1995 mustang gt help with codes

[evil9t5]

I decided to perform the Koer test and these are the codes I recieved for my 1995 mustang gt.
KOEO:
542
KOER:
312
Can anyone provide me with some help and tell me how to fix this problem?

 

[HISSIN50]

IIRC it's an A.I.R. code. You have an O/R pipe so the system is already compromised. Does the car drive ok?

 

[CC'S95GT]

When I ran the o/r h I never got a code.
My mid pipe had a port to plug in the air pump. If that makes a difference

 

[evil9t5]

It drives but not as well as it should. I think the timing needs to be adjusted. I also ran a KOEO test and got the code 542 but I was wondering if the motor has to be at operating temp to run the KOEO test?