Check engine light

ON MY 2003 AVALON, WITH 108,000 MILES, WHEN THE OUTSIDE TEMP REACHES 75 DEGREES AND ABOVE, THE CHECK ENGINE LIGHT COMES ON AND STAYS ON. THE HEAT INDICATOR IS NORMAL. THE DEALER CHECKED WITH THE COMPUTER AND IT DID NOT SHOW A PROBLEM. HE CHECKED LATER WHEN THE LIGHT WAS ON AND CONCLUDED THAT THE PASSENGER AIRBAG NEEDED TO BE REPLACED. IF THAT WERE THE CASE WOULDN’T THE LIGHT STAY ON ALL THE TIME? WHAT DO YOU THINK? ABOUT 4 YEARS AGO THE LIGHT CAME ON AND A TOYOTA SERVICE DEPT. REPLACED THE THERMOSTAT (ABOUT $300). I NOW WONDER IF I EVEN NEEDED ONE. HELP!

(SAME AS ABOVE) ON MY 2003 AVALON (108,000 MI.) THE CHECK ENGINE LIGHT COMES ON WHEN THE OUTSIDE TEMP IS 75 OR MORE. DEALER CHECKED COMPUTER AND FOUND NO PROBLEM. LATER HE CHECKED IT AGAIN AND SAID THE PASSENGER AIRBAG WAS OUT OF ORDER. IF THAT WERE THE CASE, WOULDN’T THE LIGHT STAY ON ALL THE TIME? THE LAST TIME THE LIGHT CAME ON (about 4 years ago) I WAS TOLD THE THERMOSTAT NEEDED REPLACING - COST ME OVER $300. I NEED YOUR ADVICE.