My 2010 Mazdaspeed 3 (114k miles) had recently developed a misfire in cylinder 4. Changed spark plugs and cylinder 4 was pretty rough looking. Swapped around the coils to make sure that wasn't the cause of the misfire and sure enough, the misfire stayed on cylinder 4. I knew the injectors needed to be cleaned so I went ahead and got to work on getting them taken out and replaced. This took about the span of a month as I only really had time to do it for a few hours on weekends. During this span, I would leave the battery disconnected since the battery box was out. The ECM was obviously unplugged because of this. Finally got everything back together, primed the fuel rail and everything like that and tried to start the car. It fought for a long time to actually start, and when it did it would run really rough before dying. Thought it was my battery so I would hook jumpers up and leave them connected, but that didn't really help much. Ran the CEL codes with an OBDII scanner and received the P0685 and P050A codes. I looked them up and they both seem to point to the ECM/PCM (as far as I understand that this pretty much only points to an ECU issue). I have read online that on the Mazda 3s, if you leave the battery disconnected or the ECU without power for a bit, the car has to relearn how to run well (ie, it won't idle properly and what not), but this seems to be more than this since I am receiving check engine codes.
What should I be checking to get my car started again? I went ahead and ordered a main relay (LF69-18-811) that is found in the engine bay fuse box. Is there anything else that would cause these codes? Could my ECU have possibly gone bad while it sat without being connected to the battery?
You're probably on the right track with replacing the relay. Your ecu isn't getting enough voltage. It's rarely a failed ecu, so only go to replacing that if there's nothing else left.
Check your wiring and connections carefully.
The second code is the idle air control. This explains the misfiring and stalling at idle. However, it would be unusual to have two failures at the same time although not impossible. With low ecu voltage, it may be unable to control that valve correctly and the ECU is going into limp mode. Check out your connections there.
Start doing your troubleshooting with the ecu and go from there.
You're probably on the right track with replacing the relay. Your ecu isn't getting enough voltage. It's rarely a failed ecu, so only go to replacing that if there's nothing else left.
Check your wiring and connections carefully.
The second code is the idle air control. This explains the misfiring and stalling at idle. However, it would be unusual to have two failures at the same time although not impossible. With low ecu voltage, it may be unable to control that valve correctly and the ECU is going into limp mode. Check out your connections there.
Start doing your troubleshooting with the ecu and go from there.
Thanks for the reply. Got those codes to go away after replacing the relay and also jumping my battery when trying to start the car. Now getting a P0171 code for running too lean. Car will start but won't run. Betting I have a vacuum leak somewhere or my ECM needs retrained, or both. Will update as I get further sown the rabbit hole.
So a little update. After seeing someone post a video on Facebook of having the exact issue I am having at this point, I've narrowed down what I believe to be the issue. The car starts sometimes after making sure the battery is charged and the main relay was replaced, but hitting the gas was causing it to stall and it was idling really rough. Someone on that Facebook post said to try unplugging the MAF to see if the car stays running. Tried it, and it starts up fine and would actually respond to me hitting the throttle without dying! Even with this, I am not convinced that it's the MAF causing an issue though because I tried a known good MAF and it is doing the exact same thing. With the MAF unplugged and standing in front of the car, I tried plugging the MAF in while the car was running and it killed the car once it was plugged back in (which I expected). But as the car started to die, I heard and saw some dust come out from the outlet tube that connects the TMIC to the BPV! So pretty sure it's just a vacuum leak (or leaks) that is causing my issue. I will be testing for vacuum leaks later today.
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