P0378
Timing Reference High Resolution Signal B Intermittent/Erratic PulsesP0378 is a generic OBD-II powertrain diagnostic trouble code: Timing Reference High Resolution Signal B Intermittent/Erratic 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 P0378 means
P0378 is stored when the PCM detects that the high-resolution timing reference signal on channel "B" is erratic or intermittent — pulses are being received, but their timing, spacing, or amplitude is inconsistent and falls outside the acceptable window for accurate engine position sensing. This distinguishes P0378 from P0377 (too few pulses) and P0379 (no pulses): the signal exists but cannot be trusted.
Channel "B" is the secondary high-resolution timing input, the counterpart to channel "A" (P0373). Erratic pulses are typically caused by reluctor wheel damage that produces irregular tooth spacing, a sensor that is loose and rocking relative to the tone ring, or an intermittent wiring fault that introduces noise or random dropouts. Vibration-related connector issues are a frequent real-world cause, as the fault may only appear at certain engine speeds or road conditions. Because the PCM uses the timing signal to synchronise fuel injection and ignition, erratic input directly translates to combustion instability — misfires, hesitation, or hunting idle.
An oscilloscope waveform capture is the most effective diagnostic tool; it will reveal irregular pulse spacing, pulse amplitude variation, or voltage spikes that a basic voltage or resistance test would miss.
Common causes
Most-frequently reported root causes when P0378 is logged.
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1
Reluctor wheel with damaged, unevenly spaced, or loose teeth creating irregular pulse intervals.
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2
Channel "B" engine position sensor that is loose or has excessive movement, causing gap variation.
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3
Intermittent open or high-resistance connection in the sensor signal or shield wiring.
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4
Electrical noise induced from adjacent ignition wiring, injector drivers, or alternator output corrupting the signal.
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5
Corroded or intermittently seating connector at the sensor or PCM introducing signal dropouts.
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6
Sensor internal failure producing variable output amplitude or spurious pulses.
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7
Damaged wiring harness routing the sensor signal near high-voltage ignition components.
Symptoms drivers notice
How to diagnose P0378
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and capture all stored and freeze frame data, noting any concurrent misfire or position sensor codes.
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2
Inspect the channel "B" sensor mounting — confirm it is torqued correctly and not loose or rocking.
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3
Inspect sensor wiring and connectors for corrosion, intermittent contacts, damaged shielding, or routing too close to ignition leads.
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4
Wiggle-test the connector and wiring harness while monitoring live sensor data to reproduce the fault.
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5
Use an oscilloscope to capture the channel "B" sensor waveform across multiple engine speeds; look for irregular pulse spacing, varying amplitude, or random extra pulses.
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6
Inspect the reluctor wheel for bent, chipped, or unevenly spaced teeth; check for any looseness on its shaft.
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7
Measure sensor supply voltage, signal voltage, and shield continuity per manufacturer specifications.
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8
If all hardware tests pass, check for software updates or PCM calibration issues related to timing signal tolerance.
Related powertrain codes
Frequently asked questions
Why does P0378 appear only sometimes?
Erratic pulses are often caused by intermittent faults — a connector that loses contact under vibration, a sensor that shifts position at operating temperature, or electrical noise that only couples into the signal at certain RPM. This is why the code may set during hard acceleration, over rough roads, or after the engine reaches operating temperature, but not on every drive cycle.
What is the difference between P0378 and P0377?
P0377 means the pulse count is consistently below threshold — the sensor is producing pulses but not enough of them. P0378 means the pulses exist but their timing or spacing is erratic and unreliable. P0377 often points to reluctor damage or a weak sensor; P0378 more often suggests loose mounting, wiring noise, or a sensor with internal instability.
Can I diagnose P0378 with a multimeter alone?
A multimeter can check for obvious opens, shorts, and correct supply voltage, but it cannot reveal intermittent dropouts or pulse-spacing irregularities. An oscilloscope or graphing scan tool with a sensor waveform function is required to properly diagnose an erratic signal fault.
Will P0378 damage the engine?
Prolonged operation with an erratic timing signal can cause misfires, which over time may damage catalytic converters and cause abnormal combustion. It is best to diagnose and repair the fault promptly rather than continue driving with an unresolved timing signal issue.
Disabling P0378 in software
RaceTune can permanently disable P0378 — 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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