Engine

P0377 — Timing Reference High Resolution Signal B: Too Few Pulses

Severity: medium Drive with care, to a shop

What P0377 means

The engine control module reads pulses from the crankshaft or camshaft position sensor to know engine position and speed, which it needs to time spark and fuel injection. P0377 is set when the number of high-resolution pulses on signal circuit B is lower than expected for the current engine speed. Pulses can be lost because of the sensor, its wiring, the reluctor ring or a timing drive problem.

Symptoms

  • Check Engine light on
  • Rough idle
  • Hard starting or no start
  • Misfires, hesitation or loss of power
  • Occasional stalling

Possible causes

  1. Failed crankshaft or camshaft position sensor (signal B)
  2. Damaged wiring or corroded, loose connector terminals
  3. Damaged, dirty or shifted reluctor ring (tone wheel)
  4. Stretched timing chain or a belt or chain that has jumped a tooth
  5. Electrical interference from a poor ground or nearby ignition wiring
  6. ECM fault or outdated software (uncommon)

How to diagnose

  1. Scan all codes and freeze-frame data, and note any other CKP or CMP codes.
  2. Inspect the sensors, connectors and harness for chafing, heat damage, oil and corrosion.
  3. Check sensor mounting and air gap to the reluctor, and look for metal debris on the sensor tip.
  4. Watch engine RPM on a scan tool while cranking; the signal should appear right away without dropouts.
  5. Test sensor power, ground and signal wires with a multimeter following the factory wiring diagram.
  6. Compare the signal on an oscilloscope with a known good pattern, looking for missing or extra pulses.
  7. Inspect the reluctor ring for damage and verify cam-to-crank timing (chain stretch, belt marks).
  8. If everything checks out, look for ECM software updates.

Common fixes

  • Replace the faulty position sensor
  • Repair or replace damaged wiring and connectors
  • Replace or correctly reseat a damaged reluctor ring
  • Replace the timing belt or chain with tensioners and set the timing correctly
  • Update ECM software, or replace the module only after a confirmed fault

Questions and answers

What does P0377 mean?

The engine computer counted too few high-resolution pulses on timing reference signal B. The usual culprits are the position sensor, its wiring or the reluctor ring.

Can I drive with P0377?

Only briefly and carefully, such as to a shop. The engine may stall or lose power, and if the timing drive is at fault, there is a greater risk of engine damage.

Will replacing the crankshaft sensor fix P0377?

Not always. Check the wiring, connector, air gap, reluctor ring and timing drive first, or the code will likely return.

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.