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Q: rough idle, heavy missing, fault codes P0301 and P0456. lack of power, where do i start to diagnose this?

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had been travelling a little bit today, everything was going fine, shut off the car to visit a business. came back out and it started to misfire really really bad, then my check engine light came on and I got the fault codes listed in the title. extreme loss of power, heavy shaking. cylinder specific fault code which is rare for the car so far since I've had it. the gas cap seems to be fine as well as the filler neck. Was just wondering what could cause these issues and where I would start diagnosing it.

My car has 150000 miles.
My car has an automatic transmission.

A: Hi there. From the description you have pro...

Hi there. From the description you have provided, for the P0301 rough idle and heavy misfire and P0456 evaporative small leak detected, the problems are unrelated. The P0301 is a #1 cylinder misfire code. It could be, a spark plug failure or an ignition coil has failed. It could be, a fuel injector failure or a cylinder compression problem, ie; valve train or piston issue. The P0456 is a small leak detected in the evaporative system. It could be, a faulty fuel cap. A visual inspection may appear okay but, does not seal the deal. It could be an evaporative vent solenoid not sealing or a faulty evaporative purge solenoid. With all of the possibilities that could be causing your vehicle's misfire and evaporative small leak, I recommend having your vehicle's Check Engine Light be diagnosed and repaired by a certified technician, such as one from YouMechanic.

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What others are asking

Q: Misfire code 302 cylinder 2

Hey there. Your logic is absolutely correct. This is in fact one of the most commonly used ways to diagnose a bad ignition coil. If the trouble code turns to a P0301, the ignition coil is faulty. If the trouble...

Q: Code P0456

Hi, thanks for writing in. I'd be happy to help. The P0456 code is for a small leak and the only way to find such a small leak is to have the system pressurized using the test port under the...

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It sounds like you could have a bad coil pack. Have a technician replace the spark plug wires and test the coil packs.

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