I bought a 2003 E150 witha 4.2 V6 that I plan to do a road trip with so have been trying to assess its condition. I cleaned the mass air flow sensor, and I was going to do a compression test- disconnected the ignition pack, took out two of the spark plugs, and then due to weather I packed it up and planned to get back to it another day.

The very next time I drove it I got a check engine light, I went to autozone for a readout with two codes.

First code: P1151, the autozone printout says Definition:

• lack of 02 switches-bank 2 sensor 1
• explanation, could not adjust fuel trim because of a lean or rich condition.
• probably cause: fuel pressure high or low, MAF sensor dirty/defective, vacuum leak on engine, leaking fuel injector or fuel pressure regulator.

Second code: P1131 Definition:

• lack of bank 1 02 transitions- lean
• Explanation: ECM detected bank 1 02 at its lean limit
• probable cause: misfire on bank 1, vacuum leak affecting bank 1 only, fuel injector problem bank 1
• engine mechanical condition

I decided I'd just let my mechanic look at it and get his opinion on the engine and the check engine light... however like an idiot, I forgot to tell him about my fiddling with the MAF sensor and the plugs. He did his own check, did a smoke test for leaks (found nothing) and said My o2 sensors needed to be replaced.

The light coming on after me working on it seems like a pretty good coincidence. Before I go replacing the o2 sensors... is there anything I can do to ensure it wasnt just my own screwing around that caused this? Or is it still probably the o2 sensors?

 

I'm now leaning towards bad isolator bolts and intake manifold gaskets rather than o2 sensors. Apparently it's a widespread issue on these engines.

I've done more research and I guess the way this works is like this- those codes are both essentially the same code, one is for the right side and the other is for the left. They are vague codes but in general they can mean you have a bad MAF sensor, bad o2 sensor, or a vacuum leak. Someone else said its more likely between vacuum leak and o2 sensor.

So if you get one of the codes, maybe it's an o2 sensor. And if you get both codes then it would suggest that both o2 sensors went at the same time, which is much less likely. Not only that but o2 sensors dont usually go biased (malfunction in a way that misreads too lean/rich), they are much more likely to just die altogether.
So to have both o2 sensors "go biased" suddenly is a good sign that it is NOT the o2 sensors, but a vacuum leak further upstream.

Since my mechanic says he tested for vacuum leaks with a smoke test then it shouldnt be any of the obvious places. I'm told that the isolator bolt vacuum leak wont turn up in a smoke test because its leaking internally. That along with the fact that this is a widespread issue with these ford 4.2 V6 engines. I need to replace my valve cover gasket anyway, so I'm going to do the isolator bolts/manifold gasket at the same time. If this doesn't solve my issue it will at least be done, since it seems inevitable with these engines anyway.