Toyota · 2004–2026
Common Toyota Prius problems, and how to check them
These are the 4 faults owners and garages report most often on the Toyota Prius, from the most common down. For each one: what goes wrong, how to check it yourself, and the usual fix, so you know what you’re dealing with before you buy parts.
Fact-checked by a licensed car technician · last reviewed 27 September 2026
Problem 1
Toyota Prius: Clogged EGR cooler leading to head gasket failure
What goes wrong
The EGR cooler and intake manifold passages clog with carbon, which raises combustion temperatures and knock and eventually blows the head gasket.
How to check it yourself
Misfire codes, knocking on start-up, or unexplained coolant loss. A garage inspects the EGR cooler outlet and intake for carbon and does a combustion-gas test.
The usual fix
Clean the EGR cooler, EGR valve, and intake passages as preventive work (every 80,000-160,000 km). A blown head gasket needs a head-gasket replacement.
Signs: Misfire under load, Rough idle, Coolant loss, White smoke, Check engine light
Fault codes: P0300, P0301, P0401, P0400
Problem 2
Toyota Prius: Hybrid won’t switch on: flat 12-volt battery
What goes wrong
The small 12-volt auxiliary battery (not the big hybrid battery) goes flat or wears out, especially when the car stands still or does short trips. Without it the hybrid system can’t switch on.
How to check it yourself
Measure the 12-volt battery: below about 12.2 V at rest it’s flat, and a battery that won’t hold charge overnight is worn. The hazard lights not working or a dead dashboard point to it.
The usual fix
Charge or jump-start from the 12-volt jump point (see the handbook) and replace a worn battery with the correct type. The main hybrid battery is rarely the cause.
Signs: No crank, Battery warning light, Hard starting
Fault codes: P0562
Problem 3
Toyota Prius: Brake actuator whine or inconsistent brake pedal
What goes wrong
The electro-hydraulic brake actuator and its accumulator wear; the pump then runs often and loudly, and pedal feel changes.
How to check it yourself
Listen for the brake pump running long or often; read the brake (ABS/VSC) codes.
The usual fix
Brake faults are safety-critical: have a hybrid specialist diagnose and replace the actuator or accumulator.
Signs: Brake warning light, Abs light
Fault codes: C1256, C1253
Problem 4
Toyota Prius: Inverter coolant pump failure
What goes wrong
The electric pump that cools the hybrid inverter fails, typically after 160,000 km, and the car limits power to protect the electronics.
How to check it yourself
Read hybrid codes (P0A93 points to the inverter cooling pump); with the system on, check that coolant moves in the inverter reservoir.
The usual fix
Replace the inverter coolant pump and bleed the inverter cooling loop.
Signs: Check engine light, Loss of power
Fault codes: P0A93
General guidance, not a professional inspection. Confirm faults with measurements before buying parts, and use a qualified mechanic for brakes, steering, fuel leaks, airbags, and other safety-critical work. Terms and disclaimer