Yet another P1131 problem | Ford Explorer Forums - Serious Explorations

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Yet another P1131 problem


New Member
February 3, 2008
Reaction score
City, State
Oak Harbor, WA
Year, Model & Trim Level
2002 XLT
The service engine soon light came on, and I had the code pulled from AZ. It reads P1131 Lean left. And the only sympton is the occassional rough up and down idle. But is does not do it all the time, does it both while cold and hot.

So far today I have done:

Clean MAF
Re gapped plugs
Found a soft spungy hose which I will replace tomorrow
Had the injectors cleaned at Jiffy Lube

Tomorrow I will try and located and clean the IAF, and replace that spungy hose, and new plugs, and also purchase some spray to check for any vacume leaks.

It only idles rough right when you think it has stopped.

Last year, all the O2 sensors were replaced, and the intake manifold was replaced with a new stock one. The only thing I have added is the K&N way before this code popped.

I could not located the plastic elbow from the left to right pvc, one that a previous post found cracked.

I will keep trying all the ideas I have researched in this forum.



I replaced all plugs again, the left bank (drivers side) plugs were pretty clean, the right bank (passenger side) the plugs were blackened. I was like hmm, I glanced down at the O2 senser ( I had the air box off) and I noticed this



The O2 sensor had found its way onto the back manifold. So while I was at NAPA getting plugs and carb cleaner I picked up a O2 sensor. I had to take it back and get a universal one due to both ends of the plastic connectors are melted. So, after wiring up the new O2 sensor, and cleaning just 2 more sensors, and spraying the complete engine with carb cleaner to locate any vacume leaks which I did not find any.

Took it for another 45 min drive, the dang thing is running like a champ, and is not surging anymore when coming to a stop, but the Service Engine Soon light came right back on. And it is still showing:

BBLack of bank 1 O2 transitions Lean explanation

BBECM detected bank 1 O2 at its lean limit

Possible cause

BB1 - Misfire on bank 1

BB2 - Vacuum leak affecting bank 1 only

BB3 - Fuel injector problem bank 1

BB4 - Engine mechanical condition

I'm at a loss. It is running smooth, and not surging anymore.

I think I will drive it for a while and see how the plugs look on Bank 1 after a few miles.

Speaking of Bank 1.
Would bank one be the left head as you are sitting in the car?
Or standing infront of the X looking at the engine?


If i remember right every thing is referred to as if you were sitting in the vehicle.