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Ok I bought my 2002 Cav from the State of Utah surplus. At first everything ran great. Then after a couple weeks I had a weird issues where it did not seem to want to go over 2000 RPM. You would start going and it would cut out and sometimes stall. I could also smell CNG through the AC vents 
. I read some posts on here and replaced the Low Pressure Lockoff and rebuilt the Low Pressure Regulator. The CNG smell went away and the car ran again. It did not seem like it was running like it should though. There was a hesitation when starting from idle, and seemed to have low power between 2000-3000 RPM. Also it would rarely get above 3000 RPM when accelerating and would shift hard. It was a gutless wonder, especially with the AC going. So after reading more posts it looked like the next part to replace would be the Gas Mas Sensor/Mixture Control Vale. Dealers quote this at $2600+. It is $1500 in gmpartdirect.com part #52369145. My Dad is a Chev mechanic, but does not work on CNG cars. The check engine light came on so we hooked up the Tech2 to read the code and it was P0134 saying the O2 sensor was acting up. It was a $80 part so we tried it. The car runs better than it ever has now. I get full power on acceleration and can pass semi's again on the freeway. It even starts up better, I don't have to do the 6 sec startup anymore. The part # for the O2 sensor is #12568234. It took 5 mins to switch out. If you have weirdness relating to RPM and power I would try switching the O2 sensor out 
. I read some posts on here and replaced the Low Pressure Lockoff and rebuilt the Low Pressure Regulator. The CNG smell went away and the car ran again. It did not seem like it was running like it should though. There was a hesitation when starting from idle, and seemed to have low power between 2000-3000 RPM. Also it would rarely get above 3000 RPM when accelerating and would shift hard. It was a gutless wonder, especially with the AC going. So after reading more posts it looked like the next part to replace would be the Gas Mas Sensor/Mixture Control Vale. Dealers quote this at $2600+. It is $1500 in gmpartdirect.com part #52369145. My Dad is a Chev mechanic, but does not work on CNG cars. The check engine light came on so we hooked up the Tech2 to read the code and it was P0134 saying the O2 sensor was acting up. It was a $80 part so we tried it. The car runs better than it ever has now. I get full power on acceleration and can pass semi's again on the freeway. It even starts up better, I don't have to do the 6 sec startup anymore. The part # for the O2 sensor is #12568234. It took 5 mins to switch out. If you have weirdness relating to RPM and power I would try switching the O2 sensor out
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