Faulty thermostat - What happens if I don't replace it?

I have a Mercedes-Benz E320 - I noticed the temperature stays round 50c on motorways and goes up to 70C in traffic, I was told thermostat needs replacing as it might be stuck open. My question is do I need to replace it and does it affect anything if I leave it?
If the thermostat is stuck open the engine will take a very long time to heat up from a cold start. The temperature readings are probably normal because the engine is cooled much more effectively on the motorway than in traffic.
Answered by Honest John on

Ask Honest John