Hello Stig.
Sorry for the late reply. I only just saw this. Must check my email notification settings.
Yes, I did get the board to work. I replaced two components; the blown zener, and I installed a resistor to simulate the weight of my wife’s petite bum on the seat. This resistor sits in place of a resistance from the seat pad. I had my wife sit on the pad, measured the resistance it generated, installed a resistor of that value and checked the frequency the board was sending to the car’s computer. The car is now hard wired for a light adult passenger. The airbag fault light in the dash is no more.
I’m now waiting for one of my kids, or my wife, to crash the car. I expect to see the passenger airbag, even if they’re driving alone.
Your circuit reading skills are impressive. It’s been ages but I would enjoy further thoughts you might like to share.
Regards, Rick.