Just recently bought my 2006 Rx8. 101k miles and in decent shape. I replaced the battery today so I no longer have to jump every time I start it up, however the car idles too low and will stall unless it is constantly revving.

My error OBDII codes are as following:

Heated oxygen sensor (HO2S) 2, bank 1, heater control - circuit low

U1900 Pending fault present
[Ford: CAN Communication Bus fault - receive error]


For the oxygen sensor I’m not sure if I need upstream, downstream, or both.

As for the U1900, I need some extra help figuring out what is even causing it. What are some things I can do to diagnose the issue more specifically (and fix it)?

 

You can basically ignore the U1900 code... it is a communications error that often pops up with using non-iDS scanners.

The O2 code is for the rear post CAT sensor... depending how old they are and how deep your pockets are changing both is likely a good idea

 

Also would be a good idea to inspect the catalytic converter as well. Just get a diagnoses from an exhaust shop.