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I am helping my mom with her 2003 Hyundai Elantra GLS. She has a DTC of P0133 - Oxygen Sensor Circuit Slow Response. I believe the error to be caused by a leak at the exhaust manifold. I hooked up my scan tool, a Lemur BlueDriver, and noticed that she has three data PIDs relating to fuel trim: Short Term Fuel Trim Bank 1, O2 Bank 1 - Sensor 1 - Short Term Fuel Trim, and O2 Bank 1 - Sensor 2 - Short Term Fuel Trim. I have watched several YouTube videos to learn about fuel trim and I believe I have a good understanding of it. However, I haven't been able to find any information on specific oxygen sensors having their own fuel trim values. The first O2 sensor STFT value was roughly that of the regular STFT, however the second O2 sensor had a fuel trim of 99.2%. What are these O2 specific fuel trims and do the values on my car seem okay. Thanks.
Steve
Last edit: 6 years 5 months ago by scarney. Reason: Adding [FIXED] to subject line
I know the Global OBD PID's you're looking at, and they're very confusing! The short answer is that the O2 fuel trim PIDs are meaningless, and can be safely ignored. Nothing about the vehicles fuel control is actually getting trimmed to 99% (in the case of B1S2), and the data you see doesn't reflect anything about O2 performance.
Long answer is that it has to do with SAE standards, and how the vehicle communicates with the scanner over Global OBD. The 99.2% value you saw for B1S2 is the default value for an O2 sensor that has no contribution to the fuel control strategy. The default value HAS to be there because the scanner requests the value of that PID from the PCM, and the PCM is required to send something.
Before you start chasing fuel trims start by sealing the exhaust leak and then check the response of the upstream oxygen sensor.
Graphing the oxygen sensor outputs is a great help as a picture paints a thousand words.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Thread bump! I'm glad you started this thread, scarney, 'cause this question comes up frequently. I was looking at Global data on my Escape for another thread, and caught this:
It seems that the Snap-On scanner is programmed to disregard the downstream O2 fuel trim PIDs. When actually looking at the PIDs on the scanner, it just shows a big fat 'NOT USED' next to those PIDs.
Just posting for anyone else searching in the future.
