Posting this in troubleshooting instead of where I first did.
I went to leave work today, and the remote would not unlock the car. I tried several times. I manually unlocked the car and the lights flashed. Car would not start, it did crank and fire once, but only once. I tried new batteries in the keyfob, and it appears to be ok, the red light will flash when buttons are held. The batteries that were in it tested OK as well. I am getting a ride home to get tools for tomorrow. I have actually been reading a lot about the alarm module, and still only have 1 key for the car. I have had it 2 weeks and was trying to get one from the original owner.
ANyway, I have read all the posts I could find, and learned a lot, but not enough. If everyone could give me pointers to what to disconnect/test. I tried fuse for central locking (it did stop the lights flashing when pulled) but did not disconnect battery (no tools).
The alarm light on the switch panel is always flashing. I am heading home to learn more and get tools, maybe try tonight, probably tomorrow.
So to sum up, the alarm light flashes, and the car will not crank at all, when you open the doors, the lights on the car flash for about a minute.
What can I do, what should I check, and what tools should I bring?
The car battery seems fine, and it is not original, but not new.

Thanks in advance!!!

 

OK, I showed up early today with a bag of screwdrivers, torx, wrenches, sockets, pliers, soldering iron (for RCM fix if needed)...
Car still had same symptom, I disconnected battery cable, waited 30 seconds or so, reconnected it, and the lights started flashing again. I hit the unlock button on keyfob and they stopped.
I started car, no issues, BAS/ESM whatever light was on, I rolled the steering back and forth and all was well.
So a 10mm wrench will be permanently in the cubby under the trunk as well as whatever else ends up in there.
I wish I knew what fuse(s) would have accomplished the same thing.

 

i'm sorry no one has answered you
I don't know the answer though, just wanted you to know
someone was feeling it for you. LOL

 

Thanks for the empathy, but it all worked out good. When I went to work with a bag of misc tools (I do IT work) and quoted Spicoli
"Relax, all right? My old man is a television repairman, he's got this ultimate set of tools. I can fix it."
everyone had no clue.
I also ordered a 2nd key form NeedsWings today, so hopefully I will have that soon.

 

Nice color on that XF, where in SC are you? I'm just west of Sumter.

 

Sounds like a computer glitch that you reset by removing power. You have not fixed anything (and I can tell that you already know that).

My bet is the car's battery or terminals on the battery cables - how much crud is there between the posts and terminals? How old is the battery? Is the ground cable all corroded at the bolt where it attaches to the chassis?

One thing that BUGS the HELL outta me about our cars is that there is no heavy ground cable to the engine block - the heavy ground goes to the chassis and we rely on the jumper from block to frame instead. My last car was doing a few weird things almost five years ago. I replaced the battery ground cable with one that was about 36 inches long and had the lighter pigtail for the chassis (like other cars). I installed it, with a BIG lug on the small wire and attached it to the chassis near the battery. I took the HEAVY lead and ran it to a bolt on the engine - I can't remember where I ran it, but I made sure the HEAVY cable went to the engine block.

THe car ran for over 60,000 miles like that with no more oddball issues.

I'd change the damn battery and take a GOOD LOOK at the battery cables.

 

I improved the ground from battery to chassis by making it a 1/0 gauge wire. Helped eliminating annoying quirks that the car was having

 

Underwood looks like a 6 month old car, it now has 6300 miles. Battery is 2011 I think. Zero corrosion or crud. I plan on removing and inspecting RCM tomorrow. I have ordered a 2nd key/fob from NeedsWings. Personally, I would love to disable/remove the antitheft, and install my own type of setup, that can be disabled since I installed it. I was shocked it worked for 2 weeks then flaked out.
One think of note, it was in the 3rd level underground parking garage, and a comparable era SLK was parked 20 ft away. I wonder if the other person's fob sent some screwy code to my car. I KNOW this is possible, my co-worker had a Saturn Vue, and when he locked his car, it would set off the panic alarm on a certain Subaru, EVERY TIME, even if his car was not in the garage, we walked through one day with just his key fob and set it off. Just dumb luck that it used the same frequency and code for panic that his did for lock.
Anyway, if/when it happens again, I will try fuses 1 at a time, to maybe find an easier way than killing the whole car at once. PLUS I will keep a 10mm wrench up front, I was going to put tools under the trunk floor, but I don't think I could get to them, no way to open the hatch when in theft mode, correct?
Thanks for all the suggestions, and hopefully it will hold out for Woodward.

 

Fuse #9 will disarm the alarm... I had a bad circuit board in mine. I decided to forego the alarm altogether rather than replace it. Don't know if that is your problem or not. Could be the start of a skreem module issue. Good luck.

 

Yep, tried that and the central locking one. The issue wasn't just the alarm going off, it was the car wouldn't let me do anything about it, either remote, or via ignition.

 

skreem module behind the gauge cluster could be the problem then.

 

Was this ever diagnosed? I experienced the same problem yesterday for the first time. Got it going by disconnecting the battery and waiting before reconnecting like the original poster did.