A driver drove me a FAW-Volkswagen Jetta partner car 2004 with 270 thousand km, and IMMO light always flash.
Check and analyze: I detected IMMO control unit, and found that the prompt of ground short circuit by IMMO light control terminal.
After checked the circuit diagram, I learnt IMMO light L31 unit was controlled by NO.6 foot of anti-theft control unit J301. So, I disconnected the plug of anti-theft control unit and measured the electric resistance between NO.6 foot of the plug and ground, there had no phenomenon of short circuit. Checked the function of L31, it was also normal. It meant that the problem was located in the anti-theft control unit.
Troubleshooting: replaced the anti-theft control unit and matched the keys. Then, the problem can be solved.