Well this one was a bit of a basket case, so nothing is suddenly appearing except to the extent it has to run for fault codes to "suddenly" appear.. Cleared the codes, started it this morning, and it was misfiring at idle. Did the coil disconnect test to see if I had a bad coil and all are firing to some extent (i.e. engine ran worse when disconnected). Cleared the codes again, checked, random misfire occasionally sets a misfire for 6 and 8. Sometimes both banks, sometimes bank 2 cat damaging misfire. Lambda at .999, front 02's at 2 mv. Checked the knock sensor connectors and they are solid, those codes did not return. I think they were from the miss when driving.

I have another STR that runs fine that I can swap parts with to troubleshoot, trying to think of my next move. I swapped MAF and coils 6 and 8 with no change. Previously I swapped the CPS and throttle body.

 

Don't look at the voltage on the front O2s (they're sensed using current).

Codes "suddenly" appear because that's what your laws effectively require (if curious, do some reading but I guess it's not going to help you fix the car).

BTW it's P0121 not PO121

 

Well as I said at the beginning this one was bought non-running, so no misfire codes at that time. PO also bought it as a rescue, got a few things changed before the TPS quit and then it blew the ignition fuse and he gave up. I am not sure it had misfire codes then.

I'll probably swap the rest of the coils over, they are easy enough to do and don't see any harm in it.