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P0178 OBD-II Trouble Code: Fuel Composition Sensor Circuit Low Input

Check Engine Light

P0178 code definition

Fuel Composition Sensor Circuit Low Input

What the P0178 code means

P0178 is a generic OBD-II code indicating that the diesel fuel sensor is giving a low input, which could be from an open circuit in the sensor or wiring

What causes the P0178 code?

  • The engine control module (ECM) detects that the sensor circuit is open if the sensor input is below specifications

  • The ECM detects that the circuit is open if the wiring or connection to the sensor is not making a complete circuit, such as a connection that came loose, or wiring that got cut

What are the symptoms of the P0178 code?

  • The ECM sets code P0178 to memory and triggers the Check Engine Light
  • The Water in Fuel Light illuminates on the instrument cluster

How does a mechanic diagnose the P0178 code?

  • Scans the codes and documents the freeze frame data, then clears the codes to verify code returns

  • Does a visual check of all the wiring and connections to the sensor, checking for breaks in the wiring or loose connections

  • Disconnects connection to sensor and checks to see if the sensor is testing within specifications

Common mistakes when diagnosing the P0178 code

Follow these simple guidelines to prevent misdiagnosis:

  • Do a visual check of connections to the sensor to make sure they are making good contact. The connection may be loose from someone not securing it properly after servicing the filter.

How serious is the P0178 code?

  • Engine starting and running issues could occur if excessive water is in the fuel and the sensor cannot detect it

  • The engine may misfire, hesitate and stall

If there is no problems with the fuel then the only issue will be the check engine and water in fuel light will be on.

What repairs can fix the P0178 code?

Additional comments for consideration regarding the P0178 code

Not many manufactures use the code P0178. It is set in the computer memory to indicate that the engine fuel has, or had water in it. The code is only an indicator of water in the fuel and may be damaged by fuel additives added to the fuel.

Need help with a P0178 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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