ECM Codes P0178?

Tiny
TDCHRYSLER
  • MEMBER
  • 2014 BUICK LA CROSSE
  • 3.6L
  • V6
  • AUTOMATIC
  • 130,000 MILES
Originally, I had my car scanned and received the codes 'P0178-00 Fuel Composition Sensor Circuit Low Frequency' and 'P2119-00 Throttle Closed Position Performance'. I changed the throttle body, cleared the codes and the engine light came off. Unfortunately, once I drove the car and rescanned, the engine light came back on, and the code was 'P0178-00 Fuel Composition Sensor Circuit Low Frequency'.

I also began to notice when I would go to the gas station for fuel, if I would open the tank cover before turning off, it has a rough idle/drag to turn off. Once I'm done getting the fuel and go to start the car there would be a drag/rough start. This has never happened before. I have even noticed a slight change in my fuel efficiency.
Wednesday, December 11th, 2024 AT 6:27 AM

7 Replies

Tiny
KEN L
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So this can be caused by a weak primary fuel pump which makes sense since the car is hard starting at times.

P0178 - Fuel Composition Sensor Circuit Low Input

The circuit is low due to the low pressure. Here is how to check the primary fuel pressure to help confirm the issue. Verify the fuel pressure is between 345-690 kPa (50-100 psi) with the fuel pump running.

Check out the images (below). Please let us know what happens.
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Wednesday, December 11th, 2024 AT 1:22 PM
Tiny
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Update: We checked the primary fuel pressure from the fuel pump to confirm the issue, and this confirmed what were you were saying. The pressure readings were low with the fuel pump running. I ordered the new fuel pump and having it replaced this week. Can you provide any information on the best steps for replacing the pump?
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Friday, December 27th, 2024 AT 9:12 AM
Tiny
KEN L
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Nice work! Here is the instructions for dropping the fuel tank and replacing the primary fuel pump. You will need to drain the fuel which this video will help you with, do not drop the tank with fuel in it because it can be very heavy. Be sure to clear the codes once completed. Check out the images (below). Please let us know what happens.
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Friday, December 27th, 2024 AT 11:12 AM
Tiny
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The fuel pump was replaced and codes were cleared once completed, but code P0178 still came back. Looks like I'm going to have to replace the fuel composition sensor. Also, the purge valve had to be replaced. This was causing the drag/rough start.
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Wednesday, January 8th, 2025 AT 11:17 AM
Tiny
KEN L
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Good job, here is the location and how to change out the fuel flex sensor which is the same thing. Check out the images (below). Please let us know what happens.
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Thursday, January 9th, 2025 AT 10:36 AM
Tiny
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When changing the fuel flex sensor, I’m not seeing where the sensor is, I’m seeing something different. The images show the one I bought (1st image) and what I’m seeing (2nd image). Is it in a different location on my model? Because what I’m seeing is the Fuel Pressure Sensor.
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Wednesday, February 12th, 2025 AT 3:17 PM
Tiny
KEN L
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Are the color of the wires: wht, vio/dr blu and blk? If so this is the fuel flex sensor. It might be a first gen sensor set up which might need to be adapted, I would call the dealer and explain what's going on to see if they have the correct part.
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Thursday, February 13th, 2025 AT 9:22 AM

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