Help i have a 99' F-150 4x4 Xlt with the 5.4. just recently my Service Engine Soon light came on. i had a friend read the code and we determined that is was the O2 sensor. well upon further investigation i realized i had 4 of with all have been changed but my light keeps coming back no the codes are P1151 "lack of HO2S21 sensor indicates lean" and P0155 "O2 sensor heater circuit malfunction bank2 sensor 1" but i don't understand how this can be i changed all 4 sensors any help would be great.

Thanks

 

With 2 things here, it could be a damanged HO2S harness or water in the connector, but the P1151 code says it could be a few other things. I would take a look at the cause list for the P1151 first.

Or if you take the route of the common elements between P0155 and P115, then either it is the wireing harness or the PCM that is the problem.
A few points missing, the miles on your 99 ? ( dry & cracked vac lines / EGR & PCV gummed up )
Is the xhst system stock, or have you switched it. ( xhst leaks around the O2 sensor )
Have you reciently changed the air filter ( looking for leaks after the MAFS, letting in un-mettered air ).
Does the truck idle rough ? ( vac leaks, un-mettered air in system )

While vac leaks are usually a different DTC, it is worth a check to look for vac leaks in vac lines, and on the rubber boot on the back of the throttle body for the PCV. Also if you have just changed the air filter, check that the 2 lines to the intake are connected, some times these can partically come off the throttle body elbow, and cause un-mettered air into the system.

Here is the list for the P1151 code.

DTC
P1151 - Lack of HO2S -21 Switch, Sensor Indicates Lean

Description
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.

Possible Causes
See Possible Causes for DTC P1130
Possible Causes from DTC P1130
Electrical:
- Short to VPWR in harness or HO2S
- Water in 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(s)

Diagnostic Aides
A fuel control HO2S PID switching across 0.45 volt from 0.2 to 0.9 volt indicates a normal switching HO2S.

Ususally the reporting device is not the case with O2 sensors, but depending on the millage, these have been noted as maintance items at ~ 60K miles regardless of what Ford says. Now that you replaced them you are good to go for another 60K on them.
Checking the wiring harness is also another good idea, mabye check this value at the harness input. This is pin 87, a red with a black stripe wire.

Good luck with it, let us know if you have any other questions.

 

ok i haven't got under the truck yet because its been raining but here is some info i left out. the truck has 111,000 miles on it the exhaust system is stock but the muffler was cut out so the tail pipe ends just about even with the back of the cab but it has been like that for over a year. no but i have a K&N that has been installed for over a year i cleaned it after the light came on but nothing changed. the truck doesn't miss a beat. it runs great no rough idle or change in gas milage. changed my pcv valve about 6 months ago also. when it quits raining i'll get to looking for bad wire harness or cracked vac lines but where should i start? with a certain sensor? can i ohm out the sensors or harnesses and get some reading?

thanks SSCULLY

 

Start with taking the reading at the wire harness connector for the PCM. You can get to this on the firewall, kind of behind the battery. You will need to take the battery out, to T-Tap a lead to the red w/ black wire on the pin noted above. range on the Volt Meter, is 0 to 1 V DC. If it is running in the dirt, or gets no reading what so ever, that could be a bad harness. Considering that you chaned all 4, I would say it is almost safe to rule out a 2nd bad O2 sensor, but I guess stranger things have happened. Chances of having a bad O2 sensor in positon 21, and then replacing it with another bad one, out of the 4 you got ( I think 11 and 21 are the same p/n ) is kind of high odds.

The other part you can look at first is the rubber boot on the back of the throttle body for the PCV. This cracks some times, and causes un-mettered air into the system, but I would suspect a different error rather then this error. But again it is not proven that a cracked PCV boot / hose won't cause this error, just the usual code is something else.

Kind of what you feel like hunting for out of the gate, vac leak, or electrical problem. Flip a coin on which way to go if you are un-decided.

Sorry I can't offer more on the topic, haven't seen this one to completion myself yet.

Good luck