“What is the most difficult part to replace/repair on a car?” asks some innocent on Quora. A retired engineer included this in his list:
Any repair in the engine bay of a W8 engine [Volkswagen] Passat. Really cool engine? Yes. Really impossible and/or prohibitively expensive to get any parts for it including ones that fail often? Oh yes. Don’t buy these unless you really enjoy the smell of money burning, the glow of check engine lights, and the particularly invigorating feeling of wondering what’s going to break and leave you debating whether to let it sit or spend hundreds on what should be a $5 part. The THERMOSTAT (normally a $2 to $15 part) is $260 to $682. And if it fails you will fail emissions because … the CEL will come on. Why? Because rather than make a solenoid or motor actuated valve and use it to regulate coolant flow … they took a regular thermostat and attached an actuator to it so the computer can move it too. WHEN the actuator fails, the computer notices and on goes the light!
Nissan asks a whole $27 for an OEM thermostat for my ride, but anticipated labor is only about an hour, so the bill will still come in under $200.