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NicksSticks, ASE Certified Technician
Category: Ford
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Experience:  Ford Senior Master, Including Ford Diesel Master
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1997 Ford F250 Heavy Duty 5.8L V-8 Check engine light P1151 HO2S21 P1131 HO2S11 How to

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1997 Ford F250 Heavy Duty 5.8L V-8
Check engine light
P1151 HO2S21
P1131 HO2S11
How to diagnose and fix

Hi there, sorry about the delay for the answer to your question, but I can definetely help.

These codes are caused by an excessive lean condition, in most cases this is as simple as a vacuum leak. Check all vacuum hoses for breaks, or becoming disconnected. the fact that you have a lean code for both banks points to this.

Another possible cause if no leaks is found, is your pcv valve, or mass air flow sensor. To properly diagnose, a scanner required to monitor fuel trims, along with barometric pressure are required. If no leaks found, let me know if you have a scanner capabile to do this.

Also you stated, is a 5.8 engine, in this year, only a 5.4, 4.6 and 4.2 litre is availible, so I recommend double checking engine size on emission sticker under hood as well.

Thanks! Should be a simple fix.

Customer: replied 6 years ago.


Thanks for the quick response!

I am praying that this will be a simple fix! I do have a Can ODB2 Diagnostic Tool / Code Reader with Freeze Frame. Will the readings from that help? I also double checked my engine size and it is most definitely a 5.8L V-8 351 Windsor..... I think it was the standard on a 1997 F250 HEAVY DUTY XL. Do you know where I can get a simple vacuum diagram online? If I do find a vacuum leak, what's next? Too many questions?





Hi, not too many questions at all. first, a vacuum diagram will not necessarily help. Recommend visually inspecting all vacuum lines under hood. Recommend removing cover on top of engine to ease in looking. There is a hose that goes from the passanger valve to behind the throttle body. This is the first hose I would check, use your hand to follow it. If okay, check all hoses, including before the throttle body. If all looks okay, look at your freeze frame data to see what shrt fuel trims are on both banks, and lng fuel trims are on both banks and get back to me. Will be a generic vacuum diagram on the emission sticker located on hood or fan shroud, may help.
Customer: replied 6 years ago.

Thank you! I will check all the hoses this saturday and get back to you. Just one more question. If I accept your answer( so you can get paid a.s.a.p.) before we figure out a solution, will you see this through to help me fix my problem? or once I accept your answer are you done with me? Thanks!



I am here regardless of accept status. Simply contact me on this same post, and I will answer as soon as able to without requesting anthing from you. Appreciate the the above post. If worried, wait untill tested will still be here.


Customer: replied 6 years ago.
Great to hear! I not worried just cautiously optimistic! I will post my results saturday!
sounds good, thanks!
Customer: replied 6 years ago.

All the vacuum hoses seem to be fine, not loose or cracking. Here is my freeze frame data:


STFT B1 (%) - 0.78

LTFT B1 (%) - 6.25


STFT B2 (%) 32.81

LTFT B2 (%) - 4.69


I probably should have mentioned this before but my check engine light will stay on for a couple of days and then go off for a couple of days, seems very random to me, anyway I will await your advice on what to do next!





Great information. 32.81 percent of fuel addition is extreme. These readings point to the vacuum leak being on the passenger side, or even a bad exhaust leak on the passenger side. Can you hear an exhaust leak? If none found, I am also surprised you recieved a lean code for bank one as well. Might be worth a try to remove the MAF or mass air flow sensor, and examine the little wire in the box, to see if diry or contaminated (common if you are using an k%n type air filter, or have let the air filter go). If you would like to completly verify the MAF is bad, must have a scanner able to read BARO, or barometric pressure in hrtz. A parts store may beable to read this for free.

By the way, the mass air flow sensor is the sensor located in the air filter housing, with a pig-tail connector.

Customer: replied 6 years ago.

OK..Now that you mention it I do hear an exhaust leak! How do I fix that? Is it expensive? If the MAF is dirty.... can it be cleaned with MAF cleaner? or does it need to be replaced? I'll see if I can get a BARO reading for you and post the results ASAP. Thanks again for ALL your help!






Okay great, if an exhaust leak, probably on the drivers side, this must be fixed first, and will likely fix your problem. Next, yes you can attempt to clean with a MAF cleaner, may or may not work, but definetly worth a try.


Customer: replied 6 years ago.
Yes it is on the drivers side! Is this hard ti fix? Expensive?

Great information, that will be the fault, most likely. I would only go to a exhaust shop, especially an independent shop. Check the exhaust studs, when you get a chance, to see if any of the studs have broken off, a usual sign of the exhaust manifold leaking.

If a broken stud, an exhaust shop can extract easily, by removing manifold and welding a nut to the stud, then turning out. Easiest and less time required to repair (only if a broken stud). Probably a couple of hours of labor rate to replace, once removed, manifold must be checked for straighness, if warped, can be machined, or replaced.

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Let me know what you find, and will be here regardless of accept status (gives me credit)

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