It could be multiple things, firstly do you know the software is correct in relation to the spec?

Best thing is to get it set up really as then you know your AFR etc is correct and gives you piece of mind.

Without knowing the exact van it's fitted to it's hard to say, I should imagine the wiring has shorted to ground, suffice to say it's nothing to do with the engine, that merely has a sender fitted to the head which is the ground for the standard temperature gauge.

The CO is adjustable, but you need a "exhaust gas analyzer" for this, plus this most likely is NOT the problem!

Martin