The wife's 99 Outback (100k miles, MT) just popped the "Check Engine" light. Mo OBD II scanner says it's code 141, Downstream O2 sensor heater problem. Looked in the Haynes and it has a photo of the upstream sensor but not the downstream. It also has some minimal troubleshooting checks for voltages and resistance. Anyway, I erased the code and it hasn't come back yet and I'm wondering if this is a common failure (like the EGR) or was it just a spurious reading? I'm a marine diesel mech so I know my way around an engine but I don't know that much about all this emmisions stuff. Any suggestions? How much do these sensors cost? Are they a pain in the rear to get to? Thanks! (Just discovered this forum...it's about time there was a Subaru forum!)

Well, I ended up replacing both the front and rear O2 sensors since I figured the front one is usually only good for 100k miles and I was under there already. Wasn't too hard. Just need a 22mm wrench. Turns out that my code didn't go out after changing out the rear sensor so I figured it was a wiring problem. I traced the wiring harness and found that it has a connector at the top of the bellhousing and the connector had come loose. Just pushed them back together and I was back in business. So, I'm sure the rear sensor was probably still good...but I'll rationalize this experience and just call it good preventative maintenance.