Code
P0378
GWM
P — Powertrain
- Unstable pulses of the B timer signal
AI status
Completed
Completed
100%
Causes
- Faulty camshaft or crankshaft position sensor (B circuit / secondary sensor)
- Damaged or corroded wiring or connector for the B timer sensor
- Loose or incorrect sensor mounting or air gap
- Missing/damaged teeth on the reluctor/trigger wheel (timing wheel)
- Timing chain/belt or gear damage causing irregular target wheel motion
- Intermittent ECU input or internal ECU fault
Symptoms
- Check Engine Light (MIL) illuminated with P0378 stored
- Rough idle, intermittent misfire, or engine hesitation
- Stalling or hard starting in some conditions
- Loss of power or reduced drivability
- Intermittent RPM erratic behavior during acceleration or idle
What to check
- Read freeze frame and live data from the ECU; note RPM and conditions when the code set
- Visually inspect the B timer sensor, connector, and wiring harness for damage or corrosion
- Check battery voltage and charging system during testing (ensure stable 12–14 V)
- Back-probe the sensor connector and view the waveform with an oscilloscope or a graphing multimeter
- Inspect the reluctor/target wheel for missing or damaged teeth and check for metal debris
- Wiggle-test harness and connector while monitoring the signal to reproduce instability
Signal parameters
- Hall-effect sensor: square wave 0–5 V (sometimes 0–12 V depending on vehicle reference); clean, consistent pulse edges and duty cycle tied to crank/cam rotation
- Variable reluctance (VR) sensor: AC sine/alternating voltage whose amplitude increases with engine RPM; expect smooth, regular cycles without dropouts
- Pulse spacing: evenly spaced pulses at a given RPM; any jitter, missing pulses, or amplitude collapse indicates a problem
- Signal should be stable at idle and increase in frequency smoothly with engine speed; no intermittent zero-voltage periods
Diagnostic algorithm
- Connect a scan tool and record live data and freeze-frame. Note RPM, throttle, and when P0378 set.
- Clear codes and attempt to reproduce. If code returns immediately or intermittently, proceed with hands-on tests.
- Visually inspect sensor, connector, and wiring for corrosion, bent pins, or damage. Repair as needed.
- Back-probe the sensor with an oscilloscope (preferred) or graphing multimeter. Observe waveform while cranking and at idle; look for missing pulses, amplitude drops, or excessive noise.
- Perform a wiggle test of wiring/connectors while watching the waveform and live data. Repair any intermittent circuits.
- If waveform abnormal, remove sensor and inspect mounting face and reluctor/target wheel for damage, debris, or missing teeth. Repair or replace damaged parts.
- Measure sensor resistance (for VR) and compare to specification, or check reference voltage and ground (for active/Hall sensors). Replace sensor if out of spec.
- If sensor and wheel are good, check mechanical timing (timing chain/belt alignment) and tension; correct if jumped or worn.
- If wiring and mechanical checks good, test ECU input circuit and grounds. Consider ECU swap or bench test only after all external causes eliminated.
- After repair, clear codes and verify with test drive and live data that pulses are stable and code does not return.
Likely causes
- Corroded connector or broken wire at the B timer sensor (most common)
- Contaminated or failing sensor (Hall effect or VR sensor)
- Damaged reluctor/target wheel or foreign metal debris near sensor
- Incorrect sensor air gap after recent service
- Worn or jumped timing chain/belt causing irregular pulse spacing
Fault status
Status
Unstable/intermittent B-timer signal detected by ECU — timing reference pulses are irregular or dropping out. Inspect B timer sensor, wiring/connectors, trigger wheel, and timing components.
Repair difficulty: Medium
Diagnostic time: 0.5 - 3 hours
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