P0178 Code (Symptoms, Causes, and How to Fix)

When you’re driving down the road, the last thing you want to deal with a check engine light that pops up on the dash. But when it does happen, it can be a panic-inducing.

You rush it to the auto parts store, they scan it, and give you the code. But then what?

The good news is that if you have a P0178 code, it’s a pretty straightforward process to get it fixed, though not necessarily inexpensive.

P0178 code

What Does Code P0178 Mean?

OBD-II Trouble Code P0178 Description
Fuel Composition Sensor Circuit Low

Not every car has a fuel composition sensor; in fact, most vehicles don’t. Your vehicle only has this sensor if it can utilize flex fuel, and it’s what tells your engine the current makeup of ethanol in the fuel.

When your engine throws a P0178 code, it means that the sensor is producing readings that are outside of a normal range – or that the sensor isn’t producing any readings at all.

Either way, your engine will default to a predetermined value, which means that it won’t be able to switch from regular fuel to flex fuel as it should.

Symptoms of Code P0178

fuel tank fill cap

The symptoms of a code P0178 will vary, and one of the most significant factors that will influence what symptoms your vehicle shows will be the type of fuel you’re currently using. Since the Engine Control Module (ECM) switches to a default value, you’ll usually lose the ability to use flex fuel.

But most times, your engine can run with flex fuel even if it’s not a flex fuel vehicle. You’ll just have decreased engine performance, rougher starts, and increased fuel consumption. Occasionally you won’t be able to start your vehicle at all, but this is rare.

These are the exact same symptoms you’ll experience if your car has a P0178 code.

Causes of Code P0178

By far, the most common cause of a code P0178 is a faulty fuel composition sensor. While that’s the most common problem, it’s not the only thing that could crop up.

Another major concern is wiring issues. The code is telling you that the reading either isn’t reaching the ECM or it’s not within normal limits. Of course, if you have a wiring issue, the reading won’t make it to the ECM, which will pop the code even if the sensor is working the way that it should!

Finally, there could be a problem with your Power Control Module (PCM). These problems are extremely rare, and it’s more likely that it just needs an update. However, PCMs are very durably components, and it’s even rare that they would cause a check engine light without an update.

  • Fuel composition sensor failure
  • Wiring issues
  • Power Control Module (PCM) failure
  • Power Control Module (PCM) update needed

Is Code P0178 Serious?

brake noise while driving

Yes and no. While you can run your vehicle with a P0178 code for a little while, it’s better to get it repaired as soon as possible. Until you do get it fixed, avoid flex fuel and use regular gasoline.

However, this doesn’t mean it can run that way forever. The manufacturer designed the engine to optimize to changing ethanol amounts in the fuel, and without accurate readings from the fuel composition sensor, it can’t do that.

Rough starts and poor performance all take their toll on your engine over time, which is why you should address them as soon as possible.

How to Fix

The first thing you need to do is verify if the fuel composition sensor is the problem. The easy way to do this is to purchase a fuel composition sensor and replace it. The job itself can be a bit of a pain, and the sensor isn’t much cheaper either.

In fact, you often have to spend between $500 and $600 in labor in parts to replace the sensor. To make matters worse, the only way to test the actual sensor is at a dealership.

While the sensor is the problem 99 percent of the time, it would be nice to have a few assurances before dropping a couple hundred bucks on a part.

From there, you can check the wiring to make sure everything is intact. If there isn’t a break between the PCM and the fuel composition sensor, you likely have a problem with your PCM. It might need to be updated, re-flashed, or replaced.

No matter what the PCM needs, you’ll need to take it to a dealership or somewhere else with the authorized programming tools to get it fixed. Even if you replace the PCM, you’ll still need to program it to the engine, so this isn’t a job you do at home.

Adam Mann


  1. Dsx flex fuel sensor on a 2018 camaro zl1 . I’m getting the po178 code and the milage has dropped in half . Just 14800 miles on the car . Has alot of mods and running 730 rear wheel hp . It’s throwing fuel dust all over the back bumper so it’s running to rich .

    1. If that sensor quits working, the vehicle can’t tell the ethanol percentage you’re running in the vehicle. It probably defaults to a really rich map so it is safer for the engine. The downside of doing this would be the symptoms you describe.

      The factory service manual should have a procedure for testing the fuel composition sensor. Test the fuel composition sensor and replace it if it’s bad. If the sensor tests good, check for bad grounds or bad wiring in the area. It might help to grab a wiring diagram if you reach that point.

Leave a Reply

Your email address will not be published. Required fields are marked *