Please help - I am stumped and so is my mechanic! The check engine light in my 2012 Mazda3 (110,000 miles) came on back in June. Code reader reported a cylinder 2 misfire (P0302). I had a local mechanic look at it, who replaced the spark plugs and swapped the coils, the light went off and the car was driving fine. A few weeks later the light came back on, same code, same cylinder. I then took it to my local Mazda dealer (at the mechanic's suggestion) - they performed a compression test, swapped out and replaced coils, got the same code, and finally ended up doing a carbon cleaning of all 4 cylinder intake valves. The light went away. 3 weeks later, the light came on again - SAME CODE. I had already had a truly terrible service experience with this dealership and I could not even get the service manager on the phone to confirm if they would look at the car again without charging me another $230 diagnostic fee for the job they charged me over $1000 for that didn't fix the problem, or to confirm whether they flashed the computer system after the cleaning. So I ended up taking it to another recommended local mechanic who also checked the code again (same code), ran another compression test, checked all the new coils, and reset the code. They drove it for a day trying to see if there was anything they could find - nothing - the car drives perfectly fine. A day after I picked up the car, the light came on again today. Any ideas what could be causing this or suggestions going forward?? Has anyone else experienced something similar? I'm lost and feel like I'm just throwing money away at this point.

 

Welcome to the forum,

wow what a bunch of bad service work!
You never replace random parts without confirming they are the problem and have failed or show lees then their designed specification for the component values if applicable.
I highly recommend to STOP going to "mechanics" and find a service shop that has a sign posted in the front of their build ASE Certified technicians or at least the basic ASE blue sign.
From you post none were automotive technician because your reported plan of action would not have been from any current ASE.
Not once did any mention to check the ECU monitors or the status health of the ECU?
The replacing of coils is a POORMANS or DIY to check for a fault DTC of a misfire? Most of the time that kind of coil swapping is inconclusive because very often no one does a 2-3 engine cycle test to see if any DTC returns after resetting the dash light DTC caution and going into permeant files and resetting them there. And lest not forget the monitors! If you do not reset them and let the engine, go through a couple of complete cycles you may be missing some important information to resolve the original DTC? If you came to me the first thing I would check is the counters in the ECU just to see how many times those jokers reset the ECU. It would be a clear indicator of people not being skilled in diagnostic solving.

 

thank you for your advice! I thought the dealership would have been able to run a more detailed diagnostic but they were truly incompetent. I can't even get the service manager to get back on the phone with me. Unfortunately there are few other dealerships near me, but I will definitely look for a new mechanic with your points in mind. Thank you for your response!

 

If its still misfiring on the same cylinder, get someone to swap the fuel injectors around and see if the misfire moves with the injector.