Q: Check engine light

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I recently got some bad gas in my car, so I took it to my mechanic and he replaced several fuel injectors and flushed the system and also replaced the fuel pump for the car. After all of this work, unfortunately, the check engine light is still on. He read the code, said it was the fuel cap (code indicated low emissions leakage, so he immediately said it must be the fuel cap, so I bought a new one and he cleared the code and he replaced the fuel cap). However, after replacing the fuel cap, the light would go off for a while then come back on again after normal driving (usually by second day or so). Every time it's reset, it either comes on a couple of days later during normal driving or, in one case after changing my alternator recently, it came on nearly 5 full days later. The problem is, I no longer have confidence he can locate and fix the problem, and have no intention of paying him to 'learn on the job! What else could it be? Please help! Car seems to drive normal otherwise

My car has 116000 miles.
My car has an automatic transmission.

A: Hi and thanks for contacting YourMechanic. ...

Hi and thanks for contacting YourMechanic. From what you've described, it seems that the mechanic was doing a good job. If your injectors were replaced, the fuel pump was replaced, and the fuel system was flushed out the system should be good from the bad gas.

However, the question to ask is how was the fuel system flushed. So, what you could do is remove the fuel line to the fuel injection system. Hook up a hose to the line and put it into a gas can. Then find the fuel pump relay and remove it. Grab a jumper wire and bridge the terminals on the relay ports on the fuse box and make the pump come on. Turn the key on and pump all of the fuel out. Turn the key off and put in new fuel. Turn on the key again and pump the fuel out again. This will insure that all of the fuel is clean in your vehicle.

Then remove the jumper wire and put the relay back in the ports. Turn on the key with new fuel in the fuel tank and pressurize the system. Remove the battery ground and clear the codes. Hook up the battery and test drive the vehicle for about a week. If the light does not come on, then you still had bad gas in the lines. If the light comes back on, then have the light checked for what the codes are.

Sometimes the gasket on the fuel filler neck to the fuel cap could be damaged or the neck itself could be damaged making the vapor sensor pick up a gross leak. I recommend replacing the fuel fuller cap gasket to the gas cap and retest the system before flushing the system out. Also, I recommend looking at the vapor lines under the vehicle for any cracks and replace them if they are cracked.

If you need assistance in troubleshooting your fuel issue or would like a fresh pair of eyes on the issue, consider YourMechanic, as one of our mobile technicians can come to you and properly diagnose your Check Engine Light firsthand.

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