For the past 2 weeks, the Max has been acting very funny. The idle has gotten rougher, especially after a cold start. Sometimes, the car will start up and all of a sudden die out after 10 seconds. Another crank and it'll run but will still feel rough under 1000 RPMs.

The results from using the ECU test modes yesterday gave me:

Mode 1 (Exhaust Gas Sensor Monitor)
- GREED & RED ON before starting
- NO LED until I rev it quickly to 4000+ RPMS, then it turns GREEN (NO RED still)
- GREEN stays on for 20 seconds then NO LEDs until I rev again

Mode 2 (Mixture Ratio Feedback Control Monitor)
- GREEN before starting
- RED / GREEN turn on after a few good revs, both remain constantly on as well as CEL
- GREEN blinks when high revs

Mode 3 (Self Diagnostic)
- Code 55 pre-start, idle, revving, etc

Mode 4 (Switches Diagnostic Mode)
- RED is fine, didn't test GREEN

Mode 5 (Real Time)
- No Blinks when driving, idling, revving, etc

I'm leaning toward the O2 sensor since I believe it's still original with 140K miles so far. That seems to be the one spot to look at from my results in test modes 1 & 2. I wanted to tap ECU signals but I didn't want to mess up any wires by accident. Mechanic ran codes through the system a week ago and it came back code 55 all around. I also tested voltage on MAF and it was in the required ranges.

Any ideas if a faulty O2 sensor would give me these problems? If not, any other suggestions on where to look?

 

If problem is only cold, points to TPS. - Plugs, wires, cap.
If problem is only warm, points to O2. - Plugs, wires, cap.
Still, could be 'anythin'...

ECU is a lottery machine - especially when connectors are not cleaned. Your first indicator points to smtg wrong in mentioned areas: 55=ok ECU, lol.

Check out and decipher codes: www.autozone.com http://www.cardomain.com/ride/748507/21

plus also read all pages, start http://www.cardomain.com/ride/748507/15