not sure why this is + it's not like starting the truck then you see the pale Door Ajar lite on that doesn't go off until you shut whichever door it is that's not closed properly.
instead it's while I'm driving suddenly w/o warning a red lite w Door Ajar comes on and a loud tone sound for about 3 seconds. I've opened and slammed both driver and passenger doors, sometimes this works, sometimes it doesn't.
The best I can figure is this started about the same time a few wks ago when we had some really cold weather as in -10 to -20 but that's about all I can figure out unless someone tried to break in unsuccessfuly and messed something up - but that sort of crime is rare here for anyone, much less an old beaten up truck like mine!
is this a fuse issue maybe?
any clues? immensely frustrating!

 

What YEAR Ranger???

1995 and up had an issue with the door open sensor, its on the under side of the latch inside each door, 2 sensors in most Rangers but 4 in the 4-door models, 1998 and up

The problem was the location of the sensor, any water that gets in would run down inside the latch and carry with it any dirt, this would end up in the sensor and over time it would cause sensor to short out
The sensor is a plunger type, it sends a GROUND to GEM Module when door is open, no Ground when door is closed
So a short causes GEM to activate Door OPEN light and if keys are in the ignition a "dinger" also activates, the "key in ignition" reminder so you don't lock keys in the vehicle

People often try spaying WD40 into the latch so it runs down and hopefully cleans out the dirt and short

You can pull the door panel on drivers side and reach in and unplug the 2 wire connector on the door sensor, this would remove any chance for a false door open ground to the GEM on that door
If this works then problem is in that switch/sensor
If problem still occurs try same on passenger door
Drivers door is used more so best to start there

Picture of latch with door switch at the bottom: https://i.ytimg.com/vi/HyTFsi809Hs/maxresdefault.jpg

Its held in place by "twist lock" mount so to change it you need to take the screws out of the latch to pull it back enough to twist off the old one and then install the new one

 

it's a 2001 - I've had it since 2009.

this makes total sense - we had a 1 day warm spell it got up to 39 & the car wash was packed - myself included. Next few days we were back down to zero temps, I went to open the passenger door & it was frozen solid. Later I finally got it open & there were literally chunks of ice from where water got in from the car wash, no doubt.
Thanks so much for your suggestions I will check out the video & look for those sensors. I can't help but think it's on the passenger side because that's where the ice was also that would also account for the about the time it started! Definitely was not before ! also seems to be I have to keep slamming both shut but that's only because I can't figure out which door it is but it's been my suspicion it's the passenger side!

again thanks for this! wow. nothing worse than being bugged & clueless as to what's causing something like this!

 

Go with your Gut..............its your best guess that counts, BUT...................

Try this test first
Put key in ignition and open drivers door, you should get the "dinger" for "key in ignition"
Then close drivers door and open passenger door, I don't think you will get the "dinger" sound, that was usually just when drivers door was open, but it may be for either door opening, can say on 2001 specifically

 

no, this was absolutely not the same as when one is first entering the vehicle - in fact - I've never heard this before. It's a tone sound - also - if I have a door ajar when I start, it's just an amber light, not a red one.

This would happen without warning just while driving down the highway! very bizarre!

but I just went out there and sprayed thoroughly w WD40 on both sides so hopefully that will dislodge/dry up whatever it was. I always use that early on in the season as to ward off against the key sticking when cold weather sets in, so maybe the fact there should have been a coating already there will help and is why the problem is only intermittent, not continuous.
Thanks again for your help!! I was thinking it was a bad fuse or worse - behind the dash! That it's in the door makes perfect sense!

 

In 2001 I only see one Door Ajar light, top of cluster between tach and speedo

So not sure what the other light might be

You can get PDF version of 2001 owners manual here: https://www.ranger-forums.com/genera...1-models-3747/

Makes it easier to look things up

 

thanks but I have a manual - it's a bit worn but still good.

sorry if I confused you - the light is in the same place and all but if a door is ajar when you start the engine it only glows amber/orange and no tone.

this is while driving on the highway the same light comes on bright red + the tone. have no clue where the tone is coming from - maybe the same as when you open the doors and the lights are still on?

anyway I think I'm good for now. Thanks again!

 

Assuming you have a 4 door ranger and that you live in NY, your truck has seen a fair amount of salt in its lifetime. Your bottom latch of the rear door is going bad. I had the same symptoms you had where the door ajar light would go when driving down the road. The bumps and the cab flex was just enough to "open" the door. The only fix is to replace those latches. The passenger side is the first to go as the door itself rusts out quicker than the driver side for some reason.

 

2 door + most of those yrs were in NYC w only minimal driving (less than 5k/yr) so salt damage has been minimal as well.

thanks for the heads up tho - she's pushing 177k now & we're up in the North Country where salt is definitely an issue.