My '99 4.6L has thrown code P1131 intermittently for the 6 years I have owned it. Code P1131 is upstream bank 1 oxygen sensor too lean. The code will persist for a few weeks, then it will go away for a few weeks. Change in state seems to coincide with filling the tank with gas. I replaced the gas cap and am careful to fully seat it.

Engine runs fine: no issues starting, smooth idle, 17-18 MPG.

The CEL comes on 5 minutes after I start the engine. P1131 is the only code. If I clear the code. it does not return while running. If I shut it down, let the engine cool, and restart, it returns. I don't understand the code reporting system well enough to understand whether that is significant.

There is a TSB for leaking bolts or gaskets on the upper-to-lower intake manifold area that causes similar symptoms. I'm not inclined to tear apart my manifold when the engine is running well.

Thoughts on other causes?

 

Bump

 

P1131 Lack Of HO2S11 Switch - Sensor Indicates Lean

It's slow on the switch, needs replacing. Otherwise you would most likley have 171 and 174 DTC's accompanying.

 

Sorry, bit unclear ... you mean replace the O2 sensor, right?

 

correct. make sure you get yourself a motorcraft