They can catch on fire from impact. Happens a lot as a result from accidents. Usually they generate a gas leak from the collision, and the heat from the engine, exhaust pipe, will ignite the fuel. As far as an explosion, I have never seen a vehicle "explode". Only in the movies that happens.
It is more possible to see this happen in a racing type vehicle. They use a higher octane fuel, or even race fuel, such as ethanol. They are highly flammable and possibly explosive. I still have never seen one "explode" though. Only catching on fire and spreading extremely fast.
The only time I have seen something explode as a result for an accident, were fuel trucks, tankers and so forth. The outside fire and leaking fuel gets so hot, it expands the remaining fuel in the container until it just can't take anymore, then kaboom!