Code 1131

MATTHEW SPEARS

MEMBER
1999 FORD EXPLORER
5.0L
V8
2WD
AUTOMATIC
235,000 MILES

My vehicle is throwing code p1131. I have already replaced the MAF twice and all the 02 sensors and cleared the computer. It ran like new for about a week and then started running rough again. I have checked vacuum leaks and have not found any. I have replaced the fuel pump and the inline filter also. What could be the problem? I am lost right now.

1 Reply

STEVE W.

MECHANIC
13,467 POSTS

Running rough and this code are common sings of misfire and intake gasket leaks. Another odd issue is the PCV valve that can fail and cause O2 codes to set.

The problem with the intake leaks are that they can be hard to spot because they can be hidden.
However as yours seemed to run okay and then the issue returned I would look at the actual O2 voltage numbers on a scan tool and see what happens if you use an unlit propane torch around the intake upper and lower gaskets and around the exhaust manifold as well. A small leak can be enough to trigger it and you changing the sensors may have shifted the manifold if it has a leak/crack.

Next I would look at the actual fuel trims on the scan tool. I have seen more than one case of a vehicle running rich but within tolerances on one bank and having it report the other side as lean.