I have a 2001 z24. The check engine light was on so i got it scanned and it said the problem was with the upstream o2 sensor. I changed the sensor and the light went off for one day. Then the light came back on. this time the computer said it was the downstream sensor. so i changed the other o2 sensor. the light went off for 2 whole days. Now it is give the error codes again, they are 02 sensor bank 1sensor 2 (after cat, downstream) no activity, and heater circuit malfunction. I dont know what else to do to get the light to stay off. what else could it be. by the way the only changes to the car are a ractive intake. I beleive the exhaust is all stock except for the muffler. I bought the car this way that is why i am not sure. any help is greatly appreciated