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OBD Cat monitor not ready


New Member
June 4, 2007
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City, State
chicago il
Year, Model & Trim Level
Does anybody know why my Cat monitor won't reset? i replaced the upstream o2 sensor then cleared the p0153 code with a scan tool. afterwards 3 monitors were not ready: o2 sensor, o2 heater and cat, but i had no codes. my coolant temperature wasn't heating up at all (i have a huge radiator with a towing package), so i put a piece of cardboard in front of the radiator. shortly after that my o2 sensor and o2 heater monitors reset, but i've been driving it for over a week now, even gone through the lengthy drive cycle described in the manual, and my catalyst sensor still won't reset. so i still don't have any codes, but i can't take the test until this monitor resets.

i've read somewhere that if the downstream o2 sensor is a little out of whack that it could be just enough to throw off the catalyst test, but not enough to give you an o2 sensor code. could this be the culprit? i have 215,000 miles on it.

Any help would be greatly appreciated.


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could you please describe a key cycle? i've started it and done a few minutes of city/country driving at least 10 times since i cleared the codes.

well, i changed the down-stream o2 sensor, drove it around all day for probably 100 plus miles of country/city driving, shutting the car down 5 or 6 times throughout the day, and still no luck. i'm due for my emissions test very soon, so any help would be appreciated.