2002 GMC Yukon Fuel Composition Sensor

CUBS73

MEMBER
2002 GMC YUKON
V8
4WD
AUTOMATIC
133,000 MILES

I am currenlty having an issue with the Fuel Composition Sensor and getting code F0178. I have replaced the sensor twice, they were both used parts but from a very good salvage yard. Prior to changing the sensor the first time, I took it in to a shop who read the ECM to determine the possible problem and he said 9/10 times the problem is the sensor. So, I purchased a used sensor with 31,000 miles on it and replaced the old one. I cleared the code and did not get a mile down the road and it was back on. I took the car to a shop and they checked the wiring and said I did not have a wiring problem, just a bad sensor. I called the salavage yard and told them the sensor was bad. They gave me a different sensor and I replaced it. I then cleared the code. I started it and let it idle, then turn it off. I then started it and drove about 1 mile with the light out, stopped for gas, then went about a 1/4 of a mile and the light was on again. I have had wiring checked and changed the sensor twice. I cannot imagine all three sensors would be back. I am at a loss trying to solve this problem.

1 Reply

BRIAN 1

MECHANIC
1,030 POSTS

How did the shop detemine it was the sensor?What testing did they do?You have to use a scan tool and monitor the FCS. You have to test the circuit for a short to voltage, short to ground or an open. If the circuit tests good and you replaced the FCS then the next step in the diagnostic is to replace the PCM. The fuel composition sensor is controlled by the pcm. For a shop to tell you the sensor goes bad 9 out of 10 times is a pretty poor diagnoses. Plus getting this from a junk yard is a bad idea, its called a junk yard for a reason. The older E-85 vehicles that used E-85 were not as good as the ones today, if you run gasoline in these older e-85 vehicles and then switch to e-85 is contaminates the sensor and fuel sys with corrosion, by getting this sensor from the junk yard you dont know how long it was sitting and if they ran gas or e-85. This is how the fcs works. Alcohol content and fuel temperature information is supplied to the powertrain control module (PCM) from the fuel composition sensor (FCS). The FCS has an ignition 1 circuit, a signal circuit, and a ground circuit. The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temperature, and changes output signals accordingly. The signal circuit carries both the ethanol percentage, and the fuel temperature within the same signal. The signal is both variable frequency and variable pulse width. The frequency signal indicates the ethanol percentage, and the pulse width indicates fuel temperature. The PCM provides an internal pull up to 5 volts on the signal circuit, and the FCS pulls the 5 volts to ground in pulses. The normal range of operating frequency is between 50-150 Hz. The normal pulse width ranges between 1-5 milliseconds. If the PCM detects a signal less than 45 Hz DTC P0178 will set. Post back with any other questions.