I had been having trouble with two codes; P0741 (Transmission Solenoid) and P0420 (O2 Sensor).
By moving the rear O2 sensor out of the direct flow of exhaust between the Catalytic Converter and the muffler by using a modified / deeper spark plug
non-fouler, it kept the P0420 code from returning.
Note: I regret not buying one of the adapters / bushings from online that is already modified or long enough for the O2 sensor to fit as the are difficult to bore
out with a drill. My Deville O2 sensor is the 18mm size but is with the tip it is longer than a sparkplug. I bought a "Help!" brand (#42002) from one the major
auto parts stores, but then had a machine shop drill (not the threaded part) the non-fouler so that the sensor would seat enough to get the threads started.
The P0741 would return before the required 8 of 9 monitors were "Ready". The Catalyst Monitor and The EVAP Monitors were always the last to "Ready" but the other monitors were much easier to set.
I believe that I have discovered my own somewhat modified version of a "Drive Cycle" that doesn't require going over 40mph or long distances.
In my state to pass the inspection process for my model Deville, all but one monitors must be in the "Ready" status and the "Service Engine Soon" must not be
illuminated. I was familiar with the published "GM OBD2 Driving Cycle" that I have read about on many sites but it would never completely work enough (all but 2 would "Ready") for my car to pass. So I tried driving slow for a few days in hopes that the light would go off; it didn't. I was out driving around but had
pulled over and stopped; While checking the status to see if the P0471 was current, pending or history I accidently cleared all the codes which put the car back at the beginning. Since the engine was already warm I shifted from park to "D", Pressed the "Traction Off" (button on the end of the gear selector) one time, then drove about 5 miles at under 40mph to a gas station allowing the car to coast down to 20mph just before stopping, to help some of the tests to set.
I let the car idle for a minute before shutting off and fueling. I put about 4 gallons of gas in so that the tank would be between 1/4 and 3/4 full as I had read
other OBD2 tests require. Then I restarted the engine, shifted from park to "D" and pressed the "Traction Off" button once and proceeded about 7 miles back to my home keeping the speed under 40 mph and allowing the car to coast down to 20mph just before arriving home. I let the car idle for three minutes and then checked the readiness of the monitors with a scan reader; All but three were set to "Ready". Two of those three that were not ready, I knew had been easily setting to "Ready" after a cold start. The SES was still out. Could this be?
How did the Catalyst Monitor that had been so stubborn to set before so easily have set now?
I shut the car off, took the keys, locked the doors and then restarted the next morning (after 9 hours). I did a cold start, allowing the car to warm up while
holding the brake with the car in gear with the A/C and Defroster wide open and headlights on bright until the water temperature gauge went up two clicks.
I then put the car in park and checked; The Oxygen Sensor Heater Monitor was now "Ready". I shifted from Park to "D', pressed the "Traction Off" button one
time and then drove under 40 mph for about 4 miles to the Inspection Station. I allowed the car to idle for a minute and hooked up the scan reader; the
Secondary Air pump Monitor was now "Ready". Now all but one (the EVAP monitor), were in the "Ready" mode and the SES light was still out.
My car PASSED inspection !
Later that day I drove over 45mph and the SES light came back on. I am good until next year when I will read this method and remember what to do.
Correction; I meant P0741 Transmission Solenoid and not P0471.