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P0378 OBD-II Trouble Code: Timing Reference High Resolution Signal B Intermittent/Erratic Pulses

Check Engine Light

P0378 code definition

If the PCM has stored the code P0378, it means that the system has detected an abnormal pulse reading from the high resolution timing reference signal B, which indicates that there is a fault with the system’s circuitry.

What the P0378 code means

Located in the injection pump is a sensor disk that emits pulses that the PCM uses to control fuel injection timing. The PCM tests for missed pulses, and compares those to the number of pulses it should be detecting according to the manufacturer. If this value is not correct or erratic, the PCM will store the code P0378.

What causes the P0378 code?

There are many things that could cause an erratic pulse reading between the sensor disk and the PCM. Most often, these include a defective crankshaft, camshaft, or distributor position sensors.

However, other causes may include:

  • Failure of the distributor
  • Open or shorted optical sensor
  • Faulty, corroded, damaged, open, or shorted wiring or connectors
  • Failed PCM, in rare cases

What are the symptoms of the P0378 code?

The P0378 code could be preceded by no symptoms at all besides the illumination of the Check Engine light. However, it could also be preceded by engine misfiring, knocking when accelerating, and decreased fuel economy.

How does a mechanic diagnose the P0378 code?

A mechanic should first abstract code information from the PCM with an OBD-II scanner, and check for other codes that may indicate a larger or related issue.

In order to diagnose the cause of the P0378 code, many mechanics begin by inspecting the crankshaft gear and crankshaft position sensor. This is accomplished by checking for the engine RPM signal while the engine is running. If there is no signal detected, then a visual inspection of the crankshaft and related components is necessary. The technician should repair or replace any components that are faulty. This process eliminates one of the most common issues for the P0378 code, and may help narrow the options for following repairs.

Common mistakes when diagnosing the P0378 code

When diagnosing the P0378 code, a common mistake is forgetting to clear the code and do a road test after performing each potential solution. This step should be done after each repair, to help narrow down the cause faster. By replicating the moment of failure, a mechanic may be able to discover the problem much faster.

Another common mistake with the P0378 code is automatically replacing the crankshaft, camshaft, or distributor sensors, rather than checking for mechanical distributor failure or other issues. Additionally, it is important to replace crankshaft or camshaft sensors as a set, rather than individually.

How serious is the P0378 code?

Because the P0378 code relates directly to how the PCM controls fuel injection and timing, this is a very serious issue that should be addressed immediately. Failure to address this issue when it is detected could result in the failure of other engine components when they aren’t able to obtain fuel, or correct fuel pressure.

What repairs can fix the P0378 code?

In order to address a P0378 code, there are a few repairs that a mechanic may choose to perform. These include:

In the rare case that the PCM is faulty, the mechanic may replace the PCM.

Need help with a P0378 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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