[NMT100]

CEL is back on and giving me the same P0420 code after I fixed a small exhaust leak between the front O2 Sensor and the CC. The CEL stayed off for about a week or so and came back on after a 60 mile drive the other day. I checked the FSM and found the ohms test for the O2 sensors. The front sensor spec. is 5 to 7ohms and the sensor was reading 8.9ohms (outside limitations). Anyone know how reliable that test is?????????? Also tested the rear O2 Sensor and it was well within the required spec. Recommendations????

 

[BamZipPow]

Resistance test is only an indicator...but not a true test. The key will be to hook up a diagnostics computer to see live data or data capturing of the response of the O2 sensor in question.

If you don't have the time or access to git something like that, you could just replace the O2 sensor. I would only recommend a Denso O2 sensor and nothing else. Make sure you git the correct one. I don't believe that you would have a California spec T-100 out in Maine...but you never know.

 

[NMT100]

Has anyone here ever seen a P0420 Code clear after replacing an O2 Sensor?

 

[BamZipPow]

Has anyone here ever seen a P0420 Code clear after replacing an O2 Sensor?

If that was the direct cause...yes. I replaced my friend's Buick V6 O2 sensors (pre and post) in Phoenix during my run out West. Cleared the P0420 code but it still ran pretty rough. Checked the plugs and the front bank didn't look right. The plugs were clean on one side and black carbon on the other. It ended up being the vacuum hose was disconnected to the fuel pressure regulator...which was on the front injector rail.

The code should clear itself after the O2 sensor goes through a good heating cycle. If not, you can clear it by pulling the negative wire on the battery or pulling the EFI fuse fer at least 5 minutes.

Be aware that if you have an alarm system that checks fer the battery being disconnected that it will probably enable a starter kill...go through yer procedures to re-enable the starter.