P1131!!!!
 

I have a 2000 Expedition 5.4 and I have the P1131 and was wondering if anybody has had this or can explain what causes it. When I scanned it the code description was Manufacturer Mass Air Metering, that’s all it said. I looked up more on the code and found out that it could be a bunch of different things causing this code to be set. The explanations varied from O2 sensors all the way to a vacuum leak and everything between. When I cleared the code it didn't seem like it was reset until the truck went into closed loop after the engine warmed up. I am going to fix it but first I wanted to see what anybody else thinks it is. I also am going to look a MAF to see if it is functioning correctly. Any extra info would be greatly appreciated.

My 2000 Expedition threw that code after I changed the exhaust manifold studs. In my case it was the passanger side upstream O2 sensor. I think the sheer amount of PB Blaster I used killed the sensor. From what I have read, a vacuum leak is a more common cause for this code. Hopefully someone can chime in with more info.

P1131 means the PCM hasn't seen the HO2S-Bank 1 sensor 1 switch back and forth between lean and rich within a specfied period of time. In other words the HO2S is stuck in the lean position even though the PCM has tried driving the injectors full rich and the HO2S still indicates its too lean. If you look at the HO2S-11 voltage it would continually be less than .45v.

Some things that could cause that are a wiring/connector issue in that HO2S circuit, low fuel pressure, air leak after the MAF, improperly seated engine oil dip stick, leaking EGR gasket or diaphram, exhaust leak ahead of the fuel control HO2S.

Check intake air system for leaks, obstructions and damage.
Check air cleaner element, air cleaner housing for blockage.
Verify integrity of the PCV system.
Check for vacuum leaks