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Hi, everyone, I am new to the board, and I have tried to research these topics, but I am sort of at my wits end. My car is a 1999 Boxster with manual transmission with 131,000 miles on it. All of a sudden, the windows and door locks started to act very wonky. If I try to lock the car with the fob, you can hear the mechanical whizzing of the locks and then a double beep from the alarm, and the doors do not lock The lock button on the dash does not work. The driver's side door will lock with the key, but not the passenger's side. the windows are very peculiar. If they are both down, and I put the passenger side window up all the way, the driver's side window will go up without touching the button.

My mechanic pulled codes, and this is what he got:

03 -- Airbag warning supply
29 -- ignition circuit-side airbag, driver
49 -- belt buckle, passenger
46 -- belt buckle, driver
31 -- ignition circuit-side airbag, driver
21 -- ignition circuit, driver
45 -- belt buckle, driver

Alarm System:
25 -- terminal 86 is not detected
36 -- passenger compartment monitoring sensor
42 -- wrong key or wrong transponder pill
11 -- voltage failure during alarm output
10 -- voltage failure at terminal 30 with alarm system active
46 -- control locking synchronization
34 -- passenger compartment monitoring sensor
47 -- control locking synchronization
58 -- tank servo motor not energized despite relay activation
61 -- central locking limit position unlock not reached

He suggested that I should replace the M535 ECU. I have checked all of the related fuses, and I have checked the fuse on the M535. I opened the M535 box, and cleaned everything the best that I could. There were definitely signs that there had been water in the box.

The car runs and drives, the convertible top works fine, I just can't lock the doors and set the alarm. Does this sound like it is an issue with the M535 box or more of the door locking mechanisms? Or, is it due to the electrical part of the ignition switch? Any suggestions, or ways to troubleshoot would be appreciated.


1999 Boxster, Arctic Silver over red, 5sp manual
Moisture under the seat has damaged the alarm computer
Electrical portion of switch is flaky

The latter is much cheaper (~$10 for the part and 30 minutes labor, reasonably easy DIY) but in your case it sounds like it probably is the alarm.

The old alarm can often be salvaged. Take it apart, soak the circuit board in pure isopropyl alcohol, thoroughly dry it, cross fingers. If that doesn't work there are companies that can recondition them. You should never have to pay full Porsche retail on a replacement.
Stefan --

Thanks for your reply. I did try cleaning the circuit board with isopropyl and a cotton swab. When you say soak the circuit board, do you mean remove it from the M535 box, or just take the bottom of the box with the board in it, and pour that alcohol into it?

I am talking about removing the circuit board from the M535 box and submerging it in pure isopropyl alcohol. The problem is that when it gets wet it's because it's submerged in water and the water seeps into every nook and cranny of the board, including under the components. When you soak it in alcohol, the alcohol seeps into the same areas and mixes with the water. When you then dry it, the alcohol evaporates taking the water with it.

Note also that impure alcohol (e.g. 70%) is just alcohol already mixed with water. It's best to use the highest percentage you can find.
Got it. I will give it a try. Appreciate the clarification.
Update --

I removed the ECU from under the seat, removed the board, soaked it in 99% medical grade alcohol for several hours. I then drained it and dried it thoroughly. I also changed the ignition switch.

Everything works, including the window drop, windows, interior lights. Car starts and runs without a problem. The only thing that still doesn't work is the locking/alarm system. I get what sounds like a buzzing, grinding sound coming from the control module under the seat, and then the double beep from the horn. I still have the following codes:


I called the local dealer, and they told me that it is usually an all or nothing response from the ECU, and that it is very strange that everything works except for the door locks.

Before I spend the money for a new ECU and having it programmed, is there anything else that I can check or replace? Thanks for your help.

I cleared the codes, and then tried locking the car with the key fob. I still get the grinding/buzzing sound from the immobilizer, the two horn beeps, and the doors do not lock. the only code that comes up is:


Is there anything else that I should try?

C3, B8, E8
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