Ok, first, I’m new here. This board is great, thanks moderators and contributors! Y’all have have helped me save almost $500 in just two weeks (break job, serpentine and tensioner). I just bought a few weeks ago a 1996, Explorer XLT, 4.0 w/ 88,000 miles, very good condition. Its idling roughly, hissing a bit (MAF?) and CEL light on constantly. No coolant loss that I can see, no oil loss that I can see, a bit of hesitation or weak acceleration at speed when AC is on. Autozone read the codes today: PO153 – O2 Sensor – Bank 2, Series 1 - Slow Response I know this is the Oxygen sensor problem. However, I’m unsure if this is a situation where one should replace the O2 sensor or look upstream for other problems that my cause the O2 sensor to throw that “slow response” code. I spent about one hour searching the forums for answers and I hope I'm not bringing up a dead horse topic. However, the only concrete thing I’ve found is to replace the sensor, “slow response” indicates a sensor problem. I'm not one to blame the sensor, but think this might be the sensor telling me something. But I don't know what "slow resonse" is. Is slow response an indication of a bad sensor. Or should I be looking upstream at another problem/solution?? Thanks!