HAVE A P1151 CODE... WHAT COULD THIS BE? IS IT VACCUME, MAF??

on 1999 Ford Expedition

NOT ANY MORE DETAIL

2 answers
P1151 Lack of HO2S-21 Switching, Sensor Indicates Lean (Bank 2)Possible Causes Setting Conditions
•Check for air leaks after the MAF sensor mounting location
•Check for air leaks in the EGR system and in its related hoses and/or valve
•Check for air leaks in the PCV system and in its related hoses and/or valve
•HO2S connector is damaged (check pins for damage, and for moisture)
•HO2S signal circuit open, shorted to ground, shorted to voltage
•HO2S has failed (it may be contaminated with fuel, silicone or water)
•Low fuel pressure or vehicle driven until it was out of fuel
•Vacuum hoses leaking and/or damaged
Engine running in Closed Loop; and the PCM detected that the Bank 2 HO2S-21 signal was less than 0.450v. In this case, the PCM is "correcting" rich for an overly lean condition.

This code sets in 2 Trip(s).
This code will turn on the MIL (Malfunction Indicator Lamp)
It is an oxygen sensor code.
P1151 OBD Trouble Code
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Technical Description
Lack of HO2S-21 Switch, Sensor Indicates Lean

What does that mean?
A Heated Exhaust Oxygen (HO2S) sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when the fuel control system no longer detects switching for a calibrated amount of time.

Symptoms
You will likely not notice any drivability problems.

Causes
A code P1151 may mean that one or more of the following has happened:

Electrical:
Short to VPWR in the harness or HO2S
Water in the harness connector
Open/shorted HO2S circuit
Corrosion or poor mating terminals and wiring
Damaged HO2S
Damaged PCM
Fuel System:
Excessive fuel pressure
Leaking/contaminated fuel injectors
Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system
Induction System:
Air leaks after the MAF
Vacuum Leaks
PCV system:
Improperly seated engine oil dipstick
EGR System:
Leaking gasket
Stuck EGR valve
Leaking diaphragm or EVR
Base Engine:
Oil overfill
Cam Timing
Cylinder compression
Exhaust leaks before or near the HO2S