Engine

P0378 — Timing Reference High Resolution Signal B Erratic

Severity: medium Drive with care, to a shop

What P0378 means

The engine control module tracks engine position using crankshaft and camshaft sensors. Some systems use a high-resolution timing reference signal, labeled 'B', for precise timing. P0378 sets when the pulses on this signal are intermittent, drop out or look erratic. The ECM can then calculate spark and fuel timing less accurately.

Symptoms

  • Check Engine Light on
  • Rough or unstable idle
  • Loss of power and poor acceleration
  • Hard starting
  • Intermittent stalling
  • Hesitation under load

Possible causes

  1. Failed crankshaft or camshaft position sensor (signal B)
  2. Damaged wiring, open circuit or short to ground
  3. Corroded or loose sensor connector
  4. Damaged, dirty or misaligned reluctor or tone wheel
  5. Electrical interference or a poor ground
  6. ECM fault (rare)

How to diagnose

  1. Scan for codes and freeze-frame data, and look for related CKP or CMP codes
  2. Inspect sensor wiring and connectors for chafing, oil, moisture and corrosion
  3. Check sensor power, signal and ground with a multimeter
  4. Verify sensor mounting, air gap and cleanliness (metal debris)
  5. Inspect the reluctor or tone wheel for cracks, bent or missing teeth
  6. Compare crank and cam signals on a scope to find dropouts and noise
  7. Confirm the timing chain or belt has not jumped a tooth
  8. Suspect the ECM only after everything else checks out

Common fixes

  • Replace the faulty position sensor
  • Repair wiring or replace the connector
  • Clean contacts and restore a good ground
  • Replace or correctly install the reluctor ring or timing components
  • Reprogram or replace the ECM as a last resort

Questions and answers

What does P0378 mean?

It means the ECM is receiving an erratic or intermittent high-resolution timing reference signal (channel B). The usual culprits are a sensor, wiring or the reluctor wheel.

Can I drive with P0378?

Only a short distance, for example to a shop. The engine may stall or lose power, so do not put off the repair.

Will replacing the crankshaft sensor fix it?

Not always. Check the wiring, connector and reluctor wheel first, or a new sensor may show the same bad signal.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.