The brief
Mr Wang's Volkswagen was overheating, the gauge climbing and the temperature warning coming up, so he brought it straight in. Overheating is one to take seriously, because a hot engine warps heads and blows gaskets in short order, and the repair after that is a big one. The usual suspects for overheating are the coolant pump and the thermostat, but they're not the only ones. The radiator cooling fan pulls air through the radiator when the car isn't moving fast enough to do it on its own, in traffic, at idle, crawling along, and if the fan motor seizes or the fan module fails, the engine cooks at low speed even though the coolant, the pump and the thermostat are all fine. A dead fan doesn't restart, so once it's the fan, it needs replacing.
The diagnosis
The check ruled out the obvious first, the coolant pump was circulating, the thermostat was opening, the coolant was up and clean, the radiator was flowing. The radiator cooling fan wasn't running when it should, the motor seized, which is exactly why the engine overheated in traffic and at idle but held temperature on the move. That's a cooling fan replacement, plus a coolant flush and bleed since it had been run hot, rather than throwing a pump or a thermostat at it that weren't the problem.
The work
The seized radiator cooling fan was removed and a new genuine VW-spec fan assembly fitted, the wiring and the fan module checked so the new fan switches on at the right temperature. The coolant was drained, the system flushed and refilled with the correct VW coolant, and bled the proper way so no air pockets were left. The radiator and hoses were checked over while it was apart. A road test, including a spell sitting in traffic, confirmed the fan kicking in on cue and the gauge holding steady.
The outcome
The gauge holding steady on the move and in traffic, the cooling fan switching on when it should, no temperature warning, and the coolant fresh. The Volkswagen went home with the overheating fixed. Diagnosing it properly meant we replaced the fan that had actually failed rather than guessing at the pump or the thermostat, so it was one visit and done, before the heat did any damage.
