I've had my '95 E320 Wagon for 3 years now, and love it! I have a lot of little kids, and it's way better than a minivan!

CEL recently came on right around when I noticed MPG drop from 24 to 20, and it just now failed UT state emissions, HC and CO% are high both at idle and at 2500 RPM (CO% is 4-6x the MAX and HC just over 2x).

I've built the code reader described in these forums and right now (after a recent reset) it's code 3 (LED blinks) and code 13 (using reader on pin 8). Exhaust smells like unburnt fuel (hence the MPG drop), likely indicating rich fuel mix rather than lean. Motor doesn't have any stutters and sounds smooth both at idle and rev'd.

I'm eager to dive in and figure out what it could be, but would love advice from some of you experts!

Could this be just a bad O2 sensor, or would that have triggered a different code? I tried testing that based on post (green wire under passenger-side) and found it was staying at .5v when disconnecting vacuum line, revving engine, etc. Is that a likely bad O2 sensor?

What else should I test with these symptoms? We use 10% ethanol in UT, which is what I first thought was the cause of the reduced mpg, though we drive it only once every couple weeks, so it took a while to notice.