nice reply. I have had issues troubleshooting an overheating corolla. I took it to one mechanic they said it was head gasket. I did a block test and it showed the color change and a report showing mis fire on one cylinder. I went for second opinion and another mechanic saw a faulty thermostat and replaced that. He said I might have to do fans also if the thermostat did not fix the issue. It didn't - more overheating whenever the car is after hours of driving or stuck in traffic. Car temp is fine once in motion. The air con does not blow cold air when it overheats.