Did you ever get involved with diagnostics on a car that you wish you had never taken on. Just kidding, I know we have all been there at one time or another.

Case in point, I was called in to do the diagnostics on a 2019 Mazda6 with a no start condition. The car had been involved in a minor accident which had damaged the right front corner. After the body repair the vehicle ran for a short period of time and then quit. The car had been sent to the dealer for diagnosis, the dealer diagnosed that the car needed a new PCM and engine wiring harness. At this point I was asked to come in and take another look at the car.

My initial observation found that there was a security message and a check engine light on in the dash. I hooked up a scan tool and found that I had no communication with the PCM. This explained the no start issue. Obviously the first order of business would be to establish communication with the PCM. I tested the PCM powers and grounds as well as the CAN BUS between the PCM and the DLC. No faults were found and a new PCM was called for. So far so good. After the new PCM arrived I attempted programming.

Multiple issues

The programming failed due to the programming software not being able to locate the correct file—the message I received on the scan tool when I attempted programming. I called Mazda Support and reported the error message only to find out that Mazda had multiple programming issues with this platform. The PCM was shipped to Mazda Technical Support in California where it was programmed off the car and shipped back. At this point it should have been plug and play. Unfortunately, when I attempted to marry the PCM to the immobilizer I received the same error message (cannot locate file). At this point I went back into the PCM with the scan tool and was able to read the VIN but not data. I was running out of ideas for this vehicle. It appeared after all the messing around with programming and Mazda Support, I still had a communication problem with the PCM.

Back to the drawing board

Between the initial diagnosis and all the steps in between I had been working on this Mazda6 for about a month. To say I was frustrated at this point was an understatement. All the usual tests revealed no faults and yet a new PCM did not solve the problem. I decided to go back to the drawing board and start over. I quickly checked the CAN BUS and found it to be OK. I next decided to go back and re-check the powers and grounds. Usually, I try to use the OE diagrams for this. The information system I was using only had one OE diagram for power and ground and it was only a partial diagram. I had to use the aftermarket colour diagrams. I’d run into this the first time I checked it and had the same problem. I went through the wiring diagram to locate the correct circuits, and they all checked out.

ETC Power

Right back to square one. I decided to check the wiring diagram again and to check anything connected to any type of power circuit. I eventually found a circuit labelled ETC Power (Electronic Throttle Control). I overlooked this circuit on my initial test due to labelling. When I traced the circuit back however and looked at the connector pinout it was actually a power feed from the Main Relay. When I checked the circuit, it measured 9 volts. I ran a jumper wire to see if powering the circuit properly would fix the communication problem. With a jumper wire in place the PCM communicated, I was able to do a theft re-learn, and the Mazda started and ran normally. Fixing the wiring to the ETC Power circuit fixed the car.

It took six weeks working on this car only to find that my interpretation of a PCM power circuit labelled as ETC power was the real issue with the vehicle. You can never be too careful.