I was driving on a road trip when I suddenly heard a sound like there was a hole in my exhaust. I pulled over at a gas station took a look under the car and sure enough my downstream oxygen sensor is hanging there and there is a hole in my exhaust where the sensor was. No sign of the bolts that held it in place. I did a temporary fix by stopping at at hardware store, picking up some 12 mm bolts, and bolted the sensor back in place enough to quite the exhaust. It was New Years Eve, it was below zero, and I was on a road trip, so this was good enough until I got home. I thought it was strange that the CEL did not go off. I expected at least a P0420 code which I have been getting intermittently the past few years. Eventually the CEL did go off two days later, but with a P0120 code, which is Throttle Pedal Position Sensor/Switch (TPS) "A" Circuit Malfunction. Is this related or could this be completely separate? Could my oxygen sensor been coming loose for sometime and that have been the cause of my intermittent P0420 codes? My plan is to replace the downstream oxygen sensor in a couple days, reset the CEL, and see if the code returns, unless those wiser here can tell me I should do something different.

 

Completely unrelated if your OBDII system is working correct

I would guess the loose sensor was your problem with the PO420 code

 

I agree with Wy9. The sensor was probably allowing an exhaust leak which will trigger a P0420.

I see no need to replace the rear sensor in this case if the P0420 does not return. Many will tell you to swap them at the same time as the front, but I disagree. The fronts go faster as they are in a much hotter environment. I got 80k out of my factory front and 110k out of my factory rear. I got 85k out of second front, and second rear has 75k and going strong. (My fronts are A/F sensors, and they go even faster than standard O2s.)