Stuck passenger door lock (1998 Mercury Grand Marquis LS)

Since the keyless entry system stopped working last year, I've had to use t he old-fashioned keyed locks, and number pad, to access my 1998 Mercury Gra
nd Marquis.
Driver door and trunk locks work flawlessly, but the passenger door lock is uncooperative. It seems as if it only works when the temperature is somew hat warm - let's say, above 50°F. Which it is here only sometimes in the afternoon, this being winter in my area.
It goes like this: I'll insert the key, attempt to turn it...but, it doesn' t. Maybe wiggles a little. That's it. I'm not going to force it, since I don't want to break the key off in the lock, and it probably still wouldn' t turn.
At least part of the locking mechanism does function. I can unlock and loc k the passenger door from either the driver's side number pad, or door-moun ted lock/unlock switches (both driver and passenger side). It's only direc tly turning the lock with the key which doesn't work.
I've tried rattling the key in the lock. No good.
I've tried my extra key. No difference.
I've squirted some graphite lube in the keyhole. That helped when the lock stopped wanting to turn a couple of years ago. This time, it didn't help.
I'm reluctant to put anything else in the keyhole. Someone suggested WD-40 , but that seems like a terrible idea, since it's not actually a lubricant. 3-in-1 or other oil could gunk up something sensitive.
Ours is a very dry climate, so I doubt anything is binding up due to rust a nyway.
Short of taking this to a mechanic, any ideas?
How much of a project would it be to swap out the lock myself? Am I going to run into headaches due to the vehicle's notorious first-generation PATS?
It's weird that the *passenger* door lock has seemingly worn out, since it' s the driver side lock that gets used most often.
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Jc Maxwell wrote:

Two possibilities,
Dirt/dust in the lock is mixed with the lube and causing it to gel and bind when it's colder.
Because the lock wasn't used for a long time it has a small amount of corrosion in between the cylinder and the plug (rotating part the key turns)
Either way - Go get a can of brake cleaner, make sure it has the straw. A can of compressed air - with a straw (or if you have a compressor a small tipped blow gun) and some graphite based lock lube. Now use a bit of tape and tin foil to mask around and under the lock (mainly to keep you from spraying the cleaner around and to protect the paint during the next step).
Hold open the key door, stick the straw in as far as it will go. Give it one short blast of cleaner. Now insert your key and try to turn it. It probably won't turn very well if at all. Repeat the cleaner and key until the lock frees up. The cleaner will spray through the tumblers and wash out the crud around the plug, turning the key acts like a scraper and will break loose the crud so the spray can work.
Now once you get it freed up. Hold the key door open and use the air to blow through the lock and dry it. Apply the lock lube into the plug and apply a quick shot of air to push the lube out around the plug. Apply some more lube and repeat until the lock works the way you want it to.
Don't use any type of liquid lube in the lock, it just gums them up and causes problems.
--
Steve W.

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Or, just use the locks manually on it; something most people forget is possible even on late models.
Personally, I miss crank up windows and pull-up-push down locks. The best thing since sliced bread!
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snipped-for-privacy@gmail.com wrote:

What he is saying is that the manual door lock doesn't work with the key either.
--
Steve W.

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On Thursday, February 4, 2016 at 9:42:42 AM UTC-7, Steve W. wrote:

Precisely, it is the good old-fashioned manual way that doesn't work. I wa nt to get in the car, I have to go to the driver's side & use the number pa d or door switch.
I really miss the keyless entry. It was a great help when loading or unloa ding the car. I could be carrying something burdensome & not have to put i t down to get the door open, unlock the door for someone else to get in, ma ke sure I had locked the doors after parking, etc.
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point of clarification: By "keyless entry" I mean the remote control fob, which no longer works. The keypad on the driver's door still works fine.
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On Thu, 4 Feb 2016 12:22:09 -0800 (PST), Jc Maxwell
You sure you don't just have a dead battery in the fob? Or maybe just a bad fob? It could be a cheap fix.
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On Thursday, February 4, 2016 at 2:57:56 PM UTC-7, Bill Vanek wrote:

Yes to the first - replaced the battery, to no effect.
As for the second, if I can find a fob tester at a parts store, I can find out if the fob is dead despite having a good battery.
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On Thu, 4 Feb 2016 14:48:12 -0800 (PST), Jc Maxwell

Did you program the fob after replacing the battery? It might be necessary, even though it's not a new fob. It is necessary with new ones.

I think all the national chains have those for the customers to use.
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On Thursday, February 4, 2016 at 3:57:29 PM UTC-7, Bill Vanek wrote:

Can't do it. My particular model year (1998) requires a trip to the dealer to pair new fobs or keys with the car. First-generation PATS wasn't that great.
I have previously replaced the battery in the fob. It didn't require any special procedure, it just worked.
To be clear: the fob stopped working before I tried replacing the battery, not after.

I'll have to pay Pep Boys et al a visit then.
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On Thu, 4 Feb 2016 17:49:18 -0800 (PST), Jc Maxwell

Try this: https://answers.yahoo.com/question/index?qid 100914174802AAFkfTX

I don't know how that system works, but in general, a battery that stays dead for an extended period can let a device lose its memory, and sometimes things just happen. I think that if either the fob or the module had its memory erased or corrupted, you would have to reprogram. If the method in that link works, it's a cheap fix. Even new fobs appear to be pretty cheap, and worth a try.
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On Thursday, February 4, 2016 at 7:30:08 PM UTC-7, Bill Vanek wrote:

Interesting. It's possible that you can pair a fob with the car without go ing to the dealer, if the method in that Yahoo answer works (and does not f ry my PATS module or some such). I definitely had to go to the dealer to g et a new key paired with the vehicle.
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Somehow the cell that was in the fob was apparently drained enough (measure d at 5.03 V) that the fob wouldn't work. When I put in a fresh Rayovac CR 2032, presto, the car again answered to the fob. Must have been a poor-qua lity cell the first one.
Thing is, I could swear I tried that a few months ago, to no result. I eve n wrote on the "new-old" cell with Sharpie the words "new cell" so I wouldn 't mix it up with the new-new Rayovac CR2032. Oh well, it works now.
Fob response is still not like new though. Sometimes, it takes multiple pr esses of the button for the vehicle to unlock. Could be the rubber in the fob buttons has become mushy with time and use. Lock and trunk release sti ll work very crisply. The underlying dome switches are still mechanically sound. In fact, the fob innards are in remarkably good shape, for their ag e and constant use.
The old fob casing, however, is in bad shape, with shackle and a couple of corners chipped off. It turns out you can get a new casing & rubber keys ( i.e., everything except the electronics & CR2032 cell) for cheap, so I orde red one.
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On Tue, 9 Feb 2016 09:09:04 -0800 (PST), Jc Maxwell

It's likely that the battery contacts in the fob have some corrosion, and that is why the remote started working after the battery change. Try cleaning them.
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On Tuesday, February 9, 2016 at 10:19:50 AM UTC-7, Bill Vanek wrote:

They didn't look dirty but I cleaned 'em anyway, using 91% rubbing alcohol.
Some of the switches that underlie the buttons did look a little dirty. Cleaned those too.
Fob works great now. What a relief!
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