Calorstat TH6843.91J - Engine Coolant Thermostat

Product Overview

The Calorstat TH6843.91J is an OEM-specification engine coolant thermostat for MINI Cooper models from 2002 to 2008. This thermostat regulates coolant flow through the engine to maintain optimal operating temperature, directly affecting engine efficiency, emissions control, and cabin heating performance. It fits MINI R50 and R55 hatchback and Clubman variants with the naturally aspirated and turbocharged 1.6-liter engines. Calorstat thermostats are engineered to precise factory tolerances, ensuring reliable temperature management across diverse driving conditions.

Symptoms of a Failing Engine Coolant Thermostat

• Engine running hotter than normal, with temperature gauge climbing into the red zone
• Engine running too cold, slow to reach operating temperature even after extended driving
• Inconsistent or weak cabin heat output
• Coolant leaking from the thermostat housing or around gasket seals
• Check Engine Light illuminated due to coolant temperature sensor faults
• Reduced fuel economy caused by prolonged cold-engine operation

Cross Reference Numbers

This Calorstat TH6843.91J is equivalent to or replaces the following part numbers: TH684391J, 11537596787.

Vehicle Compatibility

This part fits the popular models below. For Fitment Confidence, please use the vehicle selector tool above to verify exact compatibility for your vehicle.

• MINI Cooper 1.6 L4 Naturally Aspirated (R50): 2002–2006
• MINI Cooper S 1.6 L4 Supercharged (R53): 2002–2006
• MINI Cooper 1.6 L4 Naturally Aspirated (R56): 2007–2008
• MINI Cooper S 1.6T L4 Turbocharged (R56): 2007–2008
• MINI Clubman Cooper 1.6 L4 Naturally Aspirated (R55): 2008
• MINI Clubman Cooper S 1.6T L4 Turbocharged (R55): 2008

This may also fit other applications. Please use the vehicle selector tool to verify fitment.

Often Replaced Together

For a complete cooling system service, many owners replace the following at the same time:

• Engine coolant (flush and refill with BMW/MINI-approved mixture)
• Thermostat housing gasket or O-ring seal
• Coolant hoses showing age or deterioration
• Radiator cap (if not recently replaced)

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Frequently Asked Questions

How do I know if my MINI Cooper R50 or R56 needs a new thermostat?

The most reliable indicator is a persistent temperature gauge reading that sits too high or too low, or erratic needle movement as you drive. If your engine reaches operating temperature very slowly (especially in cold weather) or runs hotter than normal under load, the thermostat may be stuck. You may also notice poor cabin heater output in winter, which points to the thermostat restricting coolant flow to the heater core. A professional scan can reveal coolant temperature sensor codes that suggest thermostat malfunction.

What is the difference between a stuck-open and stuck-closed thermostat?

A stuck-open thermostat allows coolant to circulate continuously, preventing the engine from reaching and maintaining proper operating temperature. This causes the temperature gauge to read low, slow cabin heating, and reduced fuel economy. A stuck-closed thermostat blocks coolant flow, causing the engine to overheat rapidly. Both conditions trigger Check Engine Lights and compromise engine performance. The Calorstat TH6843.91J is calibrated to open and close at the correct temperature to prevent both scenarios.

Can I replace just the thermostat, or do I need to replace the housing gasket too?

While it is technically possible to replace only the thermostat element, professional technicians typically recommend replacing the thermostat housing gasket or O-ring seal at the same time. The gasket is exposed to coolant pressure and temperature cycling during removal, which can compromise its seal. Replacing both parts together eliminates the risk of a coolant leak shortly after service and reduces labor costs associated with a second removal.

How long does a thermostat typically last on a MINI Cooper?

Engine thermostats are not subject to a factory-recommended replacement interval on most MINI models. Under normal driving conditions, a quality thermostat like the Calorstat can last the life of the vehicle. However, thermostats may fail prematurely due to coolant contamination, mineral buildup in hard-water regions, or prolonged operation with incorrect coolant mixture. Regular coolant flushes and using BMW/MINI-approved coolant extend thermostat life significantly.

Is the Calorstat TH6843.91J a direct OEM replacement?

Yes. Calorstat is an OEM-approved supplier for BMW and MINI cooling system components. The TH6843.91J meets factory specifications for opening temperature, flow rate, and housing fit, making it a direct equivalent to the original equipment thermostat. It carries the same warranty confidence as a dealership part at a competitive aftermarket price.

What coolant should I use after replacing the thermostat on my MINI?

Consult your vehicle's owner manual or service documentation for the correct coolant specification. MINI and BMW models typically require a specific ethylene glycol or propylene glycol mixture approved by BMW. Using the wrong coolant type can damage seals, corrode internal cooling passages, and shorten the life of the new thermostat. Always flush the system before refilling with fresh coolant to remove any debris or old fluid residue.

Order Calorstat TH6843.91J today. In-stock orders ship same or next business day from our U.S. warehouse, and our fitment team is one call away if you need help confirming the right part for your MINI.

Note: All information on this page and in the FAQ is for general reference only. Verify fitment and consult a professional as needed. eEuroparts.com is not responsible for incorrect part selection or installation.