P0420 Toyota Hilux: What it Means and How to Fix

Toyota Hilux P0420

This page is meant to help you troubleshoot the Toyota Hilux 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 Hilux P0420 Diagnosis

Toyota Hilux P0420 OBDII Code Defined

P0420 is an OBDII trouble code. Here’s the technical definition for your truck:

P0420: Catalyst System Efficiency Below Threshold

Catalyst System Efficiency

Your Toyota Hilux 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

P0420 Definition Toyota Hilux
An upstream O2 sensor

When the readings from the upstream and downstream oxygen sensors show similar readings (not enough exhaust gas expelled), your truck 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 Hilux believes that the catalytic converter is no longer doing its job.

Toyota Hilux P0420 Code Symptoms

The P0420 code in a Toyota Hilux will often be thrown if one of your truck’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 Hilux P0420 code.
  • Rotten Smell– A rotten egg or sulfur smell is a telltale sign of a bad catalytic converter.

Related: P0442 Toyota Hilux

Top Causes and Solutions of P0420 in the Toyota Hilux

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 Hilux. Here are the most common ones.

Exhaust Leak (Easy to Check)

It would be a good idea to check your truck for an exhaust leak first. You should be able to hear it easily when looking underneath your Hilux.

An exhaust leak can cause P0420 since it causes exhaust gasses to escape unmetered.

If your truck 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)

Toyota Hilux Bad Oxygen Sensor Symptoms
Typical Oxygen Sensor

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 Hilux (Amazon)

Catalytic Converter (Very Common)

A catalytic converter is responsible for scrubbing out as much pollution as possible from your Hilux’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.

How to Test Your Catalytic Converter in 15 minutes (YouTube)

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 Hilux 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 Hilux. It’s most likely caused by an O2 sensor or a failing catalytic converter.

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