Cat diagnostic with bad O2 sensor?
 

I had the dealer check my catalytic at 79,000 miles, just in case I needed it replaced before the warranty expired on it. They said the diagnostic tested it OK for "efficiency."

Now (days later) they've said the rear O2 sensor needs to be replaced.

I see some people have mentioned that the rear O2 sensor has something to do with measuring catalytic efficiency, so now I'm wondering... was the catalytic diagnostic valid if the rear O2 sensor was bad at the time? Should I ask them to redo the catalytic test?

Or is the Mazda catalytic diagnostic a completely different process (nothing to do with the rear O2 sensor)?

The efficiency check is all done through the ecu. If it isn't throwing a error code, it doesn't get replaced.

Ah OK, thank you. Nope, no CELs, nothing in the memory or whatever.

Is it typical for a rear O2 sensor to fail by 80,000 miles?

They discovered it while replacing my engine for low compression (coils bad and being replaced too). Is there any relationship between compression/coils issues and rear O2 sensor failures? Or do they just "wear" and eventually go bad like most other parts?

i dont know how common it is but mine failed at 85000

Sure, but how does the ECU monitor whether the catalytic is working OK or not? I'm wondering if it's the rear O2 sensor... so if that sensor wasn't working, maybe the ECU really has no idea if the catalytic is OK or not?

The ECU checks the efficiency of the converter by comparing the readings from the rear sensor against the reading of the front sensor.

If the 2 sensors are equal, then the ECU determines that the converter is not working, and throws the code, as the readings should be different.

If the two readings are different, then the ECU determines that the converter is working, and doesn't throw the code.

Its just that simple.

BC.