Mercedes-Benz CLA: M282 thermostat failure — slow warm-up, or overheating

Thermostat failure is one of the named weak points on the M282, and it fails in two directions that feel completely different from the driver's seat. Stuck open, the engine never reaches proper operating temperature: the heater blows lukewarm, the car uses more fuel than it should, and the engine wears faster because it spends its life running cold. Stuck closed, coolant cannot circulate and the temperature climbs — which is the version that damages things. The first is easy to dismiss as the car being a bit slow to warm up on a cold morning, which is exactly why it goes unfixed for months. The part is not expensive; the consequences of the closed version are.

• Engine taking much longer than usual to reach normal temperature
• Heater blowing lukewarm rather than hot, especially at idle
• Temperature gauge sitting lower than normal, or climbing in traffic
• Engine warning light with coolant temperature codes
• Fuel economy noticeably worse in cold weather

This one is best judged on a genuinely cold start, so try to see the car before it has been moved. Note the temperature reading as you set off and time roughly how long it takes to reach normal — an engine that takes an unusually long stretch of driving to get there, or never quite gets there, is telling you the thermostat is stuck open. While it warms, put the heater on full hot with the fan low and check that the air becomes properly hot rather than lukewarm, and check it again while sitting still at idle, which is where a weak system shows first. Then watch the gauge for the opposite problem: a temperature that keeps climbing in slow traffic rather than holding steady. With the bonnet up and the engine cold, check the coolant level against the marks and look for dried coloured residue around the thermostat housing at the front of the engine. Ask whether the thermostat has ever been replaced and whether the car has overheated. If a workshop inspection is possible: ask for stored coolant temperature faults to be read.

Parts involved: Thermostat.

$400 to $1,600 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Does the heater blow properly hot, including at idle?”
• “Has the thermostat ever been replaced?”
• “Has the engine ever overheated or run unusually cold?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.