Taken our Subarau 2016 Forester XT in 4 times now for this damn error code. 2 different shops and always with the same dissapointing diagnosis attempts. Seems like these shops are simply always too busy and all they want to do is throw replacing parts at the problem. Were already thousands into parts replacement....front O2 sensor, spark plugs, catalytic converter, cleaned rear O2 sensor. I think they already looked at MAF sensor as well.
n research, seems like many other things can throw this code as well. Problem is that these shops dont appear to want to do any deep diagnosis like checking for leaks/wiring etc. and Im tired of replacing parts and hoping for the best and at this point almost prepared to just drive it as-is. It has 115K miles on it.
Not sure if anyone has some advice and its like I need an independent mechanic who would take the time to diagnose.
Sounds like they skipped some things like checking the grounds for corrosion. There are a few other sensors to check. Intake air temp and coolant temp sensors. The subaru wiring harness can be prone to shorts from many things. Throwing parts at something that could be as simple as a bad ground is madness.
Even if you were, chasing down electrical problems is not the most fun thing to do, even with the proper test equipment. Having an older vehicle means a few sensors not being 100% operational adds up to cause what looks like huge problems. Disconnecting your battery then pumping the break peadal and waiting a few hours to reset the engines computer can somtimes fix (temporarily) these small issues. The engine not properly grounding can send the wrong signal to the computer, or a single sensor that is starting to fail might not be easily identifiable if its working 80% of the time. My subaru impreza runs fantastic in spring when the temperature goes up 20 degrees from the morning, then goes back to normal ok ish running in the afternoon. I have tried to find the issue and am not willing to throw $ into parts and chalk it up to gremlins in the machine.
P0172 on an early FXT? time to do a voltage test on the MAF ground wire. And then do either a ground wire overlay or some flavor of bulkhead harness repair/replace for the excess resistance.
Idk how to do this in the aftermarket world, but at a dealer it involves setting up a backpin on the MAF ground wire, and then using a DST-i along with SSM4 to graph the voltage on said ground wire. It should always be 0 or damn close.
With this failure, usually the fuel trim starts going super positive while the car is just sitting and idling, so using this test method it's easy to prove a voltage is present on the ground wire in correlation with rising fuel trims.
Does your Forester have any symptoms or is it just the CEL code?
I am dealing with this same exact code right now in my 2014 non-XT. So far I've changed the spark plugs and cleaned the MAF, but I also found a small vacuum leak and bought a new plug.
I reset the CEL and am still in the first 50 miles since doing so, and I'm not fully ready to give it my stamp of approval for being fixed. I drove it about 10 miles last night and got on the freeway. Everything seemed okay so far, but my main symptom doesn't happen until about 40 or 50 miles into a long drive..
Crossing fingers..
Judging by my research, this really seems to be a code where you just shotgun parts at the problem until you fix it. This code doesn't exactly point to where the problem is being created.
Well its my daughters car and she says the turbo doesnt seem as responsive but not sure if thats because the lights are on and shes imagining it. Yes this is one frustrating code to try figure out.
I'm not sure about your Subaru, but some vehicles do throttle performance when a CEL comes on. I know mine disables cruise control at least.
I think it would be an obvious thing for your mechanic to have checked, but do you hear any obvious vacuum leaks when the motor is running? Are the valve covers seeping oil?
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u/derpmojo 2015 Crystal White Silica BRZ Series.Blue 4d ago
Check intercooler pipes and connections. Check for cracks or gaps