P0377
Timing Reference High Resolution Signal B Too Few PulsesP0377 is a generic OBD-II powertrain diagnostic trouble code: Timing Reference High Resolution Signal B Too Few Pulses. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0377 means
P0377 is set when the Powertrain Control Module (PCM) detects that the high-resolution timing reference signal on channel "B" is producing fewer pulses than expected during a given crankshaft rotation window. The timing reference system uses an engine position sensor — typically a crankshaft position (CKP) sensor, camshaft position (CMP) sensor, or fuel pump position sensor depending on application — to generate a precise pulse train that the PCM uses to calculate engine speed, crankshaft position, ignition timing, and fuel injection timing.
Channel "B" is the secondary high-resolution timing input, mirroring the function of channel "A" (covered by P0372). The PCM continuously compares the expected pulse count against the actual count received within each sample window. When the count falls short — caused by missing teeth on a damaged reluctor wheel, a weak or intermittently dropping sensor signal, or contamination interrupting the magnetic field — P0377 is stored. Because timing accuracy is fundamental to combustion management, even occasional missing pulses can cause misfires, rough running, or a complete inability to start.
Diagnosis should begin with a visual inspection of the sensor and its wiring, then progress to checking reluctor wheel (tone ring) condition and air gap. Waveform capture with an oscilloscope is the definitive method to confirm the pulse pattern and identify which pulses are absent or attenuated.
Common causes
Most-frequently reported root causes when P0377 is logged.
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1
Damaged, cracked, or corroded reluctor wheel (tone ring) with missing or deformed teeth on the channel "B" sensor target.
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2
Faulty channel "B" engine position sensor producing intermittently weak or absent output pulses.
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3
Open circuit, short to ground, or high-resistance connection in the sensor signal wiring.
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4
Excessive air gap between the sensor tip and the reluctor wheel reducing signal amplitude below threshold.
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5
Metal debris or oil contamination bridging reluctor wheel teeth and causing pulse suppression.
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6
Damaged PCM input circuit for the channel "B" timing signal.
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7
Loose or corroded electrical connector at the sensor causing intermittent signal dropout.
Symptoms drivers notice
How to diagnose P0377
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and record all stored and pending codes; note any concurrent misfire or other crankshaft/camshaft sensor codes.
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2
Inspect the channel "B" engine position sensor for physical damage, oil contamination, and secure mounting.
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3
Inspect all sensor wiring and connectors for corrosion, chafing, broken wires, or damaged insulation.
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4
Check the sensor air gap against the manufacturer specification — excessive gap reduces signal amplitude.
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5
Inspect the reluctor wheel (tone ring) for missing, chipped, or deformed teeth and for ferrous debris buildup.
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6
Use an oscilloscope to capture the channel "B" sensor waveform at cranking speed and confirm the pulse pattern; identify which pulses are missing or attenuated.
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7
Measure sensor resistance and reference voltage supply per manufacturer specification; replace the sensor if out of range.
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8
If wiring and sensor test good, evaluate the PCM channel "B" input circuit for damage.
Related powertrain codes
Frequently asked questions
What is the difference between P0377 and P0372?
P0372 covers channel "A" of the high-resolution timing reference signal, while P0377 covers the secondary channel "B". Both codes describe the same fault type — too few pulses received — but on different sensor inputs. Some engines use two independent timing inputs for redundancy or for dual-camshaft timing control.
Can a dirty reluctor wheel cause P0377?
Yes. Metal shavings, sludge, or heavy oil contamination can partially bridge the gaps between reluctor wheel teeth, disrupting the magnetic field and suppressing individual pulses. Cleaning the reluctor wheel and sensor tip is a necessary step in diagnosis before condemning the sensor itself.
Is it safe to drive with P0377?
Driving with this code is not recommended. Missing timing pulses can cause misfires, stalling, or engine damage due to incorrect ignition and injection timing. In some cases the PCM will enter limp mode to protect the engine. The vehicle should be diagnosed and repaired promptly.
Will P0377 prevent the engine from starting?
Possibly, especially if the pulse count drops to zero (which would set P0379 instead). A severely under-counting channel "B" signal may not provide enough timing reference data for the PCM to fire injectors and ignition reliably, resulting in extended cranking or a no-start condition.
Disabling P0377 in software
RaceTune can permanently disable P0377 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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