P0139 -- 2004 h6

[jp1203]

This started showing as a pending code on my 2004 H6 LL Bean tonight (128k miles). No light yet, but I figure I might as well gather information in case it shows up.

Not much information out there on this...most of what I find shows it with a combination of other codes! To me, based on the description of the code, it sounds like it's pretty likely that the rear O2 sensor is going bad.

Any thoughts?

Thanks!

[cardoc]

Something happened with the O2 sensor behind the CAT. Its either a worn sensor, they do wear out, or you have a bad ground connection. Given the mileage of the car, it is a good possibility the sensor is failing. The ECM detects this because it test the circuits and the effectiveness of the CAT at the same time. The computer will alter the fuel trim to force a reaction from the O2 sensor to calculate whether it is working properly. It apparently picked up during a drive cycle that the O2 is not responding well. The next time the ECM test the circuit, if the O2 is slow to react again, the MIL will come on.

Get a Denso or Bosch direct fit sensor. Do not use a Universal sensor. They won't work properly or fail prematurely.

[jp1203]

Thanks for the input! I usually go OEM on stuff like that, but good to know that Denso or Bosch are fine (maybe one of them makes the OEM sensor?).

No light so far, and I've gone about 100 miles with one pretty long drive in there since the pending code first appeared. Needless to say, I'll wait to do anything until I get a light.

[cardoc]

Denso and Bosch both make sensors for Subaru. Either one will work. NTK also makes a sensor for Subaru, but I always go with the better engineering on things like this.