Faulty camshaft position sensor or its wiring connector; Damaged or misrouted camshaft sensor harness near the ignition module; Worn or contaminated ignition control module connector terminals; Failed ignition control module (ICM) internal pulse generator; Excessive timing chain stretch or worn distributor gear causing cam-to-crank misalignment; Electrical interference from aftermarket ignition components or poor grounds; Corroded or loose ground connection at the engine block or ICM mounting surface

DTC codes / GM codes / P1377

P1377 · GMIC Module CAM Pulse To 4X Reference Pulse Comparison

Q: What does code P1377 mean?

IC Module CAM Pulse To 4X Reference Pulse Comparison. P1377 on GM vehicles means the ignition control (IC) module is reporting a camshaft pulse that does not match the expected 4X reference pulse from the crankshaft. The engine control module (ECM) compares these two signals to verify proper cam-to-crank timing and sets this code when the pulse counts or timing relationship fall outside the allowed window.

P1377 on GM vehicles means the ignition control (IC) module is reporting a camshaft pulse that does not match the expected 4X reference pulse from the crankshaft. The engine control module (ECM) compares these two signals to verify proper cam-to-crank timing and sets this code when the pulse counts or timing relationship fall outside the allowed window.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyProfessional

AreaIgnition system and misfire

Symptoms you may notice

Illuminated check engine light, often with no other symptoms
Rough idle or misfire at low engine speeds
Hard starting or extended cranking before the engine fires
Intermittent stalling, especially when the engine is cold or under load
Reduced fuel economy or poor throttle response
In rare cases, the engine may crank but not start

Common causes, most likely first

Faulty camshaft position sensor or its wiring connector
Damaged or misrouted camshaft sensor harness near the ignition module
Worn or contaminated ignition control module connector terminals
Failed ignition control module (ICM) internal pulse generator
Excessive timing chain stretch or worn distributor gear causing cam-to-crank misalignment
Electrical interference from aftermarket ignition components or poor grounds
Corroded or loose ground connection at the engine block or ICM mounting surface

How to diagnose and fix it

Inspect the camshaft position sensor connector and wiring for chafing, corrosion, or loose pins
Check the ignition control module connector and ground circuit for clean, tight connections
Measure camshaft sensor signal with a lab scope and compare the pulse pattern to the 4X reference signal
Verify base engine timing and check for timing chain stretch or distributor gear wear
Substitute a known-good ignition control module and clear the code to see if it returns
Repair or replace damaged wiring, connectors, or the camshaft sensor as indicated by the scope test
Clear codes and perform a road test to confirm the repair and ensure no pending codes return

Cam and crank signals must agree

A stretched timing chain on high-mileage GM engines often sets P1377 before any misfire code appears, so always verify cam-to-crank timing with a scope before replacing sensors.
The ignition control module generates the 4X reference pulse, and a failing module can produce a clean cam signal but a distorted reference, leading to misdiagnosis of the cam sensor.
Aftermarket ignition coils or improper spark plug gaps can introduce electrical noise that corrupts the cam pulse signal, causing P1377 without any hard part failure.
P1377 does not always mean the cam sensor is bad; a poor ground at the ICM mounting surface can shift the reference pulse timing enough to trigger the code.
On some GM engines, a worn distributor gear or excessive camshaft end play will cause this code only at certain RPMs, so a static test may pass while a road test with a scope reveals the fault.

Can I keep driving with P1377?

You can usually drive the vehicle short distances if the only symptom is the warning light, but avoid heavy load or high-speed driving until the fault is diagnosed. If the engine misfires, stalls, or is hard to start, park it and arrange a diagnostic scan to prevent further damage.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.