Hi guys, this is new to me so bear with me.
I have a 2002 4.0L 4x4 5 Speed.
I can’t for the life of me get the Check Fuel Cap Light off. I’m ready to pull my hair out. I have replaced the cap with a new oem one.
I have checked the fuel neck hoses. They are tight and no holes.
I have replaced the do hicky on the driver side wheel well and have followed the lines all the way back to the tank.
I have done the thing where you disconnect the thing on the intake manifold and my rpm’s drop to 500-600. Which I believe means I don’t have a vacuum leak?
I have removed the plastic shell under my bed in front of my spare tire and sent 12 volts to it. It opens and closes just fine. I have checked the Voltage at that source and I have power.
I have done a smoke test and can’t find anything.
I have disconnected my battery for 15 minutes to let the computer reset. But still nothing. I was told if the light stays on then eventually my check engine light would come on. But never has. This cap light has been on for 2 years and never gone away.
I took it to a shop and they said if there is no check engine light then it won’t throw a code.
Please help. I’m lost.

 

Welcome to the forum

Could just be that bulb's wire is shorted to ground some where

Below is the 2002 instrument cluster wiring diagram

Most warning bulbs get 12volts with key on and are activated by Grounding

"Check Gas Cap" Light is activated by the engine computer(PCM) via a Black/white wire on pin 10 of the cluster, wire runs to pin 82 on the PCM
So the PCM will activate that light IF................the EVAP system show a leak,there is no "cap sensor"

EVAP system uses engine vacuum to put a slight negative pressure in the fuel tank, if that pressure doesn't hold PCM will turn on "check cap" light as first thing, but WILL set a "small leak detected" code within a few drive cycles if leak persists
So no leak

Which only leaves "bulb is grounded full time", so stays on when key is on
Its possible the PCM has an internal fault
And you could start at that end of the black/white wire, by disconnecting the PCM's 104 wire connector, then turn on the key, if "check cap" light is off then PCM has an internal fault, at this time you could just CUT the black/white wire on pin 82, since it's not an operational light, i.e. a true WARNING LIGHT

If light is still on then black/white wire is shorted to ground somewhere from PCM to cluster OR cluster has an issue
Test pin 82 using OHM or continuity tester to Ground, should not be grounded of course with connector unplugged
If it is then move wiring around in engine bay and under the dash see if Ground goes away

 

So, I’m not exactly sure what you mean. If I disconnect the pcm harness the light goes off. 82 is black and white but so is the ground. With the pcm harness disconnected from the fire wall. I read continuity from 82 to ground from harness.

 

With PCM connector unplugged and key on the "Check cap" light goes OFF?

If so the PCM was turning it on
The black/white wire, pin 82 is connect to ground thru the other light bulbs on the same circuit in the cluster, so would show "continuity" to ground, if possible check it with OHM meter, shouldn't show 0 OHMS, direct connection to ground

But if black/white wire was shorted to ground then "check cap" light would have come on even with harness unplugged

So you can cut that wire at a place where it can be re-spliced, or just live with the "check cap" light

 

So, do I need to replace the pcm? Again thanks for all your help. I truly appreciate it! It’s been a thorn in my side forever. And could never figure it out.

 

Ford Dealer may be able to reload software, or use an advanced scanner to see why the EVAP system is causing PCM to ground pin 82
Not sure I would replace PCM just yet, as its just a nuisance not a operational issue

 

got same issue on my 2008, it just started happening few days ago. Since it's never been on, I blamed it on this: I gassed up the truck; it was a cold day so I left it running, which I NEVER do. So I changed my mind, and turned off the truck, but I had already taken the gas cap off. It seemed to come on sometime after that. Coincidence maybe?? :) thanks

 

If all is well with EVAP system the code should clear itself after a few days

No not a coincidence, while engine is running EVAP system is active, so removing the gas cap could cause 1 or 2 trouble codes

Check gas cap is equivalent of code:
P0442 Evaporative Emission Control System Leak Detected (small leak)

But removed gas cap could also cause code:
P0455 Evaporative Emission Control System Leak Detected (gross leak)

But at 0MPH and at idle the EVAP system is not very active, its main purpose is to pull out gasoline fumes when the fuel is sloshing around in the tank when driving, when vehicle is not moving very little fumes are generated