The "check engine" light on my '98 4.2 came on recently. They scanned the codes for me at Autozone and informed me it was likely a bad oxy sensor. To make a very long story as short as possible, I ended up replacing all 4 sensors (2 pre-cat, 2 post-).
I figured they were about due anyway since I am now at 87,500 miles, and the gas mileage of late has been worse than ever (12-13 mpg). But after installing them and clearing the codes, the light is back on again. One of the Autozone guys told me the problem could be that the dealer-supplied parts are slightly different, though this seems unlikely as the new sensors are Bosch just like the originals. Perhaps my problem is not oxy sensors at all??? Can anyone out there in Ford truckland shed some light on this topic? Thanks a lot, Andy

 

If that was your only code then disconnect the battery for about 20 minutes or so to clear the codes out of the computer, the light should go off, since it is only on when there is a code in the computer if it doesn't come back on then your all set. if it comes back on check the codes again

 

Thanks, I will try that again tomorrow and see what happens.
Usually after clearing the codes, I can drive around for 10 minutes or so before the light comes on, then it stays on.
Interestingly, I believe the actual code said, "Bank 2 lean"...
(Lean? That doesn't sound like an oxy sensor to me, as when they go the ECU usually defaults to a rich setting, not a lean one). And the gas mileage has definitely been pretty lousy lately.

 

I think I found the problem, and I don't believe it was the oxy sensors after all! I poked around the throttle body/intake boot area and bingo! found an unattended air boot nipple with a disconnected pipe below. The end of the pipe was quite grungy, indicating it had been disconnected for quite some time.

My guess is that the resultant air leak, which occurred downstream of the air flow meter, went unrecognized by the ECU, resulting in an overly lean mixture. But the oxy sensors (all 4 of them) recognized the lean condition and tried to compensate by going full-rich (which resulted in really lousy gas mileage).

I hope the above will be of use to somebody. It looks pretty easy to leave the pipe off the air boot if the boot is removed for service. I have now reconnected the pipe, reset the codes and hoping a) the 'check engine' light stays off and b) gas mileage is a little better. Andy

 

Big 6, hey I have had the EXACT same problem,..I too have replaced all of the sensors in my 98 F150 4.2Liter and STILL the stinking check engine light goes on about every two days and then cuts off and back on again. A mechanic told me it was absolutely not a sensor issue, but rather a vacuum hose leak of some sort and it would cost me about 300 buckaroos to conduct a smoke test to find the source of the leak!!

Being that I'm not a mechanic, it has been especially frustrating for me!! I see that you solved the problem,...but I don't know where to look for where you found the culprit. (throttle body/intake boot area).
Is this something I can find being an all thumbs guy? Can you send me a photo??

What do I look for?

Thanks to anyone on this!!!

Todd
TDolce(No Email Addresses In Posts!)

 

Hi Todd, the disconnected hose I mentioned goes from the driver's side valve cover to the intake boot. If you look at the intake boot, there is a nipple located between the throttle body and the air filter housing. This is where my hose was disconnected. I thought reconnecting it and erasing the codes would take care of it. But apparently that was not the problem as the light came back on!

I have since visited my Ford dealer a couple times. On the first visit, in which they charged me $54.00 to read and clear the codes, the service advisor told me I still had a bad front driver's side oxy sensor. I purchased and installed a new one. I also replaced the air and fuel filters and cleaned the MAF sensor, in response to remedies suggested to this problem by others in the F-150 forum. Someone else mentioned dirty injectors so I also added injection cleaner to tank. Cleared the codes again (disconnected battery) and thought I had solved problem as light stayed off. Next day: bing! back on again!

Went back to dealer who said codes must not have been sufficiently cleared, so he cleared them again. He said if light comes on again, it could be intake manifold leak and I would have to leave truck to troubleshoot it. Sure enough, less than 5 minutes after driving off the light is back on.

I am thinking of changing the intake manifold gasket myself as it looks pretty simple and it's relatively cheap. Besides the light on, and crummy gas mileage, the only other symptom is light pinging on steady-state throttle (regular gas). Other than that the truck runs fine. I pulled a couple plugs today, one from each bank, and they looked pretty clean. Conversely, all 4 oxy sensors were pretty sooty-looking, come to think of it. Hmmmm. Also, the plugs (Platinum Motorcrafts) look pretty worn (I measured .080 gap on one!), could that contribute to pinging etc? Probably time for new wires as well, at 87,000 miles. Sorry for this long letter, just trying to throw out as much info as possible in case it rings a bell with anyone. I appreciate this forum and the good people on it. Andy

 

