Parts for your 2006 Toyota Prius-Radiator cap

Radiator cap for a 2006 Toyota Prius (NHW20)

Based on Toyota’s technical literature, a radiator cap is absolutely relevant and fitted to the 2006 Prius (NHW20). The Toyota Electronic Parts Catalog for NHW20 lists the Cap Sub‑Assembly, Radiator (commonly referenced as 16401‑31650), and the Toyota Repair Manual for 2004–2009 Prius (NHW20), Cooling (CO) section, includes an inspection procedure for the radiator cap with a specified valve opening pressure of roughly 78–108 kPa (11–16 psi). That makes it a standard, pressurised engine-cooling component on this model.

On the 2006 Prius, the radiator cap’s job is to keep the engine cooling system sealed and at the right pressure so coolant can run hotter without boiling. By holding around 0.9 bar, it lifts the boiling point, helps prevent localised hot spots, and uses a spring-and-valve setup to vent excess pressure to the overflow tank, then draw coolant back in as the engine cools. It’s a simple part, but it has a big say in engine temperature stability, heater performance, and head gasket longevity.

Servicing-wise, it’s smart to treat the cap as a small but critical maintenance item. Under the bonnet, only open the cap when the engine is stone cold. Check the rubber seals for cracks or flattening, make sure the spring moves smoothly, and look for crusty deposits on the seal or radiator neck—any of those are reasons to replace. A workshop can pressure-test the cap with a tester to verify it holds to spec, a weak cap can cause slow overheating, coolant loss, or a collapsed hose after cool-down.

For best results, stick with the correct pressure rating (around 88 kPa nominal/0.9 bar) and a quality cap that matches the Prius neck profile. Many owners swap the cap preventatively at major coolant service intervals—Toyota’s Super Long Life Coolant schedule is long, so replacing the cap every 5–7 years or 80,000–100,000 km is cheap peace of mind in Aussie and Kiwi conditions. Remember the Prius has two cooling systems: the engine loop uses this pressure cap, the inverter/electronics loop uses a separate reservoir with a non‑pressurised cap—don’t mix them. Keep the cap clean, seated properly, and replaced if in doubt, and the NHW20’s cooling system will keep doing its quiet, efficient thing.

FAQs

Does a 2006 Prius have a radiator cap, and where is it?
Yes. It’s on the engine radiator filler neck under the plastic top cover near the front. It’s a metal, spring‑loaded pressure cap, not the plastic inverter reservoir cap.

What pressure cap does the 2006 Prius use?
Toyota specifies a radiator cap opening pressure in the 78–108 kPa range (about 11–16 psi), with a nominal 0.9 bar rating commonly supplied. Using the correct pressure value is important for proper boiling-point control.

How often should the radiator cap be replaced?
There’s no strict time-based rule, but inspecting at coolant service and replacing every 5–7 years or 80,000–100,000 km is sensible. Replace sooner if seals are cracked, the spring feels weak, or it fails a pressure test.