quinnrj

Thanks for the suggestion. I'll take a look for a loose wire, and if I have time, pick up a torch to check the sensor with the Torch test. As in that post, the sensor's voltage did increase when throttled. -Bob

He showed me on his Snap-on diagnostic machine it was between 3.5 and 4.1 Volts. The rear O2 sensor was reading between 0.0 and 1.0 V. Thanks for the quick response.

I am driving a 2001 Subaru Legacy Wagon (stick) with 137,000 miles. The CEL is on, and I'm getting the P0420 code - Cat efficiency (bank 1). I checked for exhaust leaks (I used to have one behind the cat, but sealed it) & replaced the rear O2 sensor with an OEM Denso bought from Amazon a few weeks ago. All of the pipes seem to be fine (tapped them with a hammer). I brought it into a shop today to do a diagnostic, and their equipment shows that the front O2 sensor reads 3-4 volts (BTW the same sensor was replaced in July 08 with an OEM Denso bought from RockAuto.com). :-\ The shop didn't charge me for the Diagnostic (thanks, PepBoys!), and told me that I should bring it to a Dealer to do a more detailed diagnostic. The tech that did the Diagnostic also didn't see any leaks. He suggested it could be a computer issue, or a wiring issue (melted or damaged). The car was in an accident in Sept. 04 - long before this issue came up. He suggested that the heat shields on the Cat were loose, and that Subaru will probably recommend replacing the Cat (for that reason). I'm planning on dropping it off at the Dealer's tomorrow night for their $98 diagnostic, but I thought I'd post here to see if anybody has any suggestions I could try in the meantime. I'd read on some forums that re-flashing the ECM by the dealer can sometimes fix the Po420 code. FYI: The car was winterized in July '07 & stored for a year (we moved to England for a year). After pulling it out of storage in Sept. '09, I had to replace the clutch, front brakes & the Idle Control Valve. Any thoughts would be appreciated! As an aside - I've seen software & cables advertised online for hooking one's laptop up to one's car & reading Diagnostic info. I'm enough of a geek to think that's cool. If anybody has suggestions for a cheap cable/software that I could buy, I'd be interested in that also. :-\