You have to run it through a drive cycle on both fuels.
I have an OBD Scan tool that tells me when the sensors are ready.
Some sensors only reset when driving on gasoline.
For the first drive cycle turn the vehicle off and remove the CNG 20amp fuse in the fuse box under the hood. That insures that the vehicle will run a cycle on gasoline.
Do the following:
1 - Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F and within 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.
2 - 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.
3 - Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
4 - Hold Steady Speed. Hold a steady speed of 55mph for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
5 - Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. Let the vehicle coast along gradually slowing down to 20 mph. During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
6 - Accelerate. Accelerate at 3/4 throttle to 55-60mph. This will perform the same diagnostics as in step 3.
7 - Hold Steady Speed. Hold a steady speed of 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.
8 - Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the brakes or shift gears.
Now turn the vehicle off, put the 20amp CNG fuse back in and repeat the cycle on CNG. Make sure the car is running on CNG !! You don't need to do the Cold Start again (step 1). I usually have my drive arranged so that I drive to the mechanic with the car running and leave it running until he has done the emission test.
I had the "sensors not ready" problem with the first 2001 Cavalier I sold and then I found this procedure and I've used it on every Cavalier I've taken in for emission testing and it's worked every time.
It's important to have the vehicle off when removing and replacing the fuse. You don't want to short anything out.
Hope this helps....