This page is meant to help you troubleshoot the Toyota Prius P0420 trouble code. It covers P0420 meaning, symptoms, causes, and possible solutions.
P0420 is often caused by a bad oxygen sensor, but there can be other causes as well. Jumping straight to replacing the O2 sensor can cost you time and money. P0420 is the most common OBDII code.
Toyota Prius P0420 OBDII Code Defined
P0420 is an OBDII trouble code. Here’s the technical definition for your car:
P0420: Catalyst System Efficiency Below Threshold
Catalyst System Efficiency
Your Toyota Prius has at least two oxygen sensors, one before and after the catalytic converter (commonly referred to as the upstream and downstream O2 sensors).
The upstream (before the exhaust gas hits the catalytic converter) oxygen sensors are there to measure the exhaust gasses as they exit the engine. The Powertrain Control Module (PCM) uses this data to adjust air-fuel ratio and ignition timing.
The downstream oxygen sensor’s (downstream of the catalytic converter) primary function is to verify the catalytic converters are doing their job.
Among the many jobs that oxygen sensors have is measuring the emissions levels coming in and out of the catalytic converter.
Efficiency Below Threshold
When the readings from the upstream and downstream oxygen sensors show similar readings (not enough exhaust gas expelled), your car will throw the service engine soon light (P0420).
These readings shouldn’t match, since the catalytic converter is supposed to be scrubbing the exhaust. When the numbers match or they are close, the software in your Prius believes that the catalytic converter is no longer doing its job.
Toyota Prius P0420 Code Symptoms
The P0420 code in a Toyota Prius will often be thrown if one of your car’s oxygen sensors is not functioning properly.
There may or may not be drivability issues associated with P0420. For most people, the first sign that anything is wrong at all is the service engine soon light coming on.
Here are the typical symptoms that something is wrong when you have code P0420:
- Fuel Efficiency- May or may not suffer depending on how much the catalytic converter is clogged.
- Loss of Power– It is a really strange feeling to drive a vehicle with a catalytic converter that is clogged. Typically, it’ll idle ok and drive normally under a light load. But, whenever the engine is working hard, it’ll feel like it is running out of gas. It’s very similar to the symptoms of a bad fuel filter.
- Service Engine Light– Often, this is the only symptom of the Toyota Prius P0420 code.
- Rotten Smell– A rotten egg or sulfur smell is a telltale sign of a bad catalytic converter.
Related: P0442 Toyota Prius
Top Causes and Solutions of P0420 in the Toyota Prius
Typically, the code will be triggered by the upstream oxygen sensor (before the converter).
There are many things that can cause the P0420 trouble code in your Prius. Here are the most common ones.
Exhaust Leak (Easy to Check)
It would be a good idea to check your car for an exhaust leak first. You should be able to hear it easily when looking underneath your Prius.
An exhaust leak can cause P0420 since it causes exhaust gasses to escape unmetered.
If your car doesn’t have an exhaust leak, you’ll probably need to test the O2 sensors and/or catalytic converters.
Often, the oxygen sensor is the cause of P0420. Specifically, it’ll usually be the O2 sensor(s) on the upstream side of the catalytic converter. But, don’t be surprised if it’s not.
Oxygen Sensor (Common)
Often, the oxygen sensor is the cause of P0420. Specifically, it’ll usually be the O2 sensor(s) on the upstream side of the catalytic converter.
The P0420 code is thrown when the upstream and downstream oxygen sensors on both sides of the exhaust have similar readings.
When the oxygen sensor is bad, it can cause a false reading and trigger the code. If the engine does not exhibit any of the decreased performance associated with a bad catalytic converter, it very well may be the oxygen sensor (or you haven’t noticed the performance loss yet).
How to Test an Oxygen or O2 Sensor (YouTube)
New Oxygen Sensor – Toyota Prius (Amazon)
Catalytic Converter (Very Common)
A catalytic converter is responsible for scrubbing out as much pollution as possible from your Prius’s exhaust.
Over time, they can become clogged. Although modern catalytic converters are supposed to last the life of most vehicles. There could be some underlying problem if it has clogged.
O2 Sensor Wiring
Over time, the oxygen sensor wiring can go bad. It is especially volatile since it is right next to the hot exhaust at all times. The downstream oxygen sensor wiring is also volatile since it is so far from the ECM (more distance to travel = more area to fail).
How to Test Wiring in Under an Hour (2 Car Pros)
If your Prius is misfiring or the exhaust timing is off, this can cause the catalytic converter to go bad, which in turn can cause P0420.
Engine Temp Sensor
If the computer doesn’t know what the engine temp is, it’ll keep the fuel mixture rich. When the fuel mixture is rich, the exhaust the O2 sensors see will be out of range, and it could throw P0420.
Good luck figuring out what is causing P0420 with your Toyota Prius. It’s most likely caused by an O2 sensor or a failing catalytic converter.
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