If I am welding a car, the extinguisher is next to me with the pin already out! In the seventies I had a car on its side on a carrolla, and was oxy/acet welding a sill (rocker panel in US? the bit under the doors!) A large blob of hot metal slag fell down the underside of the car, neatly severing the fuel line ( which I thought was metal) Fuel is now spewing out onto a blazing and widening pool on the floor, and my companion was halfway down the yard screaming at me to get out! luckily I had followed my rule, and the CO2 ex was ready beside me. A quick blast and it was out, stuck my thumb over the fuel line and survived to weld another day! If I had been in a pit, I could have died, but of course, the slag would not have damaged the fuel line either! My rule today is that the tank comes off, or I don't weld.
Phil
UK