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 Code (DTC P0420) and a generic trouble code that applies to all vehicle makes and models manufactured after 1996. 

The P0420 code is also known as OBD II Code P0420 Bank 1’. It alerts you when your catalytic converters efficiency is low. 

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 on the vehicle’s underside. 

The catalytic converter changes harmful compounds in the car’s exhaust emissions to safe gasses (like steam). So when they get released into the air, they don’t harm the environment or the driver.

If the oxygen levels in your exhaust system are below the required threshold in Bank 1, the Engine Control Module (ECM) triggers the P0420 trouble code — as this condition could cause damage to the engine’s fuel and exhaust system. The ECM will also turn on your check engine light, among other things.

Note: Bank 1 is the bank of cylinders where cylinder #1 is located.

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 has 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 comprises the exhaust manifold, tailpipe, resonator, car muffler, and catalytic converter. Together, these parts make sure that harmful 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 (usually of the heated oxygen sensor variety): 

  • Upstream oxygen sensor: The upstream sensor (or the front O2 sensor) is located before the catalytic converter. It monitors the level of pollutants in the engine’s exhaust.
  • Downstream oxygen sensor: The downstream O2 sensor (or the rear O2 sensor) is located after the catalytic converter. The downstream O2 sensor 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 fumes will start smelling like rotten eggs. This results from low oxygen levels in the converter, causing excess sulfur in the fuel tank and exhaust fumes

4. Check Engine Light (CEL) Turns On

The most prominent sign of any engine code diagnosis is the check engine light turning up on your dashboard. While the check engine light 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.
If your check engine light is lit, be sure to have the problem diagnosed!

Can I Still Drive?

Yes and no. 

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

So it’s best to diagnose correctly and fix the trouble code before returning to your regular 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: An oil leak in the exhaust system causes a faulty catalytic converter

◾ Faulty rear oxygen sensor or front oxygen sensor: A damaged rear O2 sensor or the upstream sensor

◾ Damaged oxygen sensor wiring: Damaged, misplaced, or incorrect O2 sensor wiring or connectors. 

Bad spark plug: A spark plug is causing a misfire, allowing unburnt fuel into the catalytic converter.

Leaking fuel injector: A damaged fuel injector can leak fuel past the ignition coil into the cylinders, causing a failed catalytic converter.

◾ Using leaded fuel: Using leaded fuel instead of unleaded fuel.

Dirty air filter: a dirty or clogged air filter can restrict airflow to the engine, causing all sorts of problems and signaling the engine code

◾ Faulty engine control module: A faulty engine control module that incorrectly signals fault codes to the engine. 

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


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 fuel system. 

Once you know what caused the diagnostic trouble code, it’s time to fix it. Check for other engine codes as well and repair as needed. 

However, diagnosing the exact cause of any trouble code can be tricky. So if you do not have decent mechanical knowledge, it’s best to leave the diagnosis to your mechanic. 

Common Diagnosis Mistakes

One of the most common mistakes to avoid when fixing the P0420 code is not fully diagnosing the problem before replacing any parts. People often instinctively change the O2 sensor or oxygen filter when remedying the OBD II code P0420

However, the problem could be something totally different, like a bad spark plug. So, make sure you properly diagnose all potential causes before deciding on a fix. Better not to assume it’s simply an upstream oxygen sensor or downstream oxygen sensor issue.

Possible Repairs for P0420

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

Your mechanic must first 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
  • Shore up any leak and replace fuel injector
  • 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 ECM. However, that’s very rare.

Your mechanic may then take a test drive to see if the OBD II code P0420 is resolved and troubleshoot for other problems if not.

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