Then perhaps as Johny99 said the lambda snesor could be the one.

It's relatively easy to check - I think anything under 0.5V gives it the "Lean" signal (ie it's running lean) and over 0.5V the "Rich" signal.

I await to be corrected on this !! But i'm pretty sure that the above are the figures to use. Find the pins on the ecu which are the signal from the Lambda - get your mate with a multimeter in the footwell and go for a short drive - you should see if this is giving the correct signal. There maybe a standard resistance "At rest" if you like which could give you a good idea as to the condition of it.