and Signal “B” data, when available
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:
- Which sensor creates Signal “B”
- Whether it is optical, Hall-effect or magnetic
- Sensor location
- Power, ground and signal terminals
- PCM input terminal
- Expected pulse count
- Expected voltage
- Correct waveform
- Sensor air-gap specification
Testing the wrong “B” sensor is an impressively efficient way to learn nothing.
3. Check for Manufacturer Bulletins
Search for technical service information concerning:
- Known sensor failures
- Injection-pump wiring faults
- Encoder-disc contamination
- Harness-routing problems
- Connector revisions
- Low-voltage starting problems
- PCM calibration updates
- Required relearn procedures
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:
- Battery state of charge
- Battery load capacity
- Cranking voltage
- Starter current draw
- Positive-cable voltage drop
- Ground-cable voltage drop
- Engine-ground condition
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:
- Loose connectors
- Broken locking tabs
- Bent terminals
- Backed-out pins
- Corrosion
- Water intrusion
- Oil or fuel contamination
- Chafed insulation
- High-resistance splices
- Pinched wiring
- Heat damage
- Missing shielding
- Previous repairs
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:
- Correct supply voltage
- Stable reference voltage
- Reliable sensor ground
- Low ground-side voltage drop
- No voltage fluctuation
- No short between sensor circuits
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:
- Continuity
- Excessive resistance
- Short to ground
- Short to voltage
- Short to another signal
- Poor terminal contact
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:
- Correct pulse count
- Stable voltage amplitude
- Clean switching edges
- Consistent spacing
- Smooth frequency changes with RPM
A P0377 waveform may display:
- Missing individual pulses
- Repeating gaps
- Missing pulse groups
- Weak transitions
- Reduced amplitude
- Rounded waveform edges
- Pulses disappearing during cranking
- Low-amplitude sections at specific shaft positions
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:
- Pulses missing at both points: Inspect the sensor, encoder, reluctor, air gap and mechanical drive.
- Complete waveform at the sensor but missing pulses at the PCM: Inspect wiring resistance, shielding and connector terminals.
- Correct waveform at both points: Investigate PCM connector tension, software or internal processing.
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:
- Pulse count
- Voltage amplitude
- Frequency
- Phase relationship
- Behavior during cranking
- Response to RPM changes
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:
- Sensor lens
- Light source
- Encoder slots
- Encoder surface
- Fuel or oil contamination
- Debris
- Cracks
- Warping
- Internal alignment
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:
- Reluctor-wheel condition
- Missing or damaged teeth
- Metal debris
- Sensor-tip contamination
- Sensor mounting
- Specified air gap
- Reluctor runout
- Shaft movement
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:
- Encoder or reluctor rotation
- Injection-pump drive
- Timing belt or chain condition
- Distributor-shaft operation
- Pump-shaft play
- Crankshaft or camshaft end play
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:
- Internal optical sensor
- Encoder disc
- Pump connector
- Internal wiring
- Pump-mounted control electronics
- Shaft and bearing condition
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:
- Correct sensor power and ground
- Proper mechanical rotation
- Correct sensor air gap
- A valid signal at the sensor
- Signal integrity at the PCM
- PCM power and grounds
- Connector terminal tension
- Applicable software updates
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:
- Cleaning and securing the sensor connector
- Replacing damaged connector terminals
- Repairing high-resistance wiring
- Repairing sensor power or ground
- Restoring signal-wire shielding
- Replacing timing-reference Sensor “B”
- Cleaning an optical sensor
- Cleaning the encoder disc
- Correcting the sensor air gap
- Replacing a damaged encoder
- Replacing a damaged reluctor wheel
- Removing metal debris
- Repairing excessive shaft movement
- Replacing a weak battery
- Repairing battery cables or grounds
- Repairing injection-pump electronics
- Updating PCM software
- Replacing and programming a confirmed defective PCM
- Performing a required timing relearn
If repairs require a crankshaft-position relearn, see the P0315 crankshaft position variation guide.
Estimated P0377 Repair Costs
| Repair | Estimated 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:
- Stalls
- Hesitates heavily
- Loses power
- Runs roughly
- Becomes difficult to restart
- Produces abnormal combustion noise
- Enters limp mode
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:
- Clear stored and pending codes.
- Confirm the battery is fully charged.
- Monitor cranking voltage.
- Observe Signal “B” with an oscilloscope.
- Confirm the correct pulse count.
- Compare Signal “B” at the sensor and PCM.
- Compare Signals “A” and “B.”
- Allow the engine to reach operating temperature.
- Reproduce the original freeze-frame conditions.
- Rescan for pending codes.
- 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.
