The catalytic converter error code: P0420

The P0420 (whose official onboard diagnostic definition is ‘Catalyst System Efficiency Below Threshold’) code is a sign that a vehicle’s catalytic converter is subject to one or more problems. Such problems are usually in relation to the catalytic converter failing (or simply struggling) in its intended ability to reduce and oxidate (more commonly called oxidise) what will otherwise be the vehicle’s toxic exhaust fumes.

Catalytic converters are responsible for cleaning an ICE vehicle’s exhaust fumes (insofar as ICE emissions can be cleaned) following its combustion cycle, so being notified of a failed or failing catalytic converter may be a sign of an overabundance of engine pollution. The significance of this will be covered later, but for now, it’s important to consider the details of a catalytic converter before discussing what the implications of its error message are.

A close-up of a modern vehicle’s catalytic converter 

What is a catalytic converter?

A catalytic converter (CC) is in almost every modern ICE vehicle, namely in the undercarriage, and a typical example of one is pictured above. It is made of ceramic honeycomb structures which are coated with precious metals (usually platinum, but they can also be palladium, and rhodium) due to their oxidation properties.

As the name catalytic converter reflects, it uses catalysts to convert the vehicle’s three main harmful compounds, namely hydrocarbons, carbon monoxide, and nitrogen oxides.

There are two catalysts within a catalytic converter: the reduction catalyst and the oxidation catalyst. To achieve its intended redox (reduction oxidation) process, the former reduces nitrogen oxide by counteracting the amount of oxygen in the car’s exhaust fumes; and the latter converts carbon monoxide into carbon dioxide (CO2) – by, opposingly to the reduction catalyst, adding oxygen (O2).

For this process to be monitored, two oxygen sensors – namely the upstream and downstream O2 sensors – are respectively positioned at the front and back of the CC. Those sensors’ communication with the vehicle’s ECU (electronic control unit) determines whether the car may ultimately inform the user of a P0420 code. The process that leads to this problem will be discussed in more detail next.

When a faulty catalytic converter leads to a P0420 code

As mentioned, the upstream and downstream O2 sensors are both responsible for gathering the data needed to ascertain whether a vehicle’s catalytic converter is faulty. This is why the positioning of the two sensors is vital: there is a specific point of comparison between the two components that the ECU must determine.

By being situated at the front of the CC, the upstream sensor is there to monitor the concentration of pollutants in the engine’s exhausts before it reaches the catalytic converter. And after the exhaust fumes have exited the catalyst converter and reached the downstream sensor, this sensor is there to measure what should be the CC’s clean output.

But again: the ECU has to be there to gauge the relationship between both the former and the latter sensor’s output. If they are similar – or, even worse, identical – that means the catalytic converter has failed in its purpose: its input has not been suitably treated for the fumes to safely exit the vehicle’s exhaust pipe. The two sensors’ dedicated electronic control unit will then have the driver informed of the P0420 error code, which is, to reiterate: Catalyst System Efficiency Below Threshold.

It is then of course the driver’s responsibility to act accordingly: thankfully, most P0420 instances are manageable. Yourmechanic.com even stresses that people shouldn’t fall into the trap of replacing their oxygen sensors straight away when they see the error code. There are many reasons for an P0420 code to light up, the most common of which is simply a poor catalytic converter – in which case, a mechanic may simply replace the CC itself. Nevertheless, given that the problem(s) may also be down to a dirty CC (consider oil contamination, for instance), faulty O2 sensor wirings, or even a poor ECU, and so on and so forth, the driver must be willing to explore the many possible faults and potential solutions.

On top of all this, the very fact that the P0420 code needs to exist may be considered a problem in itself! After all, if ICE (internal combustion engine) vehicles were more energy-efficient, they wouldn’t have to have such a filtration system in the first place. Perhaps faults such as the P0420 can be seen as yet another sign that the dawn of electric vehicles not only will happen – but has to happen.