Explained DTC: P0420 Catalyst System Efficiency Below Threshold Bank 1


When the internal combustion process takes place, it creates a lot of toxic and dangerous pollutants. Vehicles have something in them called a catalytic converter which breaks down these exhaust emissions so that air pollution can be prevented or reduced. This component relies on a catalyst to break down these harmful pollutants. Some of which include nitrogen oxides, carbon monoxide, and hydrocarbons. The catalyst itself may be made from rhodium, platinum, gold, or palladium material. Basically, these meshes serve as a filter for the fumes of these pollutants. That way, fewer emissions are released out of the exhaust pipe and into the outdoor environment.

Trouble Code P0420

When you scan your power control module for a diagnostic check, it may give you trouble code P0420. If it does, then it means that your catalytic converter is not filtering the emissions as well as it should be. More than likely, the catalyst is too worn out or dirty. You can try cleaning it to see if that works but if it doesn’t, then you will need to get the converter replaced.

First, you should check for leaks in the exhaust system because that may cause trouble code P0420 to generate too. Also, check your oxygen sensors because they may need to be repaired or replaced if they’re in bad shape. There are two oxygen sensors in every catalytic converter. There is one sensor located behind the catalytic converter and another located in the front of it.

The front sensor is an upstream sensor and the rear sensor is a downstream sensor. If just one of these sensors fails to function, then trouble code P0420 will come up.

Read also: Explained DTC: P0171 (system too lean bank 1) and P0174 (system too lean bank 2)

Causes & Symptoms

Aside from a bad catalytic converter, there are other causes of P0420. For instance, your vehicle may have exhaust pipe leaks, exhaust manifold leaks, engine misfiring, catalytic converter oil contamination, bad oxygen sensor, bad engine coolant temperature sensor, fuel injector leak, high fuel pressure, and bad oxygen sensor connectors. Sometimes even putting the wrong type of fuel in your gas tank can make trouble code P0420 occur.

You will notice when you have problems with your catalytic converter because certain symptoms will arise. The Check Engine light can be expected as the first symptom. You may experience a temporary power reduction while you let the vehicle warm up in the cold. The worst symptom will be not being able to accelerate faster than 30 or 40 miles per hour. On top of that, your exhaust pipe may emit a nasty rotten egg odor. If you notice any of these symptoms, take your vehicle into the shop immediately and have the mechanic replace the necessary components.


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