Originally Posted by
NomakeWan
It's not wiring, no. The 1995 definition specifically says, and I quote:
"During start-up, the CCM requests and receives information from the PCM or ECM as long as a DTC 41 is not present. This information is compared to configuration information store(d) in the CCM. This determines if the CCM has been programmed for the current vehicle's engine configuration."
"DTC 81 will set when start-up has occurred, a current DTC 41 is not present, and the CCM's stored engine configuration does not match the configuration information received from the PCM or ECM."
The above is in the 1995 service manual. I will note that my Arduino sketch does not trigger DTC 81, so the answer must lie within the $41 reply. If it were anything else, that DTC would set because my sketch is only designed to reply to $40 polls and nothing else. So if the answer were in some other handshake, then I would have experienced DTC 81 while testing the sketch.