Are you sure its the engine malfunction warning light that the customer is seeing & not something daft like the handbrake / low brake fluid warning light?

It wouldn't be the first time a customer is misreporting a problem & sending a mechanic off on a tangent.

I too am pretty sure that those A/C codes are displayed when A/C is not present. I seem to remember that the old Snap-On Hermitage would even say "Ignore if A/C not fitted)