Q: AC failed and started running rough and the Check Engine Light came on. But NOW its fine.

asked by on September 08, 2016

I have a 2007 Reno and was taking a long trip. I left home on a full tank of gas and about an hour and 20 minutes into the trip It started running rough and I noticed the check engine light had come on. Also the A/C, which had been working was no longer blowing cold at all. I stopped at a Auto Parts store and checked the codes, which were as follows. P2106, P0452, P0405, P0107, P0651, P0606 and misfires and all 4 cylinders P0300 and P0301-302-303-304. I want to point out that the car ran fine for 130 miles before I had any issues. Googling and asking advice from non-mechanics seemed to indicate that bad gas may have caused these issues so I added some fuel additive (sea-foam motor treatment) and added gas up to 3/4 of the tank. That was almost 2 weeks ago. Since then it has been running a little rough and with no power. I was prepared to drain the gas tank and replace the spark plugs when today noticed it was driving normally and the CEL had gone off, but the A/C isnt working. WTF?

My car has 90000 miles.
My car has a manual transmission.

I wouldn’t concern yourself with the timing of your cars symptoms. When a car fails, I liken it to when a light bulb decides to burn out. It was just its time. Everything will eventually fail. I also would strongly disagree with the bad gas theory. This idea is a common cause that is often pointed at, but the reality is it is rarely if ever true. The only time you could get bad gas is if the fuel you had used had been sitting in an unsealed container for several years. It just isn’t very likely.

I can speak to all the codes you have listed. Thanks for listing them by the way. I can derive a few conclusions with them. When we see this many codes, it is time to consider what could be responsible for all these codes. It extraordinarily unlikely that all these components related to these codes failed at exactly the same time. The one common thing between all these components is the PCM. In particular, the 5 volt reference, which is supplied by the PCM. Without explaining the purpose of the 5 volt reference, I will say it is integral in the accurate measuring of the vehicles sensors. If this signal is corrupted, it will cause many failures and codes as you see here.

The 5 volt reference is most commonly corrupted by a sensor that is failing. In your case, I would clear the codes and see if they all come back or if one or a few appear. This will often set us down the correct diagnostic path. Most often, there is a sensor that has a short in it that caused the 5 volt reference to be corrupted. The PCM will not have accurate information and all the systems will go into a default mode.

It could also be the PCM. Diagnosing such a problem requires knowledge of every system in the car. So if you are not intimately familiar with them, or don’t want to take on the learning that will be involved to diagnose your symptoms, I recommend having one of our mobile technicians diagnose the trouble codes for an accurate repair.

Good luck! These can be tricky.

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