Temp light coming on intermittently, mk2 jetta

[Blocksmith]

My brother's mk2 does this intermittently, and I've found that cleaning the probe ends in the overflow tank resolves the issue, at least until they get dirty again. And like Vanbcguy said, if your coolant tank is dirty enough that it's difficult to see into (like the one on my brother's car), there's probably sludge in the coolant system that's built up on the probe.