ok, here goes. just became proud papa of o3' F 150 4x4 5.4L. runs good but ck engine light wont stay away long. codes po171 & po174 system lean bank 1 & bank 2. 02s12 & o2s22 are down stream sensors that are not switching properly. ltft is about 33%. not to good. have replaced fuel filter & ck'd fuel pressure. 35 psi. hard to see all vacume lines. any help greatly appreciated. thanks in advance.

 

Click on SEARCH and enter your two fault codes.

This is probably the most asked question on this forum. Do a few minutes of poking around first before hollering.

 

100% agree.
I went through this myself a year ago and started a thread and I recieved alot of help and answers. You should be able to find that thread by typing in the P0171/P0174 codes in the search section. You will find your answer in there.

 

Clean the mass air flow sensor and check the PCV hose for holes or any abnormalities in the hose.

Leak on bank 1 and 2 means either vac leak like the PCV colapesed in on itself and created a hole in the hose, allowing vaccum into the manifold, or your Mass Air Flow Sensor is so dirty you truck is only 'reading' so much air, however it is actually getting alot more air into the intake.

 

thanks for all the feed back

 

ok, so i've cleaned & than replaced maf (due to inconsistant baro/voltage readings) & traced all hoses including this pvc elbow that everyone has told me to check. sprayed 2 cans of brake clean around all hoses, egr, lower plenum- to head area, checked for cracked boot after maf, i also dissconnected battery for about 10 mins & drove to reset values. as soon as the temp dropped 2days later the light came back on again. i'm going to due somthing i probably should have done 1st. replace the pvc valve. however my biggest ? is regarding both down stream o2 sensors b1s2 & b2s2. ive noticed since ive owned this truck that niether of these down stream sensors switch high/low mv signals like the 2 up stream o2 sensors do. they seem to both stay around the 600 mv range. even if the mv signal does change from that area they still dont switch high/low like they should. it sxeems like they should change from high to low rapidly like the b11 & the b21 sensors due. any input greatly appreciated.

 

This may help -

http://www.fordscorpio.co.uk/ho2smonitor.htm

 

Awesome link !!!

 

When I have my scan-tool plugged in while I am driving, my downstream O2's do not switch radiply like the upstream sensors. If I remember right, the downstreams show mild/slow flucuation between 450-550. My upstream O2's are constantly flucuating and taking readings. Hope this helps.

 

That is normal behavior for down streams. If they rarely switch, the cat is working. If they switch at or near the same rate as the up streams, the catalyst is toast.

 

Thanks for info !

 

thankyou jbrew & projectsho89. that was a very informative link especially the O2 sensor theory part. as tjkinnyc said. since this AM i took a MUCH closer look at things & found slight cracks in pfe sensor lines (both) couldnt see cracks without actually moving & bending them & also found broken vacume nipple where rubber hose meets small diameter formed white plastic hose on pass. side near or on top of frame rail.it is actually a multi-vacume connector. canister purge? im not sure. also replaced pvc valve so well see what happens. thanks again.

 

Glad your finding things as you go. I tried just replacing the PCV valve and that did not work out long. I pulled my PCV Assembly twice and thought it was good. Finally, I said the heck with it and purchased a NEW one from the dealer ($28.00 + gov.) and it corrected my last P-Code. I have had none come back. I am guessing that maybe there was a pinhole or slight crack in the black, rubber elbow that connected to the throttle body. I know it was really spongy and aged. Anyways, it fixed my problem.

 

oops! i meant "CNY".sorry. thats probably like someone thinking I'm from Detroit. anyways its funny that after all the comments I've heard about the pvc elbow & those other 2 elbows @ the rear of the throttle body about spongyness & being soft, I'm surprised I dont show those signs especially w/ 260000 miles.I tried sliding that elbow of the vacume port & it just didnt want to come off very easy so I just left it alone. I know the fella who had this truck prior & I'm certain he stayed on top of reg maintainance including lof's maybe thats why. made a few trips earlier today, so far so good but it will take a while if the light is gonna come back.

 

well it came back. 171 1st than a few miles 174 came back. im reading from a snap-on 2500 brick. wondering if its actually possible that both sides of intake are leaking? I once had a missfire #3 cyl. but it hasnt popped up since. right side leaking more ?? if so #3 miss code back then would make sense. if you guys have any thoughts I greatly appreciate your input otherwise it just might be intake time. thanks again,