When it should be changed
Every engine has a mileage and a time interval, and the earlier of the two applies. Age matters because the belt hardens and cracks whether the car is driven or not, so a low-mileage older car can still be overdue.
What affects the cost
- Engine layout and how much has to be removed to reach the belt
- Whether the water pump is belt-driven and replaced at the same time
- Tensioners, idlers, seals and auxiliary belt components in the kit
- Whether the engine is a wet-belt design running in oil
- Coolant, oil and filter needed to complete the job properly
Why the water pump goes with it
On a belt-driven pump, the labour to reach it is the same labour as the belt. Leaving an old pump in place risks a coolant leak that contaminates the new belt and repeats the whole job.
What happens if it fails
On most modern engines a snapped belt allows valves and pistons to meet, which commonly means bent valves and sometimes a replacement engine. It is the clearest example of a service item that is cheaper than the consequence.