HI yall, I have a 1999 explorer with a 4.0 ohv auto with about 174,000 miles on it. the PO changed the coil pack,wires,plugs,MAF and cat on it trying to get rid of codes. I know the from searching that they are both precat 02 codes. So im planning on looking for vacumn leaks with propane around intake and egr and checking on the fuel pressure and wiring and connections. Is there any thing else I should check out? for both sensors to throw this code? The truck runs ok just has a shutter when going up hills and stuff.
Per the factory service manual, the following are possible causes of the P0133:
Contaminated HO2S sensor.
Shorted /open wiring.
Deteriorating HO2S sensor.
Inlet air leaks.
For the P1151, the manual references P1130:
Short to VPWR in harness or HO2S
Water in harness connector
Open/Shorted HO2S circuit
Corrosion or poor mating terminals and wiring
Excessive fuel pressure
Leaking/contaminated fuel injectors
Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system
Air leaks after the MAF
Improperly seated engine oil dipstick
Stuck EGR valve
Leaking diaphragm or EVR
Exhaust leaks before or near the HO2S(s)
The descriptions for the P0133 relates to frequency and amplitude, for P1151 it seems to be more focused on amplitude, and specifically a rich condition. In the list above for P1130 you can probably ignore situations that would cause a lean condition, at least for now. If it were me, I'd start by checking fuel pressure along with how quickly the rail pressure drops when the key is turned off.
What's a bit confusing about the P1151 is if you only look at the code name versus the code description it could send you down the wrong path. The code name states:
"P1151 - Lack of HO2S-21 Switch, Sensor Indicates Lean"
While the code description is listed as:
"A HEGO sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when fuel control system no longer detects switching for a calibrated amount of time."
Well my harber freight fuel pressure gauge gave me different readings. I hook it up and started the car reading 30 tapped the gauge and it went to 65. Cut the car off and pressure dropped. Retested pressure 60 cut the car off and pressure stayed. So have to get a better tester. Could find no leaks with the propane.