P0179 Code: Fuel Composition Sensor Circuit High

What the P0179 code means

The fuel composition sensor (flex fuel sensor) is mounted in the fuel line, under the vehicle or in the engine bay, on flex fuel vehicles (vehicles set up to run on gas and ethanol). The computer uses it because it measures the percentage of ethanol, which the computer needs to adjust fuel injection and spark timing. It usually measures the fuel temperature too.

A signal above the maximum (close to 5 V) means the signal wire is open, the sensor ground is broken, or the signal is shorted to the 5 V reference or to 12 V. While the fault lasts, the computer uses a substitute value, and that is when the P0179 code is stored.

Keep in mind that the code points to the circuit where the computer saw the problem, not necessarily the part that failed. Check the wiring before you replace the sensor, and treat the P0179 code as a starting point for testing rather than a parts list.

P0179 code symptoms

• Check engine light on and steady.
• Hard starting, especially on ethanol and when the engine is cold.
• Higher fuel consumption or lack of power.
• Wrong ethanol percentage on the scan tool.
• Possible companion codes from the same sensor (low, high or intermittent signal).

Likely causes of the P0179 code

1. Unplugged or poorly seated connector, for example after a repair or after changing the air filter.
2. Signal or ground wire cut inside the insulation or at the connector itself.
3. Signal shorted to 5 V or to 12 V by rubbing against another wire in the harness.
4. Corroded or open connector pins that no longer make contact.
5. Sensor open inside, so the signal stays at its maximum value.
6. Also check: Fuel with water in it or contaminated fuel can give out-of-range readings even when the sensor is not damaged.

How to diagnose the P0179 code step by step

1. Read the live data from the sensor and compare it with what you expect. The ethanol percentage should match the fuel you put in the tank, and it should only change after a fill-up.
2. Inspect the connector. Unplug it, look for green, backed-out or oily pins, and plug it back in firmly.
3. Check for 12 V and ground at the sensor connector.
4. Measure the signal frequency with a multimeter that has a hertz scale or with an oscilloscope: 50 Hz with pure gas and up to 150 Hz with pure ethanol.
5. Do a wiggle test. With the engine running, move the harness and the connector while you watch the live data. A sudden jump gives away the spot where the fault is.
6. Check the continuity of each wire between the sensor connector and the computer connector with the key off.

How to fix the P0179 code

• If the connector was loose or corroded, clean it with contact cleaner and secure it.
• If the checks show the wiring is good, replace the sensor.
• If the signal is missing at the connector, repair the open wire by soldering it and sealing it with heat shrink.
• Use a quality OE or equivalent sensor. Cheap generic ones often have different response curves.
• Clear the codes, drive a full drive cycle and check in the live data that the reading makes sense.

Technical reference data

• 12 V supply, ground and a frequency signal.
• Approximate frequency: 50 Hz with gas that has no ethanol and 150 Hz with pure ethanol (E100), and proportional in between.
• The pulse width usually indicates the fuel temperature.
• These are approximate values. Check the manufacturer data for your engine.

Brand-specific notes

Many flex fuel vehicles in Brazil also set this code. If your vehicle is not a flex fuel model, this code should not show up at all, so check the computer configuration.

P0179 on GM

This is a typical code on flex fuel GM vehicles in the United States.

P0179 on Ford

This is a typical code on flex fuel Ford vehicles in the United States.

P0179 on Chrysler

This is a typical code on flex fuel Chrysler vehicles in the United States.

Frequently asked questions about P0179