why does my 1999 pontiac grand prix loss all power when the tempature light comes on

Check your battery connections. A loose or dirty connection can also cause this type of issue.

Found this thread on another site. Might be same issue as yours ---My 1999 Grand Prix has been acting up with all the same symptoms many people described. Engine cut out at highway or low speed; hard to start after a hot shut down; the Trac light stayed on after the engine cut out and immediate restart in neutral etc. It was getting worse lately with the hot weather. I contemplated replacing the crank position sensor, mass air flow sensor, PCM one at a time, and hundreds of $ at a time, like many people before me. Then, I found the way to fix my problem at ZERO DOLLAR! I figured that it could be a voltage problem. I used a multi-meter. I measured the voltage across my battery, which show the normal 12.7 V. But from the positive terminal junction to a good ground on the engine (e.g. Al engine mount) the voltage has dropped down. I also measure the conductivity as well, and I did not get zero Ohm between the negative terminal to the good ground spot on the engine. I removed the grounding cable attached to the body panel and found that it was bolted on to a part with original body paint! What a bad design! Paint is not the best conductor in the world! After years of dirt and rust build-up, the conduction between the bolt threads won't be as good as before. So you get intermittent voltage drops that stalled the engine. I gave the contact a good rubbing with sand paper. I check voltage and conductivity after and it was good as you would expect. Since than, I never had a single staling. It has been three weeks with lots of kilometers in hot weather. I trust that the same remedy will solve a lot of your problems. I apologize to many garages for reducing tens of thousands of dollars of their potential business.