This is a very simple mode. There are no PIDs. A mode 4 message tells the ECU to clear all DTCs.
Under the hood, the way it works is as follows. There are a slew of subroutines that periodically monitor the car's health and set DTCs as necessary. If you send a mode 4 message (with no PID - if you send a PID, it won't work), the ECU sets a bit (108E7, bit 2) to let these subroutines know that they should clear their associated DTCs. At the same time this bit is set, a countdown timer at 1094E is set to 30. When it counts down to zero, the "clear DTCs" bit is cleared, and the system returns to normal operation. I haven't yet worked out how long the ECU takes to countdown from 30 to 0.
Last time I said that I would post some details on the immobilizer system. I have been working on it, a lot, and have learned a lot, too, but I don't feel like I have enough of the big picture to share yet. So, maybe next time. Otherwise, the next post will be on OBD Mode 5.
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