GT40 diagnostic codes

Seems like your mechanic should have a manual for the code reader with a code list.

I bought a Innova Ford Code Reader so it may have no relevance to what your guy used.
Plus I have used it once so I'm not an expert.

My manual does not have a code 40 listed in the Fault Code Definitions list. No 10,20,30,50,60,70,80 or 90.

It does describe a cylinder balance test procedure. The computer compares the power contribution that each cylinder makes to engine operation. If a cylinder isn't contributing at the same level as the others then the computer sends a 2 digit code to the Code Reader.

10 Cylinder 1 has a problem
20 Cylinder 2 has a problem
40 Cylinder 4 has a problem
90 System is OK
Left off the other 30, 50 etc for brevity.

Seems like your mechanic would have to know how to get into the proper test sequence to perform this test. There are several pages to it that I didn't bother to read. I see one of the things is to tap on the accelerator pedal 3 times.

Not sure if this is any help or not.