P0177 Code: Fuel Composition Sensor Circuit Range/Performance

What the P0177 code means

The fuel composition (flex fuel) sensor sits in the fuel line, under the vehicle or in the engine compartment, on flex fuel vehicles (built to run on gasoline and ethanol). The computer uses it because it measures the percentage of ethanol so it can adjust injection and spark advance; it usually measures the fuel temperature too.

In this case the signal is inside the electrical limits, but it is not believable. The computer compares the fuel composition sensor with other data: the ethanol percentage should match the fuel you put in and change only after a fill-up. If the difference goes over what is allowed for the programmed time, it stores the P0177 code. It is not a cut wire: it is a sensor that reads wrong, freezes at one value or reacts too slowly, or else a real abnormal condition.

Remember 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.

P0177 code symptoms

• Check engine light on steadily.
• Hard starting, especially on ethanol and when cold.
• Higher fuel use or lack of power.
• Wrong ethanol percentage on the scan tool.
• Companion codes for the same sensor (low, high or intermittent signal) may show up.

Likely causes of the P0177 code

1. Sensor out of calibration or degraded, giving a believable but wrong value.
2. A real abnormal condition: fuel contaminated with water, or a blend outside specification.
3. Dirty sensor or one with deposits, which makes it respond slowly.
4. High resistance in the connector from corrosion, which shifts the reading.
5. Computer software. Some manufacturers have released updates for false range codes.
6. Additional 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 P0177 code step by step

1. Read the sensor live data and compare it with what you expect: the ethanol percentage should match the fuel you put in and change only 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 on pure gasoline and up to 150 Hz on pure ethanol.
5. Do the wiggle test with the engine running: move the harness and the connector while you watch the live data. A jump gives away the fault location.
6. Check the continuity of each wire between the sensor connector and the computer connector with the key off.

How to fix the P0177 code

• Correct the real condition if there is one (thermostat, level, leaks, fuel).
• If the sensor reads wrong against a reference thermometer or gauge, replace it.
• Look for technical bulletins or software updates from the manufacturer.
• Use a quality original or equivalent sensor: cheap generic ones often have different response curves.
• Clear the codes, drive a full cycle and verify in the live data that the reading makes sense.

Technical reference data

• 12 V supply, ground and a frequency signal.
• Approximate frequency: 50 Hz on gasoline with no ethanol and 150 Hz on pure ethanol (E100); in between, proportional.
• The pulse width usually tells you the fuel temperature.
• Approximate values only: check the manufacturer data for your engine.

Brand-specific notes

If your vehicle is not flex fuel, this code should not appear: check the computer configuration. It is also common on many flex vehicles from Brazil.

P0177 on GM

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

P0177 on Ford

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

P0177 on Chrysler

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

Frequently asked questions about P0177