2004 Chevrolet Tahoe, Code P0178

This 2004 Chevrolet Tahoe came in with the SES light on and complaints of poor fuel economy and a “strange feel to the engine”. I checked codes and found a code P0178 stored along with misfire and lean codes. P0178 is set if the frequency reading in the fuel composition sensor is below 45 Hz for 12.5 seconds or more. The normal frequency readings should be between 50Hz and 150Hz. The frequency reading indicates the ethanol percentage and the pulse width indicates the fuel temperature. The FCS is located near the driver’s frame rail, in front of the fuel filter.

There is a three wire harness connector.

The white wire, terminal “A” has a five volt reference signal sent from the ECM. The FCS job is to cycle the five volts to ground and create a varying square wave as it relates to both ethanol content and fuel temperature. With one lead of my multimeter attached to ground, I lightly touched the terminal for the white wire. The correct signal was present. Notice I stress “lightly touched the terminal”. Anything other than a light touch could damage the spring tension of the terminal.

Next I checked for 12 volts across the pink(terminal “B”) wire and the black(terminal”C”) wire. Again I used a light touch and I also used a fused jumper wire, with the correct terminal end, as part of my test set up.

The full picture to show how the parts are connected for testing.

This is one of my favorite tests for testing current carrying capabilities of a circuit. I have read about people using sealed beam headlights, small motors and other items to place a load on a circuit see see if it is okay or weak. Those are okay but seem too cumbersome to me. I simply do this. First I look at wiring diagrams and determine that I am at the correct terminals and wire colors. Next using diagrams again, I make sure that I am only testing a directly fused circuit and a direct ground. I DO NOT USE THIS TEST FOR CIRCUITS COMING OUT OF SOLID STATE DEVICES, such as computers. Actually, I do but it is with a deep understanding of automotive electronics and intensive studying of a particular circuit to make sure that I will not damage anything. This is what I do: I install a fused jumper wire with the correct sized terminal ends for what I am working on. I remove the factory fuse for the circuit and go up to a larger fuse. I then place a fuse with the original fuse rating in the in line fuse holder and in this case I turned the ignition on. The fuse blew immediately. This quick test, tells me that the power and ground side of the circuit will both handle to factory load rating. I put the original factory fuse back in its place and my testing is done.

Of course I also have the luxury of using a Tech 2 scan tool. This coupled with the tests that I had already shown indicates that the fuel composition sensor is faulty. 42 HZ is below the minimum 50 Hz.

The sensor is pretty easy to change. Disconnect the two fuel lines. Of course after relieving pressure at the service port according to manufactures instructions. Remove three bolts and of course disconnect the electrical connector.

The tool needed to disconnect the fuel lines.

Another view of the tool.

The new unit installed along with a new fuel filter. One trick that I have learned, since it is almost impossible to get the forward fuel line off of the fuel filter is this. I remove the rear line from the filter and trace the front filter line around to the FCS. I disconnect the line there and the pull the line and filter out together. Once at the bench I cut the fuel filter tube and then I can use my tool to remove the remaining fuel filter tube from the fuel line.

The sensor is installed and the FCS frequency reading is now within specifications. Also notice that the fuel alcohol content is now at 8%. Earlier it was at 38% and had gone as high as 42% while I monitored the data.

7 discussions on “2004 Chevrolet Tahoe, Code P0178”

  1. Hi guys, I know this is a pretty old thread but am coming for some advice and hopefully some answers. I have a 2004 Chevy Tahoe Z71 that I took in for fuel line repairs maybe two weeks ago. Over the last week it started running very chuggish and then started throwing the P0178 code. When I got under the vehicle to see if the harness was loose or what the sensor even looked like I noticed that the lines they ran completely bypassed both my fuel filter(Which is now GONE) and the sensor itself. You go up top and the lines that ran off the sensor into the engine are cut and just sitting there. Is this right? Is there some way that the truck should run with NO fuel filter or FCS??? Please help, this is my only vehicle and it now is not running at all.

    1. Wow, seems like someone messed up. There are shops out there that could do that and it be a correct repair but this is what would have had to be done. The lines could have been bypassed as you described as long as the fuel pump was updated to a newer design with a built in fuel filter and the PCM calibration changed to remove the flex fuel coding. All of that would have been detailed in conversations and documented on the invoice. The problem is that you have the code P0178 which means the PCM calibration has not been changed or not been changed correctly.

      The problem that you have now is that you have unfiltered fuel going to the injectors and that will eventually plug them up. Also without fuel going through the sensor the computer does not know how much ethanol is in the fuel or what the fuel temperature is. Without that information the PCM cannot properly control the fuel going into the engine and it will run poorly or not at all.

      Your first step would be to contact the shop that did the repairs and see if they are willing to rectify the situation.

      If you are forced to take matters into you own hands, I would recommend installing a fuel composition emulator/simulator in place of the original sensor. That should get it running again. Then you would need to have a fuel filter re installed in the system. This would be the least expensive option for you.

      The next option would be to have a “Tuner” shop do a delete in the PCM calibrations for the FCS system. That kind of a shop may be very difficult to find.

      Then lastly you would need to restore the system to factory design. This would likely be the most expensive route since it is highly probable that your old FCS is now no good.

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