Oh I see,...well,...my check engine light comes on for about a day or two, and then goes off for a day or so also. I don't think I have any gasket trouble only because I have already had some major work done on that already prior to this. I have already done the O2 sensor dance and what followed was the engine rebuild. Now this latest problem was diagnosed by one mechanic as a vacuum leak. My gas mileage seems to be consistent and above average while I lalso do not have any pinging or stuttering while driving. My only issue comes when the truck idles at a light. I sometimes get a sensation of misfiring as if the truck may want to stall out or hesitate. So maybe my issue truly is a vacuum leak. I will be taking it in next Tuesday for a brake job and will have it looked at then perhaps. I heard too that spraying carb cleaner on the hoses while the truck is idling is a poor mans way to test for leaks as the truck will surge when the spray hits the target where the leak is. Not sure how true this test is though and if I could do more damage by doing this....

td

 

Todd, I think the pinging on my truck is simply from carbon buildup combined with warm weather (I'm in Fla.).
As far as your truck, I didn't know the 'check engine' light was able to go on and off like that. My understanding (from the dealer) is that once a trouble code is stored the light comes on and stays on until the code is cleared. (Anyone out there know about this?)
It does sound like a vacuum leak to me in your case, not sure about mine. I have also seen mechanics do the 'spray test' to find a leak as you described, and I have done it myself. A word of caution if you use carb cleaner: DON'T! That stuff is highly combustible, as you will find out if you hit the hot exhaust manifold. Wd-40 is a much safer alternative, but still be careful.
Hope you find it, and please let us know if you do. Cheers, Andy

 

Yeah,

I'm in a warm and humid climate myself (Memphis). As I write this, my check engine light is off and it's on it's usual 2nd day. Probably tomorrow it will return back on. It has done this for 3 months. I will be finally addressing this issue Tuesday. My work has me going so much that I actually have to plan time off to get anything done. I may just let them spray the hoses as I like my eyebrows the way they are. I'll tell ya what though, that da&@! check engine light plays on my nereves though. I have had that stinking light come on from day 1 AND 4 O2 sensors and an engine rebuild later!!! I have had the doors worked on as at one point the interior lights would not go off when the doors shut. They fixed the driver side door switch and two days later the right door went and the problem returned. I swear, this was my first Ford and my last! I bought it because I wanted a reliable bum around work truck and my father in law raves about Fords. Now I'm stuck in this black hole of repair bills. I swear if I ever get to the bottom of it all,...I won't sell the truck because I will at least want to enjoy it for a few years!!!

td

 

I know what you mean. The 'check engine' light is driving me a tad loco as well. This weekend I took about a 200 mile round trip (to the 12 Hours of Sebring Road Race) and I recorded about 17.5 mpg overall, quite an improvement over the last tankful but it should be as it was mostly highway miles. My cruise is out and I lost my owner's manual so not sure which fuse(s) to check. I also think my front brake rotors are warped as I get the pulsing pedal despite new drums/shoes in the rear. Like you, I had also bought this truck with high expectations as I have owned a succession of 4 Rangers over the years with almost nary a problem. I still think the F-150 is a good vehicle and built quite well in general, but I'm ready to shoot that light out! Please let us know what happens on Tuesday, perhaps there will be a large mushroom cloud over Memphis!

 

This thread is very old, but thought I would contribute again in case anyone is having the same problem. It's been half a year, but I finally found the problem!!! It turned out to be a small "L" shaped vacuum hose on the rear side of the passenger valve cover that was split, admitting unregistered air to the engine.
Since replacing it, the light has not returned, and coincidentally the pinging has stopped. Yahoo!

 

I have to give you a personal THANK YOU!!! I have a 99 F-150 4.2 with 110,000 miles they are my own miles so I know what the truck has went through. My check engine light came on the other day, I went to autozone they told me bank one was to lean. Then told me to change the O2 sensors, called the local Ford dealer he told me the same. Took your advice and checked that 90 degree line, nothing. Followed it to where it hooks up under the intake and waala the 90 degree rubber hose has a hole in it. I didn't make it to get O2 sesors so I guess you could say I've gotten 110,000 with a rubber hose, and regular mantainance. Again Thank you for your advise.

Mike Curry


99 F-150 4.2 (Soon to have a Thunderbird Suupercharger on it, I'm working on it now)
Cold air
Throttle Body Spacer
Not alot of mods YET

 

Way to go, Mike! Don't even think about replacing any oxy sensors until you have to, yours may last another 50K. I only wish I had mine back that I threw in the trash! (Along with the time and money I spent to replace the intake gasket). Live and learn I guess, and I'm glad we can profit from each other's experiences in this forum. Andy