P0420 Code – Catalyst System Efficiency Below Threshold (Bank 1) | Meaning, Causes, Symptoms & Fix

The P0420 diagnostic trouble code indicates that the catalytic converter on Bank 1 is not operating efficiently.

In simple terms, this means the catalytic converter is not properly reducing harmful exhaust emissions.

When the P0420 code appears, your Check Engine Light will turn on.
In most cases, the vehicle may still drive normally, but the issue should not be ignored.

P0420 is one of the most common emissions-related trouble codes worldwide.

What Does P0420 Mean?

The code P0420 stands for:

Catalyst System Efficiency Below Threshold (Bank 1)

Modern vehicles use two oxygen sensors:

• Upstream (before the catalytic converter)

• Downstream (after the catalytic converter)

If both sensors show similar readings, the ECM determines that the catalytic converter is not working efficiently and triggers P0420.

What Is Bank 1?

Bank 1 refers to the side of the engine that contains Cylinder 1.

On inline engines (4-cylinder), there is only one bank.
On V6 or V8 engines, Bank 1 is one side of the engine.

Symptoms of P0420 Code

Common symptoms include:

• Check Engine Light ON

• Reduced engine performance (sometimes)

• Rotten egg smell from exhaust

• Poor fuel economy

• Failed emissions test

In many cases, there may be no noticeable driveability issues.

Common Causes of P0420

1. Failing Catalytic Converter (Most Common)

Over time, the internal honeycomb structure can degrade or become clogged.

2. Faulty Oxygen Sensor

A failing downstream O2 sensor may give inaccurate readings.

3. Exhaust Leak

Leaks before or near the catalytic converter can trigger false readings.

4. Engine Misfires (P0300 Related)

Unburned fuel entering the catalytic converter can damage it.

5. Running Rich Fuel Mixture

Long-term rich conditions may overheat and destroy the converter.

How to Diagnose P0420

Step 1: Check for Other Codes

Look for:

• P0300 (Misfire)

• P0172 (Rich condition)

• O2 sensor codes

Step 2: Monitor O2 Sensor Data

Compare upstream vs downstream sensor waveforms.

If downstream mimics upstream, catalyst efficiency is low.

Step 3: Inspect for Exhaust Leaks

Check welds and joints.

Step 4: Temperature Test

Measure temperature before and after catalytic converter.

Outlet should be hotter than inlet.

Can You Drive With P0420?

Yes, in most cases the car will still run.

However:

• Emissions will increase

• Fuel efficiency may drop

• The vehicle may fail inspection

Ignoring the problem long term can cause further damage.

Catalytic converter replacement is often expensive due to precious metals inside.

Vehicles Commonly Affected

P0420 frequently appears on:

• Toyota

• Honda

• Nissan

• Chevrolet

• Ford

• BMW

• Hyundai

High-mileage vehicles are more likely to develop this issue.

FAQ

Is P0420 serious?

It is not immediately dangerous, but it affects emissions and long-term engine health.

Can I clean my catalytic converter?

In most cases, no. Replacement is usually required.

Can bad spark plugs cause P0420?

Yes. Misfires can damage the catalytic converter.

How long does a catalytic converter last?

Typically 100,000+ miles depending on maintenance.

Final Thoughts

The P0420 code indicates reduced catalytic converter efficiency.

While the vehicle may still drive normally, repairs should not be delayed to avoid costly damage and emissions issues.

Proper diagnosis is essential before replacing expensive components.