No signal to the pcm

how is that posisble? the car runs and drives fine other than being a little rich.

What causes this problem?

 

Code 12 is all systems are good,for the rich part change your O2 sensor.

 

Why would I have an SES light then if that's the only code?

 

Are you reading all of the codes? 12 will flash first and then you will get the next one ,and so forth until all codes are shown. Be sure you are holding the scan long enough to get all. If that IS the only code then I am stumped. It could be a stuck relay also.

 

it just repeatedly flashes 12... no other codes...

 

pretty sure that means everything is looking good...

with the car RUNNING, is the SES light on?
If so, then code 12 is probably there, for you

if not, then it's supposed to flash
12-12-12 <--- starts code sequence
any other codes
12-12-12 <---- ends codes


if the SES light is NOT on when the car is running, then your car is fine