Estimates Trouble Codes P0420

P0420: Catalyst System Efficiency Below Threshold

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What is P0420?

Error code P0420 is defined as “Catalyst System Efficiency Below Threshold.”

It’s an OBD-II Diagnostic Trouble Code (DTC) and a generic powertrain code that applies to all makes and models of vehicles manufactured after 1996. 

Code P0420 is often also referred to as ‘Code P0420 Bank 1’. It alerts you when your catalytic converter becomes less efficient at controlling or reducing pollution. The catalytic converter is a device that was invented to meet the standards set by the National Emissions Standards Act (1996). It’s a small device with two pipes located on the vehicle’s underside. The catalytic converter uses the ‘catalyst’ chamber that changes harmful compounds from the car’s emissions (carbon monoxide) to safe gasses (like steam). So when they get released into the air, they don’t harm the environment or the driver.

To be specific, trouble code P0420 suggests that the oxygen levels in your exhaust system are below the required threshold in Bank 1 (the bank of cylinders where cylinder number 1 is located). 

This condition could cause damage to the engine’s fuel and exhaust system. So, the Powertrain Control Module (PCM) triggers error code P0420, turning on your check engine light, among other things. 

Common symptoms

A bad catalytic converter generally doesn’t reflect any indicators. However, you can look out for these common symptoms if you suspect your car is facing problems with its exhaust system. 

They are: 

1. Lack Of Power

If your vehicle fails to gain power or operate smoothly after warming up, it could point to a faulty catalytic converter. This could be due to uneven fuel pressure caused by the worn-out converter. You may also notice jerky movements while driving or complete stall-outs.

2. Inability To Speed Up

A bad catalytic converter will cause exhaust buildup in the exhaust pipe, reducing performance. So you may notice that your car doesn’t speed up beyond 30-40 MPH, or has diminished fuel efficiency.

3. Foul Smell From The Exhaust Pipe

The exhaust system consists of several components that help reduce exhaust noise and ensure the exhaust gas safely exits the engine. It consists of the exhaust manifold, tailpipe, resonator, car muffler, and catalytic converter. Together, these parts make sure that harmful gasses and emissions from the engine don’t enter the vehicle and make occupants sick. 

In most cases, the exhaust system also cleans out the pollutants and protects the environment against carbon monoxide emissions. 

It typically contains two to four oxygen sensors, which fall into these categories:

  • Upstream oxygen sensor: The upstream sensor (sometimes called the front O2 sensor) is located before the catalytic converter. It monitors the level of pollutants in the engine’s exhaust and sends this information to the ECU.
  • Downstream oxygen sensor: The downstream O2 sensor (also called the rear O2 or heated oxygen sensor) is located after the catalytic converter. It measures the level of pollutants passing through the catalytic converter.

When engine code P0420 is caused by a worn-out or failed catalytic converter, there’s a chance that the exhaust pipe will start smelling like rotten eggs. This results from low oxygen levels in the converter, causing excess sulfur in the fuel tank, leading to the odor. 

4. Check Engine Light (CEL) Turns On

Perhaps the most prominent sign of any engine code diagnosis is the check engine light turning up on your dashboard. While the CEL doesn’t necessarily indicate DTC P0420, it could be one of the fault codes keeping your check engine light on. 

You may also see a failed emissions test due to a bad catalytic converter.

Can I still drive?

Yes and no. 

Engine code P0420 doesn’t cause any drivability issues. However, neglecting the trouble code may cause serious damage to other engine parts, leading to costly repairs. 

So it’s best to perform a proper diagnosis and fix the trouble code before returning to your normal driving routine.

P0420 causes

The root cause for DTC P0420 is usually a catalytic converter failure. However, there can be other underlying reasons for this trouble code to show up. 

Some of them are:

  • Damaged exhaust manifold or exhaust leak: A clogged or damaged exhaust manifold that prevents exhaust gas from exiting the engine, affecting converter efficiency.
  • Leaking exhaust pipe: An exhaust leak that draws excess oxygen into the exhaust system, triggering a lean oxygen sensor reading.
  • Oil contamination in the catalytic converter: Oil leaks in the exhaust system, causing a faulty catalytic converter. 
  • Faulty rear oxygen sensor or front oxygen sensor: A damaged downstream oxygen sensor (rear O2) or upstream sensor (front O2). 
  • Bad spark plug: A spark plug is causing a misfire, allowing unburnt fuel into the catalytic converter.
  • Damaged oxygen sensor wiring: Damaged, misplaced, or incorrect O2 sensor wiring or connectors. 
  • Leaking fuel injector: A damaged fuel injector that leaks fuel past the ignition coil into the cylinders, causing a failed catalytic converter.
  • Using leaded fuel: Using leaded fuel instead of unleaded fuel.
  • Faulty engine control module: A faulty engine control module that incorrectly signals fault codes to the engine. 

Keep in mind that the main causes behind the P0420 engine code can vary from engine to engine, although it’ll likely be concerned with the catalyst system efficiency. 

So let’s understand how this code manifests itself in various vehicle types.


The best way to diagnose a trouble code is to use an OBD-II scanner. For DTC P0420, you may have to inspect the front and rear oxygen sensor, catalytic converter, and the fuel system. 

However, note that diagnosing the exact cause for any trouble code can be tricky. So if you’re not someone with decent mechanical knowledge, it’s best to leave the diagnosis to your mechanic

Once you know what caused the diagnostic trouble code, it’s time to fix it.

One of the most common mistakes to avoid fixing the P0420 code is not fully diagnosing the problem before replacing any parts. 

Most people often instinctively change the O2 sensor or oxygen filter when they diagnose an error code P0420. 

However, the problem could be something totally different, like a bad spark plug. So, make sure you properly diagnose all potential causes (on top of the upstream sensor and downstream oxygen sensor) before deciding on a fix. 

Check for other engine codes as well and repair as needed. And if you’re unsure, it’s best to call a mechanic instead of replacing any parts yourself.

Possible repairs for P0420

Since the code P0420 can be caused by several different issues within the exhaust system, it doesn’t have a single solution. 

Your mechanic will first have to identify the problem and then conduct the appropriate repairs. For instance, if you have an exhaust pipe leak or a leaking fuel injector, your mechanic will need to replace them with new parts. 

In general, your mechanic may perform any of the following procedures to try to clear the trouble code: 

  • Clean the catalytic converter
  • Repair any damaged sensor wiring
  • Replace the engine coolant temperature sensor
  • Replace the oxygen sensor and get a new catalytic converter if needed
  • Fix any oil burns or misfiring circuits
  • Replace lean or rich fuel mixture

In some cases, your mechanic may advise the replacement of a faulty Engine Control Module (ECU). However, that’s very rare.

Your mechanic may then take a test drive to see if the trouble code is resolved and troubleshoot for other problems if not.


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