This is for the newer 900 cars, I believe post-1990. It also applies to some 9000 cars. The part number of my ECU was 9529975. I'm not sure about the earlier cars, but failures are rarer in these anyhow. Some of this may be applicable to the earlier cars, but take this with a grain of salt if working on one of those ECUs.

Difficulty: 1.5/5

Parts Required:
-Maybe capacitors: 150pf or 120pf and 100uf/35v

Tools Required:
-Maybe the 10mm and extensions to remove the knee bolster or a Torx for the right side heating duct if you can't get at the central lock ECU by hand.
-Maybe two bladed screwdrivers to pry the end cap off.
-Soldering iron

Removal and disassembly:
The central locking ECU is easy to remove and take apart. To remove it, just unscrew the right side heating duct and pull it down from under the knee bolster and then unclip the ECU from its gold zinc chromate plated holder and unplug it from its harness. From there, you'll need to carefully pry the end cap off (end with the plug) and then you can pull out the board.

Resolder bad joints:
The central locking ECUs used lead-free solder. This is commonplace and works fine nowadays, but back in the early '90s they hadn't quite perfected it and solder joints cracked or "went cold" after some time. The common remedy for a non-functional central locking ECU is to re-solder the relay pins to the board. You may also have to solder the chip pins, resistor pins, and capacitor pins, but there are not so many pins anyhow, so it's easy enough to do by hand. Some say to bake the thing at 375F in an oven for 10 minutes. This may also work to re-flow the solder, but it could heat-damage your components.

But what do you do if this still does not solve your problem?

I have found a solution. My 900 had intermittently functional central locking even after the re-solder. It turned out there were bad capacitors inside. It contains two capacitors: a small ceramic capacitor that was only marked "n15" which according to the internet means it's a 150 picofarad unit, and a bigger electrolytic capacitor clearly marked 100uf 35v.

First capacitor (ceramic):
The first hint of what was amiss was that if I flexed the circuit board a little, it would make the relays twitch or stop the central locking from functioning. I did some poking around and found that when I wiggled the tiny ceramic capacitor (located near the resistors) the relays would twitch. I had already resoldered the pins, so I decided to replace the capacitor. Problem is, I only had a 122pf capacitor lying around. What the hell, I put it in. These are not polarity-sensitive so can be soldered in either direction. Bingo, now the thing worked. Some of the time. It wouldn't randomly twitch anymore with flexion of the board, but it would still fail to lock and unlock after sitting for a minute.

Second capacitor (electrolytic):
I decided it could be that the electrolytic capacitor (located near the chip) was self-discharging with time. I replaced this capacitor with a higher-rated unit (all I had) that was 100uF, 100v. Stripe (negative lead, shorter pin on a new unit) faces toward the end of the board, not the end with the plug. It seems to consistently work now.

Hopefully I don't have to replace the resistors or the chip. That would just be too much fun. If you don't see another post from me, it likely kept on working.