P0420: Catalyst System Efficiency Below Threshold

What Does the P0420 Code Mean?

Error code P0420 is defined as “Catalyst System Efficiency Below Threshold       (Bank 1).”

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

The P0420 code is triggered when the oxygen levels in the exhaust system are below the desired threshold in Bank 1. 

Here’s how this code is triggered: 

The catalytic converter is a small device that converts harmful pollutants in the car’s exhaust emissions, such as unburned fuel or hydrocarbons, to safe compounds like carbon dioxide, nitrogen, and water. 

The catalytic converters’ efficiency relies on two oxygen sensors: the upstream sensor and the downstream sensor (rear sensor). 

The upstream sensor monitors oxygen levels in the exhaust gases before they enter the catalytic converter, while the downstream sensor measures oxygen levels after the exhaust gases pass through the converter. This helps maintain catalytic converter efficiency and reduce emissions.

However, if both the sensors show similar readings, it indicates that oxygen levels in your exhaust system are below the required threshold in Bank 1. 

As a result, the catalyst monitor in the Engine Control Module (ECM) or Powertrain Control Module (PCM) triggers the P0420 trouble code — as this condition could damage the engine’s fuel and exhaust system.

Common Symptoms of P0420 Code

A bad catalytic converter generally doesn’t have any direct indicators, making the diagnosis tricky. 

That said, here are some common symptoms that show up when your car’s exhaust system is malfunctioning: 

1. Check Engine Light (CEL) Turns On

The most prominent sign of any error code is the check engine light coming on. While this warning light doesn’t necessarily indicate DTC P0420, it could be another fault code keeping your check engine light on. 

If your check engine light is lit, be sure to have the problem diagnosed!

2. Lack of Power

If your vehicle fails to gain power or operate smoothly, especially after warming up, it could point to a faulty catalytic converter. A worn-out converter can create uneven fuel pressure, leading to reduced engine performance, jerky movements during acceleration, difficulty maintaining speed, or even complete stall-outs.

3. Inability to Speed Up

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

4. Foul Smell from the Exhaust Pipe

The exhaust system consists of several components, like the exhaust manifold, tailpipe, resonator, car muffler, and catalytic converter. Together, these parts prevent harmful emissions from entering the engine and making occupants sick. 

When engine code P0420 is caused by a worn-out catalytic converter, the exhaust fumes may start smelling like rotten eggs. This odor results from the catalytic converter’s inability to convert hydrogen sulfide into sulfur dioxide. The smell is often more pronounced if there is excessive sulfur in the fuel or if the catalytic converter is no longer efficiently processing exhaust gases.

5. Failed Emissions Inspection

Without proper converter efficiency, your vehicle will emit higher levels of pollutants, which can exceed the allowable limits set by emissions standards. If you fail an emissions inspection, it’s a strong indicator that the catalytic converter needs inspection or replacement.

P0420 Error Code Causes

The most common cause for DTC fault code P0420 is catalytic converter failure or reduced catalyst efficiency. However, this trouble code can also show up due to: 

• Damaged exhaust manifold 
• Exhaust leaks
• Oil contamination in the catalytic converter
• Faulty upstream O2 sensor or downstream O2 sensor (rear O2 sensor)
• Faulty oxygen sensor wiring or connector
• Bad spark plug
• Fuel injector leak
• High fuel pressure
• Using leaded fuel
• Dirty air filter
• Faulty Engine Control Module (ECM) or Power Control module (PCM)

NOTE: A faulty MAF sensor can also cause an incorrect fuel trim (air-fuel ratio), straining the catalytic converter. Likewise, a faulty engine coolant temperature sensor can indirectly hamper the fuel trim, affecting the converter’s catalyst efficiency.

How Serious is Code P0420 and Can I Still Drive?

While the P0420 code doesn’t typically affect drivability, it’s still serious. 

A faulty catalytic converter can cause harmful emissions, like carbon monoxide and nitrogen oxides (NOX), which contribute to pollution and health risks. 

Although you can drive temporarily, it’s important to address the issue to avoid engine damage, failed emissions tests, and any resulting costly repair.

How Easy Is it to Diagnose Code P0420?

Diagnosing the exact cause of any trouble code can be tricky. So, if you don’t have decent mechanical knowledge, it’s best to leave the diagnosis to a mechanic or your dealership. 

Moreover, diagnosing the P0420 code can vary depending on your vehicle’s make and model. But here’s the general process:

1. Retrieve the code: Mechanics will use an OBD-II scan tool to pull the code from the ECM.

2. Check the O2 sensor: If the downstream O2 sensor‘s voltage fluctuates similarly to the upstream O2 sensor, it indicates high oxygen levels in the exhaust. This points to a faulty catalytic converter.

3. Inspect for other issues: If each O2 sensor appears fine, they’ll check for engine misfires, ignition problems, high fuel pressure, or a fuel system leak.

4. Test drive and check freeze-frame data: They’ll do a test drive while monitoring the freeze-frame data to ensure proper sensor function.

5. Check for ECM issues: Finally, the mechanic may update the ECM to the latest version to rule out software-related issues.

Common Diagnosis Mistakes

A common mistake when fixing the P0420 code is replacing parts like the O2 sensor without a complete diagnosis. The issue might not be the upstream or rear oxygen sensor but related to an exhaust leak, faulty spark plug, or ignition problems. 

Always diagnose all potential causes before replacing parts to avoid a costly repair and ensure the right fix.

Possible Repairs and Costs for P0420 Trouble Code

Since the P0420 code can be triggered by multiple issues within the exhaust system, there is no single solution. For example, if there’s an exhaust pipe leak or a faulty fuel injector, these parts will need to be replaced.

In general, a mechanic will diagnose and perform any of the following procedures to clear the trouble code: 

• Clean the catalytic converter
• Fix exhaust leaks 
• Repair upstream or rear sensor wiring and connector damage
• Replace a faulty oxygen sensor 
• Replace a faulty fuel injector
• Install a new catalytic converter
• Replace a faulty Powertrain Control Module (rare)

Your mechanic will then take a test drive to see if the OBD-II code P0420 is resolved.

Here are estimates for some of the repairs and replacements mentioned above: 

• Wiring damage: $100 to $1500
• Catalytic converter cleaning: $100 to $300
• Exhaust leak: $220 to $330
• Fuel injector replacement: $335 to $2000
• Engine coolant temperature sensor replacement: $400 to $500
• MAF sensor replacement: $250 to $470
• Upstream or rear oxygen sensor replacement: $360 to $520
• Catalytic converter replacement: $450 to $4200
• Engine or Powertrain Control Module replacement: $800 to $1500