'01 Toyota Corolla LE

This car had only 70K miles. Body is OK with minor dings and scratches. There was a check engine light that was illuminated in the instrument cluster.

I checked it out with my scan tool. The codes were P0300 and P0303. P0300 is a random misfire and P0300 is a misfire at cylinder 3.
I was told before that the car overheats. So I took my radiator pressure tester and tested the radiator.

First, I looked at the radiator cap to see in the rubber seal was good, it wasn't. If you see any rubber that had come off, it is already bad. I took note that it needs to be replaced.
Then I installed my radiator pressure tester and started pumping air into the radiator. I found out that the radiator neck base was leaking air.

I couldn't find any other area of coolant leaking because the neck base was corroded to even hold pressure. So what I looked for is the water pump located at the serpentine belt. It was leaking too. It was three things that would not hold pressure. 1. radiator cap 2. radiator 3. water pump.

If the engine is too hot the ECU(computer) will know from the sensor on the engine. That's why it set off the codes.

If your car is overheating. You need to check if the radiator is full of coolant. If it is then check for leaks. If you don't see any leaks then I suggest to replace the radiator cap. And if it still overheats. Bring it to the professionals. You might have a head gasket leak.