After two days of rain, car started in the morning and ran fine for a short trip. Later in the day it wouldn't start. It cranked briskly but wouldn't catch. Gas tank full. After ignition switch is turned on for a few seconds, the colored lights on the rear view mirror start flashing. Remote seems to work, locks/unlocks fine. I disconnected the negative terminal of the battery and reconnected, then reset the alarm. No change. Googling around indicates it may be the crankshaft position sensor, but would this sensor go bad abruptly with no warning? Car has 85k miles, but considerable rust. Maintenance is up to date. Any advice would be appreciated.

I searched on "lights mirror flashing" and found this thread (and many others):

W202, C220 Start Problem

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Thanks for the tip on the alternately blinking colored lights on the rear view mirror. I did the following, which worked in my car. I don't know if it will work for any other alarm system.

1. There is a little toggle switch hidden in the left well of the driver's seat. The forward position is for resynch in my car.

Be careful with this switch, because a combination of the switch movement with the driver door open or closed or some other things can change the alarm setting to "valet" and do other unwanted stuff. I don't have a manual for my alarm system, and it took me a hours of googling around and trial and error to figure out how to fix this last time I messed with an alarm and got it upset.

2. Removed negative terminal of battery (I actually already had a knife switch installed on that terminal, so the battery doesn't discharge if I leave the car sit a long time).

3. Wait 10 seconds.

4. Replace negative terminal. Alarm will sound.

5. Within 5 seconds, press lock button (the II botton on a two-button remote).

6. This should resync the remote. In this case, I used a different remote with a better battery for the resync.

7. Then I locked and unlocked the doors with the remote.

8. Then I locked and unlocked the doors with the regular key.

9. When I turned the key in the ignition (driver door closed), the rear view mirror lights didn't flash, as they had before. So I turned the key to start, and it fired right up.

10. I put the remote into the glove box. From now on I intend to use the regular key to lock and unlock, and forget the remote and the alarm. I have a steering wheel lock bar that I will use if I take the car anywhere it might get stolen.

11. I called the local Mercedes dealer, who says that they can't remove the alarm, but they can disarm it is such a way that the remote will still control the door locks, but that everything affecting the engine starting will be disarmed permanently. They estimate about $200 for the job. I figure it will be worth it not to worry about the alarm acting up and stranding me someplace if my resynch trick doesn't work.

In the above I modified several variables at once, including the remote and using both the remote and the key to lock/unlock the doors, so I don't know which of all the things I did actually cured the problem. But from now on, no alarm for me, unless I replace the current one with a new one where I can actually have a manual and technical support.

Unfortunately, my problem is not solved. I started the car today, no problem, and left it idle about 15 minutes. Then, just as I was getting ready to take it to the Mercedes dealer, the engine stalled out from idle. Now the colored lights on the mirror blink alternately when I turn on the ignition, and the car turns over but will not catch, same symptom as before.

Have tried two different keys and two different remotes, both with key lock/unlock and remote lock/unlock. Have tried resynch with battery negative terminal off/wait/reconnect. No luck.

I will let it sit for a while. Maybe I can get it started again and running long enough to get it to the dealer. Otherwise I will have to get it towed.

Car was towed to dealer. Dealer did the following (from the invoice -- paraphrased).

Performed short test. Found no codes. Removed battery to get to locking control module. Test for power and ground and tried test module. Made no difference. Removed side panel in trunk to see if wet. No signs of water. Bypassed aftermarket alarm system and didn't make any difference. Replaced K 40 relay and car started. Performed short test and cleared codes from unplugging control modules and test drove auto.

Total cost was just over $500.

The K 40 relay cost just over $100.

Service manager said the K 40 relay was shorted, and that they had also cleaned some contacts in the wiring harness.