Most likely, you don't have the power supply wired properly. Check your connections.
Today i got around to installing my mac mini car computer with P2140 PSU/controller
When i tested it with a powersupply a few weeks ago everything was working as it should
now when i turn on the ignition, the p2140 powers up and starts flashing but it never triggers the mini, i can manually short the pins on the switch cable and it will start no problems
any ideas why it would not be automatically starting?
The 2140 can sometimes get 'scrambled' -especially during testing or if your battery goes dead. Hook it up to the USB and reflash it. That may do the trick.