Code
P1377
BUICK
P — Powertrain
IC Module CAM Pulse To 4X Reference Pulse Comparison
AI status
Completed
Completed
100%
Causes
- Timing chain, belt, or gear jump/slip or incorrect timing installation
- Failed or weak camshaft position sensor
- Failed or weak crankshaft position sensor (4X reference)
- Faulty camshaft actuator (VVT) or solenoid causing incorrect cam timing
- Damaged wiring harness or poor connector connection between sensors and control module
- Intermittent sensor signal due to contamination, broken tone wheel, or physical damage
Symptoms
- MIL/Check Engine lamp illuminated
- Hard start or no-start
- Rough idle, stalling, or misfire
- Reduced power or limp mode
- Intermittent running issues that may vary with engine speed
What to check
- Retrieve freeze frame and all stored codes with a scan tool; note conditions when code set
- Visually inspect cam and crank sensor connectors and wiring for damage or corrosion
- Check battery and charging system voltage during cranking
- Use a lab scope to capture cam and crank (4X) waveforms simultaneously
- Inspect timing components: timing chain/belt, guides, tensioner, and timing marks
- Inspect VVT actuator/solenoid operation if equipped
Signal parameters
- Crank 4X reference: typically four evenly spaced pulses per crankshaft revolution (square wave TTL style); amplitude commonly ~0–5 V depending on sensor type
- Cam pulse: one (or two depending on engine) reference pulse per two crank revolutions, square wave or hall/VR style; amplitude ~0–5 V
- Expected relationship: cam pulse occurs at a defined angular position relative to the 4X crank pulses (one cam pulse every two crank revs); deviation beyond manufacturer timing window triggers the code
- Signal frequency increases with engine rpm; waveforms must be checked across idle, accelerating, and cranking conditions
Diagnostic algorithm
- Connect a scan tool, record freeze frame data and all related codes. Clear codes and attempt to re-create; note when code returns.
- Visually inspect cam and crank sensor connectors, harness and engine grounds. Repair obvious damage and retest.
- With a lab scope or appropriate oscilloscope, capture simultaneous cam and 4X crank waveforms at key conditions (cranking, idle, rev). Verify clean signals and proper amplitude.
- If signals are noisy, intermittent, low amplitude or missing — focus on sensor and wiring replacement/repair. Wiggle test harness to check for intermittent faults.
- If signals are present but timing relationship is incorrect, verify mechanical timing: align cam and crank timing marks per service manual. Check for timing chain/belt stretch, jumped tooth, or damaged guides/tensioner.
- Check VVT actuator operation and oil control solenoids (where applicable) for proper movement and oil feed. Replace or repair if stuck or inoperative.
- After any repair, clear codes and perform a full functional test/road test. Re-scan to confirm code does not return.
- If all sensors, wiring and mechanical timing are correct and code persists, consider module diagnosis or replacement as a last step (follow manufacturer procedures).
Likely causes
- Electrical fault at cam or crank sensor connector (corrosion, loose pin, broken wire)
- Mechanical timing slipped (worn timing chain or jumped tooth)
- Faulty camshaft position sensor
- Faulty crankshaft position sensor (4X reference)
- VVT actuator stuck or failing and changing cam timing unexpectedly
Fault status
Status
IC module detected cam pulse to 4X crank reference pulse correlation outside allowed tolerance. PCM set P1377 when phase/sequence of cam and 4X signals did not match expected relationship.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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