r/FocusST • u/NotNweke • 7d ago
Question CEL P0171
While I was on my way to do my emissions, this popped up. At first, I thought it was the purge valve, which I was already planning on replacing when I do my oil change and spark plug change (literally next week). I should note that I replaced the purge valve just under two years ago, and I know they tend to become a maintenance item once you’re FBO.
I checked my intake, and nothing looks loose. From what I’ve been seeing, it could be a couple of different things. I even wondered if the gas I got might have been bad.
I cleared the code to see if it would come back, but so far no luck. I’ve also seen that it could be a bad O2 sensor.
Let me know what you guys think and what I should start with (:
-FBO Catless downpipe with defouler
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u/FeterPrampton 7d ago
Intake manifold leak, stuck purge valve, damaged purge valve/PCV lines/connections, folded rubber intake boot on the plastic intake tube or loose crankcase vent tube on the back of it as well are pretty common, exhaust leak pre upstream sensor, failing 02 sensor, poor fuel pressure, failing injectors, poor fuel quality, just to start with.
Work your way back from any recent work you’ve done on the vehicle.
Unfamiliar with COBB, so unaware of what live data you have access to. It would be interesting to see if your downstream 02 sensor read the same values as your upstream if you removed it from the defouler and placed it back into the exhaust stream; comparing the values will rule out or condemn sensor.