I've had my '96 F150 ( 4.9L/5 speed with approx. 180,000 miles ) for about 18 months and have had to replace both O2 sensors twice in that time . I recently replaced the front one ( on the exhaust manifold ) as a CEL was showing and the code indicated a problem with it . That was two weeks ago and the light has not reset itself yet . The auto parts guy said it had to go through an undetermined amount of heat cycles before it would reset and that legally he wasn't supposed to turn the light off with his scanner ? I am retired so I don't drive long distances in the truck normally but according to him , it's the number of heat cycles , not mileage that cancels the code ? To compound my problem , in the area where I live you must pass an emissions test before you can renew your license plate decal ! My registration ran out at the end of May and it's now July ! I have cleaned the MAF sensor with MAF sensor cleaner as suggested by the printout given to me at the auto parts store to no avail ! Any suggestions or real life experience anyone can pass on to resolve this matter ? The truck runs good and has a fairly new catalytic converter ( less than a year old ) installed . The engine seems to have a slight miss " sometimes " at idle but really runs very well . I installed new plugs , plug wires , distributor cap , ignition rotor , PCV valve , fuel filter , and a serpentine belt shortly after buying the truck . can anyone enlighten me on the direction I need to go ?

 

You are being mis-informed. The codes that trigger the CE light do not automatically go away they have to me manually cleared, and if the problem that is setting the codes isn't fixed correctly the codes and the light come back. Codes can be cleared with a code reader/scanner or you can simply disconnect the battery for at least 15 minutes. I suggest you do this and then wait to see if the CE light comes back on, and if it does post the codes(not the code description) you get here so we can help you diagnose the problem.

 

The " check engine " light continues to burn ! Don't remember the code # but it indicated a " lean " condition for bank #2 , sensor # 1 . That is the sensor located on the engine exhaust manifold . What would cause a lean condition ? I have cleaned the MAF sensor as suggested by the auto parts guy but no luck ! In my area , we have to pass emissions testing to renew your auto registration and mine ran out at the end of May ! Been driving cautiously trying to avoid being stopped and need to find out what is causing this CEL to continue to burn . The truck runs great and seems like there are no problems as the CEL suggests ? Any suggestions !

 

As Paul said before did you have someone clear the codes?

A vacuum leak can cause a lean condition. An exhaust leak can trigger a lean code as well. Perhaps one of the exhaust manifolds or exhaust pipes has a slight leak?

 

See.. that is exactly what I was talking about. Those codes do not necessarally indicate a sensor problem, they tell you the computer has detected a lean condition which could be caused by any source of fresh air in the exhaust upstream of the O2 sensors.. or in other words a cracked exhaust manifold, leaking gasket, or broken or rusted smog plumbing.

 

I have checked all vacuum lines , can't find any that appear to be bad ? There is a slight hissing sound at idle but wonder if I'm just hearing the air being pulled through the filter and on through the MAF sensor ? I have cleaned the MAF sensor with the proper cleaner . All the air inlet structure seems to be sound with no obvious holes or leaks . The truck has a fairly new converter ( within the last year ) and there's no obvious exhaust leaks ? The converter was welded in place and I wonder if there could be a slight unnoticeable leak ? But the CEL code indicates a " lean " condition at the front manifold mounted O2 sensor so wouldn't that eliminate the exhaust pipe leaking as a source for the code ? I've been told a vacuum leak , manifold or gasket leak , or a dirty MAF sensor as the likely cause of the code . I've tried everything possible with no luck . My truck registration ran out the end of May and it must pass emission testing before I can renew it . Money is too tight to make a professional intervention possible at this time ! Any one have more suggestions ? Meanwhile , the truck runs great with no obvious problems ( other than the CEL ) All and any help will be appreciated !

 

I question the hissing noise you mentioned. I took some "heat" on another forum recently where the issue turned out to be an improperly routed PCV intake hose. Because the hose was open to the atmosphere it was allowing unmetered air into the intake causing a lean condition. Where the debate came into play was whether or not this "vacuum leak" would cause a lean code on one O2 sensor or both. After the OP corrected the hose routing the lean code on bank 1, sensor 1 went away. My theory was it could be detetced by only the one, fortunately my hypothesis held true.

Keep in mind a vacuum leak can be caused by a vacuum reservoir as well as the vacuum lines. You stated you checked the lines, but how was this performed?

 

The vacuum lines were visually inspected and all appear to be good .

 

Another suggestion as well is to remove the fan blade, and see if you can pin-point the hissing sound. Ive found this has helped many times. If you cannot remove the fan blade, you could perhaps remove the serp belt and then let the motor run perhaps a few mins to see if you could track down the hissing. This isn't a prefered method since the water pump, alt are not being driven. Also, you can try using a piece of hose to help identify where the hissing is coming from.

 

In my experience a visual inspection will only reveal if a line is completely cracked in two or not connected. Beyond that you are missing most of the story. A more in-depth inspection of the vacuum circuits are in order, IMHO.

I use a small handheld vacuum pump/gauge to check each vacuum circuit. I am not an advocate of spraying anything on the lines to see if the engine RPM increases. Others swear by that method, I swear at it.