They will pull the part number from the ECM (so have that ready if your ECM is buried in the dash), check what vehicle it came from and then proceed to go over the vehicle with a fine tooth comb to make sure that your vehicle has EVERY piece of emissions in place and functional that came with the vehicle the ECM is from.
You can help yourself by making sure that there are no trouble codes stored, and that all the monitors have run. Any of these that are off, will be an automatic fail.
They will not check brake lights and things like that. Just emission related items.
what exactly are you swapping from and what is your vehicle?
