# O2 Sensor Problem **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** 850, volvo\ **Created:** [December 18, 2009, 4:17am UTC](https://community.cartalk.com/t/o2-sensor-problem/40909 "2009-12-18T04:17:42Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Lynne123](https://avatars.discourse-cdn.com/v4/letter/l/b77776/32.png) [@Lynne123](https://community.cartalk.com/u/Lynne123)\ **Post date:** [December 18, 2009, 4:17am UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/1 "2009-12-18T04:17:42Z")
Check Engine is still on after O2 sensors (front & rear) were replaced. Brought back to mechanic and change the rear O2 sensor (under warranty)as required by computer read out. Also, welded holes in exhaust in front of converter and repaired pin holes in vacuum hose. Car will idle intermittently and did stall recently. Also, how do I get ride of a mold smell when the air is set at defrost and moisture on windows. Car is a 1996 Volvo 85 (5 cylinder) w/188,000 miles. ---
**Author:** ![Manolito](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/manolito/32/3070_2.png) [@Manolito](https://community.cartalk.com/u/Manolito)\ **Post date:** [December 18, 2009, 5:54am UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/2 "2009-12-18T05:54:37Z")
You are on the right track. Keep looking for vacuum and exhaust leaks until you find them all. Also check the air tube between the mass airflow sensor and the throttle valve. As for mildew smell, try removing the cabin air filter (should be under the hood in front of the passenger) and then spraying Lysol or a commercial product made for this purpose like Wurth into the air intake at the base of the windshield while the defroster is blasting full force. Any coolant loss? This car is prone to heater core failures. The good news is that they are not too expensive and they are super easy to replace on this vehicle. ---
**Author:** ![cigroller](https://avatars.discourse-cdn.com/v4/letter/c/c68b51/32.png) [@cigroller](https://community.cartalk.com/u/cigroller)\ **Post date:** [December 18, 2009, 12:40pm UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/3 "2009-12-18T12:40:50Z")
Your oxygen sensors probably got replaced - erroneously and ironically - because they were working well. Its strange, but their basic job is to report on the amount of oxygen in the exhaust. If the amount gets out of line with what it should be the sensor sends a signal to the computer to turn on the engine light. A lot of mechanics then decide to replace the sensor - weird. Do you know what the actual codes were? They would be in the format P0123. ---
**Author:** ![hellokit](https://avatars.discourse-cdn.com/v4/letter/h/6a8cbe/32.png) [@hellokit](https://community.cartalk.com/u/hellokit)\ **Post date:** [December 18, 2009, 1:04pm UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/4 "2009-12-18T13:04:10Z")
Cigroller, Sir, Your question, “Why do mechanics (or, non-mechanics), heedlessly, change oxygen sensors?”, deserves its own discussion post. Let’s DO that, OK? Hellokit ---
**Author:** ![cigroller](https://avatars.discourse-cdn.com/v4/letter/c/c68b51/32.png) [@cigroller](https://community.cartalk.com/u/cigroller)\ **Post date:** [December 18, 2009, 4:34pm UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/5 "2009-12-18T16:34:16Z")
hellokit, sir - I see that you started that thread - a fine idea, though I wonder if it isn’t preaching to the choir. Perhaps you could get a spot on the evening news instead? Alas, wisdom only comes to those who seek it. BTW, my own understandings of these things is partly drawn from discussions on these boards. A very very long time ago it may, in fact, have been one of your posts on this kind of topic that gave me my own “aha” experience. So thank you! And, of course, there are plenty of others still here and yet to come who are not in the choir. ---
**Author:** ![tardis](https://avatars.discourse-cdn.com/v4/letter/t/f0a364/32.png) [@tardis](https://community.cartalk.com/u/tardis)\ **Post date:** [December 18, 2009, 4:59pm UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/6 "2009-12-18T16:59:03Z")
Yes, the all to common solution of “shoot the messenger.” The O2 sensors where warning about a problem, so instead of looking for the problem, they replaced the sensors. ---
**Author:** ![Bladecutter](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@Bladecutter](https://community.cartalk.com/u/Bladecutter)\ **Post date:** [December 18, 2009, 9:42pm UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/7 "2009-12-18T21:42:23Z")
Chances are it is the MAF or MAP sensor that has gone bad, and is reporting the O2 sensors are out of spec, and/or that the Fuel trim is too high/low. Replace the MAF sensor, and this issue should be corrected. BC. ---
**Author:** ![Lynne123](https://avatars.discourse-cdn.com/v4/letter/l/b77776/32.png) [@Lynne123](https://community.cartalk.com/u/Lynne123)\ **Post date:** [December 20, 2009, 2:25am UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/8 "2009-12-20T02:25:18Z")
Fault Code ECM436 Rear O2 Sensor ---
**Author:** ![euryale1](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@euryale1](https://community.cartalk.com/u/euryale1)\ **Post date:** [December 20, 2009, 5:55am UTC](https://community.cartalk.com/t/o2-sensor-problem/40909/9 "2009-12-20T05:55:19Z")
Unfortunately it could mean a bad catalytic. If the rear has been replaced twice the, front 02 is running the show for the mix and the rear was mandated to alert that the cat was no longer working.