What causes fault code P0377?

• Most likely: Faulty optical sensor or crankshaft/camshaft position sensor (Signal B circuit) • Likely: Wiring issues (intermittent connection, corrosion, or short circuit in signal wire) • Possible: Damaged reluctor ring or debris/metal shavings on the sensor tip • Rare: Faulty Engine Control Module (ECM) or electromagnetic interference (EMI) from ignition components

How do you fix fault code P0377?

STEP 1: Inspect the connector for the specific sensor (crank/cam/distributor) for corrosion or loose pins. Clean as necessary. STEP 2: Check the sensor's ground connection and reference voltage (typically 5V or 12V). Measure resistance in the wiring harness between the sensor and ECU; should be < 1 Ohm. STEP 3: Use a digital oscilloscope to monitor Signal B. Verify that the pulse train is consistent without drops at various RPMs. Square-wave signals should have sharp transitions. STEP 4: Physically inspect the reluctor ring/trigger wheel for damage. If wiring is intact but signal pulses are missing or erratic, replace the sensor.

Powertrain (Engine & Gearbox)Standard EOBD

P0377 - Timing Reference High Resolution Signal B Too Few Pulses

Q: What does fault code P0377 mean?

The P0377 diagnostic trouble code (DTC) indicates that the Engine Control Module (ECM) has detected too few pulses from the high-resolution timing reference signal 'B'. This signal is critical for the ECM to calculate precise fuel injection and ignition timing. The 'B' signal usually refers to a secondary high-frequency pulse stream from a crankshaft, camshaft, or optical distributor sensor. When the pulse count falls below the calibrated threshold relative to the master reference signal, the ECM loses its ability to perform fine-timing adjustments, often resulting in a Limp Home mode to prevent engine damage.

Q: What are the symptoms of fault code P0377?

• Check Engine Light (MIL) illuminated • Engine hard to start (extended cranking) • Engine stalling or hesitating during acceleration • Reduced engine power (Limp mode) • Rough idling

Last updated:1 February 2026

Related:Crankshaft Position Sensor (CKP), Camshaft Position Sensor (CMP), Engine Control Module (ECM), Reluctor Ring, Distributor Pickup Coil

System

Powertrain (Engine & Gearbox)

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Random or Multiple Cylinder Misfire Detected

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Cylinder 1 Misfire Detected

P0302

Cylinder 2 Misfire Detected

P0303

Cylinder 3 Misfire Detected

Technical Description

The P0377 diagnostic trouble code (DTC) indicates that the Engine Control Module (ECM) has detected too few pulses from the high-resolution timing reference signal 'B'. This signal is critical for the ECM to calculate precise fuel injection and ignition timing. The 'B' signal usually refers to a secondary high-frequency pulse stream from a crankshaft, camshaft, or optical distributor sensor. When the pulse count falls below the calibrated threshold relative to the master reference signal, the ECM loses its ability to perform fine-timing adjustments, often resulting in a Limp Home mode to prevent engine damage.

Possible Causes

Faulty optical sensor or crankshaft/camshaft position sensor (Signal B circuit)

Wiring issues (intermittent connection, corrosion, or short circuit in signal wire)

Damaged reluctor ring or debris/metal shavings on the sensor tip

Faulty Engine Control Module (ECM) or electromagnetic interference (EMI) from ignition components

Symptoms

Check Engine Light (MIL) illuminated

Engine hard to start (extended cranking)

Engine stalling or hesitating during acceleration

Reduced engine power (Limp mode)

Rough idling

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P0377 - Timing Reference High Resolution Signal B Too Few Pulses

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P0377 - Timing Reference High Resolution Signal B Too Few Pulses

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