My WRX (750 miles) has me somewhat confused. Some of you will remember that I posted regarding my CEL coming on two times in a week's time. I brought my car to the dealer and they diagnosed the problem as a bad rear O2 sensor (slow to respond code). They ordered the part and told me to call back and make an appointment to have it installed. The CEL was turned off by the dealer when they diagnosed the problem that day. I've driven the car for about 250 miles since the trip to the dealer, and the CEL hasn't come on again. That surprises me in itself. On top of that, my car seems kind of sluggish. It used to just take off at 3500 RPM (especially in second gear). Now it just seems to continue reving casually once I get to that RPM. I thought this was related to the heat and humidty. However, I took the car out tonight in the cool weather and it still feels sluggish. I called the dealer today and made an appointment to have the new rear O2 sensor put in.

Would a faulty O2 sensor cause this kind of sluggish performance, and if so, why hasn't the CEL come on again since the trip to the dealer? Thanks once again guys!

 

FWIW the rear sensor will not effect performance but will make a CEL come on. The rear one only tells how your cat is working efficancy wise. The front one will help control the air fuel mixture. That one can mess things up. From what I've been seeing around, there is a high failure rate of the front sensors. I know a guy that got 4 of them in a short period of time.

As for the light coming on, you have to have a particular number of the same codes hit in a period of time to have the light come on. for instance (making up numbers) 15 codes in 30 seconds.

 

Thanks a lot for the detailed response. I really appreciate it.