Need some advice on how to proceed with multiple codes. Here is the history
1) Three months ago I got a P0134 code ( O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 1)). Ignored as MPG seemed ok
2) 2 months ago the bottom of main catalytic converter heat shield came loose and was removed. Noticeably hotter in engine bay.
3) Last week I got P0685 (Main Relay Ecm Fuse Wiring Problem) and P2251 (O2 Sensor Negative Current Control Circuit/Open Bank 1 Sensor 1)
4) 3 yo Battery died a couple of days ago. I replaced the battery and checked cables but still showing P0685 and P2251. Not seeing P0134 but havent been driving very far to see it tripped and an disconnecting battery when parked overnight.
Was going to replace O2 sensor but after reading through threads Im seeing a lot of other issues that may be the problem.

 

In terms of P0134 and P2251, most likely replacing the sensor will fix the issue. Get something decent, NTK or Denso. Stay away from Bosh at all cost. Rockauto should have those sensors at the cheapest price. NTK should be in the car from the factory, although the latter is also good.

P0685 might be a glitch. ECM has two ways of getting power, through key-on circuit and through one of the relays.



This relay not working would cripple a lot of systems, including injectors and crank sensor, so engine wouldn't run it something was bad with that relay or fuse. But having that relay stuck on might cause problems.
If this code comes back after replacing the A/F sensor, I would bet something with electrical system is bad. Crappy battery, faulty alternator, bad grounds, or corroded positive cable.
Visual inspection doesn't always tells you condition of the cables / connections, and voltmeter might be not enough to check if alternator is working properly.

Why do you keep disconnecting the battery?

Heat shield shouldn't be a problem unless it melts wiring... or anything around it.

 

Thanks for the reply. I got an NTK sensor from Oreillys and was planning on replacing it this week. Will still do so but have been reading lots of posts saying O2 sensors rarely go bad and its usually something else, vaccum leak, exhaust leak, bad wiring, spark plugs, cat converter. Willing to take the chance as checking all the others issues may be beyond my abilities/patience

Ive been disconnecting the battery for years now as I only use the car a couple of times a week and cant seem to find the parasitic drain. (did disconnect hands free but still have issue). But now with P0685 Ive read that the ECM may stay on and will drain battery quicker as I experienced the other day,

 

Just wanted to update the thread. Replaced o2 sensor a few days ago and so far so good, no more codes. Thanks again @peter6 for the right call. fyi disconnecting the battery doesnt clear P0685 and P2251, only P0134. Had to go through XM diagnostics to delete the codes.

 

@peter6 is my new hero. TY for sharing your knowledge with the forum!