Overheating problem iMac G5 question

Hi guys. I've recently fixed my iMacs power supply which had gone bad. But when I plugged it back in and started it up I got the chime, LED 1 and 2 come on and then LED 4 comes on. Think I read somewhere that if led 4 does come on it means something is overheating or the temperature is high.. Now I did take my MOB out so I'm guessing maybe it's the thermal paste ? I've never changed that in the ten years we've had the machine so that is my guess.. But any feedback is welcomed