I have a '99 Trans-Am that is due for smog testing, and I can't seem to get the OBD-II catalyst monitor to run. I've driven the car over 400 miles and through 5 cold-start cycles. I've also been following the recommended drive cycle (cold start, idle for 3 minutes with A/C and defrost on, then accelerate at 3/4-throttle to 55 mph and hold a steady 55 for 7 minutes, then coast to 20 mph). I've done the steady-speed cruising/coast-down thing a whole bunch of times. I still can't get that catalyst monitor to run. All the other monitors ran a long time ago. I've got no codes. I've got a 224/230 cam with a stock exhaust system and some tuning. Any thoughts?

 

Did the previous tuning perhaps disable (rather than
enabled, no error) some of the rear O2 sensor or
catalyst codes? In some cases disabling a "sub-test"
will defeat the larger diagnostics that look at them.
Might look at the Helm book for that diagnostic and
check off all the things that it says can inhibit.