Do you really have to replace an O2 sensor when you do the cat converter?

The mechanic who replaced my catalytic converter also insisted that it was standard procedure to replace the upstream oxygen sensor when you did that. For an addtional $144. Is this true? Or was he padding his pockets?

Unless there’s a code indicating that there’s a problem with the O2 sensor, there’s no reason to replace it.


Sometimes the upstream Oxygen sensor is fused in place from heat and can’t be removed from the old catalytic converter. Unless that is the case, use the old one and replace it later if it causes a problem.

Another possibility is that you’re shop isn’t entirely certain that it was your cat that was a problem, so that their real idea was to replace both as part of throwing parts at a code.

Do you know what was done to actually make sure that what you had was a problem with the cat rather than just an O2 sensor problem?

Yes. The code read “Catalytic converter efficiency low (front bank)”. I took it to three different mechanics (including the dealer). The dealer and the mechanic who fixed my car (I hate dealers and don’t trust them, so I didn’t have them fix it) both said they did a “flow chart” of diagnostics after getting this code, and that the end result was that the converter was bad. But the mechanic called me and said that it’s standard to replace the front (upstream) O2 sensor when replacing the converter. When I said the dealer hadn’t mentioned this (despite their usual greed), he said that mine was reading as “lazy” and that this might have caused the converter to blow in the first place. I’m doubtful, since the third mechanic interpreted that scanner code I mentioned as the upstream sensor.

How many miles on this car? Unless the mileage is really high, something caused your catalytic converter to fail. The front O2 sensor is a good suspect sense it didn’t warn you of any problems before the cat failed. (Unless my assumption that there were no prior check engine codes or light on is incorrect.)

87,000 miles on the car. There was one check engine code alert last year, but it had to do with a sensor in the gas tank.

Looks like he sold it to you,

From his point of view he needs to cover his tracks, If the check engine light came back on after he installed your cat, it could have costed you a lot more money.

I think you were better off with the $144 new 02 censor.

In the early days of 02 sensors, they were replaced often. I would recommend changing it because of its age. For the price, you might be getting the right one.