I'm 99% sure it's something that's not set right as the car has been totally overhauled and most of the parts are new. I don't think there is actually a fault anywhere.
The CO reading disagrees
If it was OK before the overhaul, and isn't now, then something has changed - possibly a replacement part (like a sensor) is defective.
Is the fuel pressure OK at normal and at fast idle?
Is it a green or red top ECU? Are you using a lambda sensor if it's a red top ECU? If so, is the lambda sensor new?