How do i check to see if my cars running in closed loop.

 

The ALDL is located under the dash to the right of the steering wheel. Use the ends of a paper clip to jumper pins A & B (upper right) and start the engine. This will put the ECM into "diagnostic mode".

The SES will flash at a fast rate till the engine is warmed up and in closed loop mode. Then the On/Off flash duration will indicate Righ/Lean condition; so should be of equal duration. Bring the engine from idle up to several RPM levels and monitor the flashes. If flashes level out to equal duration in each case, all is well.

If the SES flashes never slow down to about 1Hz, the ECM is not going into closed loop mode. In that case suspect the O2 sensor or the coolant temp sw. Here a scan tool would be very helpful.

 

OK, I know this is a dead thread but i couldn't find what I was looking for and I have a question about this. When I jump the A & B terminals and start my car (after it's warmed up) the SES light flashes rapidly while idling. when i rev the engine it flashes alowly as described above. when it returns to idle it flashes slowly for maybe 15 seconds then it goes back to rapid flashes. I already detrmined the temp gauge is not reading accurately but i'm not sur eif it is because of a bad sending unit or gauge as my Tach doesn't read right either and now i am wondering about my oil pressure too. so do i need an O2 a Coolant sensor, or and IAC??? my initial problem is the cars starts and rins great. but after it has been warmed up and shut down for a while when i restart the engine it just stalls until i gas it a bit then it clears up and runs great again.

 

I suspect the O2 sensor but you may have grounding/power problems with the dash.

If idle is ok for a while I think the IAC is fine.

A scan tool would tell you if the O2 sensor seen by the ECM is ok.

Have you pulled the any trouble codes to see if the ECM is detecting/recording a sensor problem?