Robert Callaway

So I am getting a CEL - P0140 for Bank 1 Sensor 2. I replaced both downstream sensors and reset the light using my iCarsoft MBII. Drive the car for a week or more and the light comes back on, same code. I run all the O2 sensor diagnostics and everything checks out. Reset the light. Days later it's back. Then, a few days after that, it goes out again. Took it to my favorite MB indy, he runs all his MB diagnostics, everything checks out, wiring all looks good. Cannot find an issue. Drives it for three days. No issues. Pick up the car. The next day....CEL comes on again, same code.

What the heck? He is frustrated as much as I am and we are brainstorming.

Any ideas as to what is going on here? About 135k mi.

This car has had most everything redone on it. Just had him do the balance shaft, and all sorts of "while you are in there" items. That was a few months ago. Runs like a top; super smooth.

Any thoughts appreciated !

R.

F1Fan

What year and engine type is it?

This might be a sign of a bad catalytic converter.

Robert Callaway

Sorry. 2006 E350

Wouldnt readings from the O2 sensor confirm that? I think the iCarsoft shows those for each sensor.

cetialpha5

Have you checked for a wiring fault? Maybe clean the MAF also and I'd also change the upstream sensors. Those are the ones that usually go, not the downstream ones. When you have a problem with the downstream ones, it usually indicates a problem somewhere else. Plus the usual check for an air leak somewhere.

Robert Callaway

All the wiring was checked by my indy. He said that is fine.

I ran diagnostics on all the sensors with iCarSoft and they test good. My indy did the same with his MB diagnostics. Do we really need to swap the front O2 sensors? Seems questionable.

Where are we looking for "air leaks"? Intake was just off and on with new gaskets.

cetialpha5

The front O2 sensors get lazy as they get old, but it will get worse before it triggers a CEL. At that mileage, you're not just throwing parts at it, they're probably worn out anyway or close to it. They can be replaced anywhere from 60-100k and you're already beyond that anyway.

As for the wiring, I'm thinking of rust in the connectors, wire may look fine. It's always a challenge to find an air leak, no idea where one might be, but it can also lead to problems like this. You'd do things like a smoke test to find one if you had one. It's just citied as a possible reason with your error code.