I have heard these terms a few timees a nd tyhought it was time I figured out what they mean. What do open or closed loop mean in context with the PCM? Thanks
Bad Andy

 

Closed loop is when the PCM monitors the output of the O2 sensors in the exhaust and adjusts the fuelling to keep the O2 levels at optimum values (for emissions), but the O2 sensors take a while to warm up at the beginning, so the car runs in open loop, with the PCM predicting the required fuelling from stored data. The 'feedback' of the output value (exhaust gas composition) to control the input value (fuel) 'closes the loop'.

Also, the PCM switches to open loop on WOT, where emissions are not regulated.
Link

 

I have also heard that the car runs in open loop while the cats heat up to operating temp. I have deleted my cats , but have not gotten O2 sims yet. Since I have the no cat function code, does the PCM continue to go into closed loop when temp has been reached, or stay open?

 

No, once the front O2 (upstream of the cat) is warmed and producing 'rational' readings (near what is expected) the PCM goes closed loop, the downstream O2 monitors the effeciency of the cat's conversion, which takes a while longer to 'light off', though with the AIR that's only a minute or less. Without cats it'll still run in closed loop.

 

Great, Thanks