Parts for your 2021 Mitsubishi Asx-Thermostat

2021 Mitsubishi ASX Thermostat – What it does and when to sort it

According to Mitsubishi Motors’ ASX/RVR/Outlander Sport service manual for the GA/GS platform (covering 2010–2022) and the Mitsubishi ASA electronic parts catalogue, the 2021 Mitsubishi ASX is fitted with a conventional, wax‑pellet engine coolant thermostat. It’s housed in the thermostat housing at the engine end of the lower radiator hose on the 4B11/4B12 petrol engines used in this model year, and it’s a key part of the cooling system design.

The thermostat’s job is simple but vital: it regulates coolant flow so the engine gets up to operating temperature quickly, then holds it steady under all sorts of driving. That quick warm‑up helps reduce engine wear, improves heater performance on chilly mornings, and keeps emissions and fuel use in check. Once temp is reached (typically in the low‑to‑mid 80s °C for this family of engines), the thermostat starts to open, sending coolant through the radiator to shed heat, then modulates as the load changes.

While thermostats aren’t a routine “every service” replacement item, they benefit from a quick check whenever coolant is changed or any cooling work is done. Many workshops will proactively replace the thermostat and its O‑ring if the vehicle is older or has higher kilometres, or when tackling related jobs like a water pump or radiator swap, simply because access is already sorted and the part is inexpensive.

Common signs the ASX thermostat needs attention include:

• Slow warm‑up, temp gauge sitting low, or poor cabin heat (stuck open)
• Overheating or a rapid rise on the gauge (stuck closed)
• Temperature fluctuating under steady cruise
• Cooling fan running more than usual, or a P0128‑type fault code recorded

When replacing, use an OEM‑quality thermostat with the correct temperature rating for the ASX, and always fit a new gasket/O‑ring. Work on a cold engine, capture and refill with Mitsubishi‑approved long‑life coolant (often the blue SLLC type) at the right mix, and bleed air from the system. Refit the housing carefully and tighten fasteners to the workshop manual spec to avoid leaks or cracked housings. After refilling, bring the engine to temperature with the heater on, confirm steady gauge behaviour, and check for leaks. For many owners, a precautionary replacement around the eight‑to‑ten‑year mark or north of 150,000 km—especially if any cooling system work is being done—adds handy peace of mind.

Popular questions

What temperature does the 2021 ASX thermostat open at?
For the 4B11/4B12 petrol engines, the factory thermostat begins to open in the low‑to‑mid 80s °C range. Exact specs vary by part revision, so the service manual or parts listing is the best reference. Using the correct temperature rating is important for drivability and emissions.

Where is the thermostat located on a 2021 Mitsubishi ASX?
It’s mounted in the alloy thermostat housing at the engine end of the lower radiator hose, near the water pump side of the engine. Access typically requires removing undertrays and the hose, then refitting with a new O‑ring and bleeding the system after.

Should the thermostat be replaced during a coolant change?
It’s not mandatory every time. Most workshops test and inspect, if the thermostat shows age, corrosion, leaks, or there are temperature control issues, replacement makes sense. Many choose to replace preventatively when doing bigger cooling jobs because the extra labour is minimal.