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P0178 Fuel Composition Sensor Circuit Low Input

OBD-II Trouble Code Technical Description

Article by
John Ingalls
John Ingalls
Former Service Manager & Air Force Mechanic

Fuel Composition Sensor Circuit Low Input

What does that mean?

This generic powertrain diagnostic trouble code (DTC) typically applies to all OBD-II equipped vehicles including but not limited to Ford, BMW, Chevy, Pontiac, Mazda, VW, Honda, Scion, Land Rover, etc.

There are four trouble codes commonly associated with fuel composition sensor circuit malfunctions. This circuit is also referred to as the as the flex fuel sensor circuit and the OBDII codes that are triggered by the Engine Control Module (ECM) when this circuit malfunctions are P0176, P0177 and P0179.

The fuel composition sensor circuit monitors the amount of ethanol in the gasoline on a flex fuel engine. The amount of ethanol can very every time you fill up the tank requiring appropriate adjustments for optimal performance and fuel economy. The fuel composition sensor circuit sends a signal to the ECM based on the ethanol level. The ECM adjusts the ignition timing and fuel injector pulse width accordingly to burn the fuel more efficiently. The end result is improved performance, reduced pollution and increased fuel economy.

Code P0178 is set when the ECM detects a low voltage situation within the fuel composition sensor circuit. In most circumstances the presence of a P0178 or P0179 is a very good indication that the problem is electrical in nature.

Code Severity & Symptoms

The severity of this code is moderate.

Symptoms of a P0178 engine code may include:


Typically, potential causes for this code include:

What are the most common repairs?

Diagnostic and Repair Procedures

Note: The first step in the troubleshooting process for any malfunction is to research the Technical Service Bulletins for the specific vehicle by year, model and power plant. In some circumstances this can save a lot of time in the long run by pointing you in the right direction.

Basic Steps

Advanced Steps

Note: The advanced steps become very vehicle specific and require the appropriate advanced equipment to perform accurately. These procedures require a digital multi meter and the specific technical references for the vehicle. Voltage requirements will very based of the specific year, model and engine in the vehicle.

Circuit Checks

With the key in the off position and with the sensor and ECM connected check for power and ground referencing the appropriate technical data. Ground should always read 0 volts and power should be 5 volts or 12 volts based on the vehicle and the system configuration. The specific technical manual or online reference material for your vehicle will guide you through this process. If all of these readings are correct the associated component most likely requires replacement. Frequency checks are recommended in some troubleshooting procedures to specifically pinpoint the source of the malfunction.

If this process identifies the absence of a power source or ground, continuity testing may be required to check the integrity of the wiring. Continuity tests should always by performed with the power removed from the circuit and the normal readings should be 0 ohms of resistance unless otherwise specified by the technical data. Resistance or no continuity is an indication of faulty wiring that must be repaired or replaced.

Hopefully, the information in this article has been helpful to point you in the right direction to correct the problem with your fuel composition sensor circuit malfunction. This article is strictly informational and the specific technical data and service bulletins for your vehicle should always take priority.

Related P0178 DTC Discussions

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