onecrazyfoo4u

When I was under my car awhile back, I noticed the rear O2 sensor (by the midpipe cat) had one of the two wires that was completely severed it looked like...probably from chaffing. Yet, I've never had any check engine lights...and my torque app says there are no codes.



How do you check for sure if the o2 sensor is working? Is there a voltage or some other reading you can do with a multimeter, or anything I can monitor on the torque app?



I ask because I just replaced my midpipe catalytic converter a few months ago because the honeycomb was completely melted/broken apart, blocking almost all exhaust flow. I'm thinking maybe the o2 sensor was bad and the engine was running too hot, melting the cat? I don't want this to happen again obviously! Thanks guys.

SR Tech

O2 sensors read in mV



Downstream O2 does not affect AFR, only monitors cat effiency.



If the wire was broken you would get a code, it may be a stored code that's not illuminating the CEL, or your bulbs blown, OR the wires not broken.

onecrazyfoo4u

So the downstream O2 sensor wouldn't affect engine performance or fuel mileage at all? I checked again and the black wire is completely disconnected. One of the white wires is just partially hanging on. But no check engine light or codes! And it passed smog just fine.



Just want to know if I should replace it or not.

SR Tech

Nope, downstream only monitors the cat. That's it. The PCM compares upstream and downstream readings to determine how well the cat is working. Upstream 02 controls AFR along with other sensor variables.



I'm surprised it hasn't thrown a circuit code with a broken wire.



I would still replace it anyway

onecrazyfoo4u

Hm...interesting. I don't think I'll replace it if it isn't causing any harm. I'm wondering if I should replace my upstread O2 sensor though, since somehow my downstream cat had melted after 150k miles. Maybe the upper O2 sensor wasn't working properly and the exhaust was burning too hot and melted the cat?



What else would cause a cat to melt almost completely shut like that? I just don't want that to happen again!

SR Tech

Cats are melted by raw fuel, caused commonly by misfires that arn't addressed. Or a mixture that isn't burned completely like having weak spark from lack of maintanance on the plugs/wires.



But an extremely rich mixture over a long period of time would eventually eat at it.



Also leaded fuel will melt it but that's hard to find these days and I doubt you've been filling your tank with it.



Without being able to watch the live o2 data, i wouldn't just throw parts at it. Just make sure your tune up items have been changed plugs/wires/filter