Parts for your 2020 Mazda 3-Radiator cap

2020 Mazda 3 radiator cap: what’s actually fitted and where to find it

Technical documentation for the BP-series 2020 Mazda 3 shows it does not use a traditional cap on the radiator neck. Instead, the pressurised cap is fitted to the coolant reserve tank (also called the degas or expansion bottle). This layout is noted in the Mazda 3 (BP) Workshop Manual under Cooling System Cap Inspection, which specifies removing the cap from the coolant reserve tank, not the radiator. The 2020 Mazda 3 Owner’s Manual diagrams likewise point to the reservoir cap under the bonnet, and the Mazda parts catalogue lists a “coolant reserve tank cap” for BP models rather than a separate radiator neck cap. Aftermarket catalogues for caps also list the application as “reservoir” for the 2020 Mazda 3.

Why the change? Modern Skyactiv cooling systems are designed as closed, remote-pressurised systems. The radiator itself is a sealed unit, and the pressure control happens at the reservoir. That means the “radiator cap” as older drivers know it isn’t used on the radiator in this Mazda — the working pressure cap is on the bottle.

• Improved packaging: deleting a filler neck on the radiator gives more space and cleaner airflow up front.
• Better air separation: the remote degas bottle continuously bleeds trapped air, improving cooling performance.
• Service access and safety: owners check and top up at the translucent bottle, and the warning label and pressure cap live in one easy-to-see spot.

Owners looking to replace a “radiator cap” on a 2020 Mazda 3 are actually chasing the cooling system cap on the reservoir. Only remove it when the engine is completely cold, and confirm the pressure rating printed on the cap when sourcing a replacement. If there are signs of a tired cap — perished rubber seal, crusty deposits, hard starts after heat soak, or coolant pushing past the cap — it’s smart to replace it. A weak cap can drop system pressure and raise the risk of overheating.

When topping up, use Mazda Genuine FL22 long-life coolant (the same specification already in the car) and don’t mix types. Fill to the “F” line on the bottle when cold, start the engine with the heater on, let it warm through, then recheck the level once cooled. If pressure loss or coolant loss continues after cap replacement, have the cooling system pressure-tested by a technician.

Does a 2020 Mazda 3 have a radiator cap?

No. The radiator itself is sealed. The pressure cap owners service is on the coolant reserve tank under the bonnet.

Which cap should be replaced if there’s coolant loss?

The cooling system cap on the reservoir. Replace it with a like-for-like pressure rating as printed on the original cap, and only open it when the engine is cold.

How should coolant be topped up on a 2020 Mazda 3?

Top up via the reservoir only, using Mazda FL22 coolant. Fill to the cold “F” mark, run the engine to temperature with the heater on, allow it to cool, and recheck. Avoid mixing different coolant types.