What's the penalty for an expired 6 month tow-car permit?

Thanks for the various advice. Don't think I want to chance having the car seized. Might try to find a way to get an extension in Vallarta

But another question. If we drove the car across the border with the aim of renewing the permit for another 6 months (and renewing my visa as well), then re crossed back into Mexico in another 2-3 days, is there any regulation against crossing and re-crossing in a short period like that? Perhaps if we crossed north at Nogales, then came back in via Lukeville?