This post is just to pass on my experience to others really.
I've been trying to track down an error detected by the engine management system: the red warning triangle appeared (the one in the rev counter), but the orange engine icon light didn't light as well. Once I'd put a bulb into the relevent slot for the orange light, the bulb still didn't light. Turns out that the engine light (at least on my car) doesn't light up at all when the car's immobilised. Since I was fiddling around with the dash whilst doing this, the immobiliser had well and truly timed out, leaving me scratching my head wondering why the orange light wasn't lit (I was considering dodgy solder joints: I've done my homework!), when all the time all I needed to do was turn the immobiliser off. Hey presto, orange light appears on ignition sequence, allowing me to pull off the error code from it.
Hope this helps someone.