The check engine light is on, the car drives the same, and READY comes up as usual. On the Prius, Auris, Corolla and CT that light is very often code P0420: catalyst efficiency has dropped below the threshold. It is an exhaust code. The HV pack does not set it and does not, by itself, light the hybrid-system warning.
What code P0420 reports
The controller compares the signal from the sensor before the catalyst with the signal from the sensor after it. A healthy core stores oxygen and smooths the signal from the rear sensor. When the rear one starts copying the front one, the computer stores P0420 and lights the engine lamp. The car often drives normally, because the mixture and the battery carry on as before. That is why it is easy to take the lamp for a hybrid fault, while the hybrid system stays quiet.
Why it comes back on a hybrid
- The engine stops at the lights, the core cools down, and the efficiency monitor completes only on a longer, steady stretch
- Worn plugs or a coil cause a misfire, and unburnt petrol finishes off the ceramic
- On a third-generation Prius, coolant from the head gasket enters the exhaust and floods the core
- A leak ahead of the rear sensor lets air in and falsifies the reading
On short local trips alone the monitor often does not run at all. The code confirms only after a longer, steady stretch, once the core is hot. A lamp cleared with a scanner comes back after such a drive if the cause is still there.
What not to replace first
A new catalyst is sometimes needed, but not as the first step. First the spark plugs and coils, then flange tightness and the signal from both sensors. On a third-generation Prius, before anyone prices the core, we check whether the engine is losing coolant — the text on the head gasket describes that. A core fitted while coolant is entering the exhaust comes back with the same P0420.
A battery code is a separate story: P0A80 and the module codes. Replacing the pack does not clear P0420. At the inspection a lit engine lamp means a fail, no matter how quietly the hybrid runs.
How we tell them apart in the workshop
We read the code and the data from the moment it was stored, compare the sensors at idle and on the road, and look at the exhaust under the car. A lazy rear sensor with a lively front one and a dry system can be just the sensor. When both sensors are trustworthy and the core does not smooth the signal, we replace the catalyst — not the battery. A cheap universal core often misses the threshold, and the code returns after a few thousand kilometres.
Check engine with the hybrid warning off is the exhaust. The HV pack speaks in other codes.
Check engine without the hybrid warning? We will separate P0420 from the battery.
Visit Green Serwis in Krakow — we specialise in Toyota and Lexus hybrids.
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P0420 is about the catalyst or about what finished it: the plugs, a leak, coolant from the head. The HV battery does not store this code. We diagnose the pack when a battery code is present as well, or when the car loses electric driving. The offer is on the hybrid service page.