I have a 97 El Dorado. I had a rash of several problems that have now been cleared up. What started this was the infamous emissions tests. To make a long story short I now have all of the offending DTC'* taken care of. My question is this, after clearing all of the codes there is a little matter of onboard diagnostics that have to run and complete in order to pass emissions. The last test (catalyst) is still either failing or has not completed yet. There are no DTC'* pointing to a problem. Is there a pointer to what this test really is and how I can get this test to complete. I've looked high and low and have found little or no information in this area. BTW both O2 sensors on either side of the CAT were changed some time back. Thanks in advance for any information about these onboard diagnostics.

Brad

i have tried doing the gm official drive cycle and have never got it to work. to get the cat to go ready it takes several times of warming up from cold, drive it for a while, shut it off for a few minutes, drive it home. that seems to do it for me if done a half dozen times from code clearing. if your codes weren't cleared by a scanner it will take longer because the number of cycles is increased before it tries the test again

Thanks for the advise. I'll give it a shot.

What gets me is the lack of information about these onboard diagnostics.
I couldn't even find any info. on the official GM drive cycle that you mentioned.

Brad

General Motors Driving Cycle

A complete driving cycle should perform diagnostics on all systems. A complete driving cycle can be done in under fifteen minutes.

To perform an OBDII Driving cycle do the following:

Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50°C (122°F) and within 6°C (11°F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire and if closed loop is achieved, Fuel Trim.
Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 32km/hr (20 mph). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-60mph). This will perform the same diagnostics as in step 3.
Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

a few books can help Haynes tech book obd-2 & electronic engine management systems
Obd-2 repair strategies incl. state inspections - obd2-en by Mandy conception