Old thread, but I figured I would help out as I can since I'm a license fire extinguisher technician.
You have nothing to worry about on heat with a dry chem extinguisher like most people use. If you had a CO2 extinguisher, it could pop the safety in high heat, but they are high pressure extinguishers, where dry chems like you find in vehicles and most homes/businesses are low pressure (195 psi typically)
I would like to add how scary it is to see so many people running extinguishers from places like Walmart. These are typically plastic head pieces of junk from Kidde or First Alert, but there are many other brands that suck equally. Most of these extinguishers are unable to be serviced, and have seen an innumerable amount of these leak down before ever being used.
Here is a link to a thread I started on another site that has much more detailed information about extinguishers. It is mostly in reference to extinguishers for the home and/or garage, but the majority of information applies to the same extinguishers used in vehicles.
Fire Extinguisher Info - The Garage Journal Board
If you have any more questions about extinguishers, please let me know and I'll be more than glad to help you to the best of my ability.