Today I was working on a 2001 workhorse, owned by one of our shops fleet customers. The complaint was the infamous service engine soon light was on. So as usual I started my diagnosis with scanning the codes. The code P0140 Bank 1 Sensor 2 oxygen sensor "no activity detected" was stored.
So I check the data, and it looks like B1S2 is reading a lower than normal voltage. Ok so maybe we have an O2 sensor that has failed or is starting to fail. My next step is back probing the signal at the O2 sensor connector to see if it is producing a signal. On this vehicle Bank 1 is on the driver's side (Bank 1 is the same side as Cycl #1). I follow the exhaust back to find the O2 after the cat (Sensor 2).
I then connect my lab scope to observe the signal. and this O2 is actually producing a signal. So I think hmm, maybe there is a harness or PCM fault. Next I back probe the signal wire for this sensor at the PCM and I see the same voltage reading on my scope as I do on the scan tool, interesting. So I then scope the Bank 2 Sensor 2 and it is not producing a signal. I go back and compare both B1S2 & B2S2 actual readings with the data on the scan tool. I find that at some point the connectors on the post catalytic converter O2's had been reversed. Which are of course mounted near each other.
In all the fix was replacing the Bank 2 Sensor 2 O2, and connecting the O2 sensor harnesses to their correct locations. This one is just another example as to why we need to stay on our game. Had we scanned the data, and only back probed the PCM signal feeds the wrong sensor could have easily been replaced and the harness would have stayed incorrectly routed. So that's my story of the day...
(Sorry No Pictures Today too busy...)