Singer

Are there any other indicators that will let you know if your catalytic converter is clogged? I have a 2000 ES300 with 168k miles and it overheats on a hot day with the A/C on full blast on the freeway. Once I turn off the A/C, the gauge goes back down to normal range. I have taken the car in, but the mechanic cannot duplicate the problem. I've thought of flushing the coolant (since it needs it anyway) and possibly changing the thermostat, gasket, o-ring and radiator cap at the same time. No check engine lights. I was just fortunate that I looked at my temperature gauge as the needle started getting close to the red mark. Could have been dangerous for my engine. Also, it amazes me that car manufacturers do not connect any audible warnings when the temperature rises above the safe zone. I guess they want to sell more cars or more replacement engines??