Originally Posted by pismojeff
i think that once your CEL goes on, the car disregards the o2 sensor if it is indeed the o2 sensor that is defective. i find that after i decarbonize effectively with in tank cleaners, the car surges because a bit because of the carbon that ends up on the o2 sensor. id wonder if your mechanic did something at the last minute to help your car pass emissions that dumped a bit of the carbon onto it. i find that mine cleans with a couple of passes in 2nd gear at 60 mph (around 5700 rpm) for 1.5 minutes, followed by a couple in 3rd at 70, repeat. it was probably due for replacement anyway, but the 1221 definately DOES NOT mean that it is indeed the 02 sensor that needs replacement (i've had several air leaks and air restrictions do the same thing). has your gas mileage gone down since the CEL came on as compared to when it was surging?