P0420 Catalytic Converter Trouble Code

What is the P0420 code definition?

P0420 - Catalyst System Efficiency Below Threshold Bank 1
P0430 - Catalyst System Efficiency Below Threshold Bank 2

What does P0420 code mean?

DTC P0420 sets when the catalytic convertor fails to efficiently remove harmful pollutants from the engines exhaust gas. The catalytic converter is a vital part of the vehicles emissions control system and is used to clean up the exhaust emissions that are produced during the combustion process. The engine control module (ECM) monitors the catalytic converter using feedback from oxygen and/or air/fuel ratio sensors. With one sensor placed before the catalytic converter and one after, the ECM measures the input and output exhaust gases and determines the catalytic converters efficiency. If this efficiency level drops below the ECMs programed threshold, the ECM will set a P0420 DTC

P0420 code symptoms

• Check engine light

• Excessive exhaust smell

• Poor fuel economy

• Low engine power

What causes P0420 code?

Failed catalytic converter: A failed catalytic converter is the most common cause of a P0420 code. While some catalytic converters fail from age or high milage, the majority of failures are a byproduct from an underlying issue. The most common issue being an excessive caused by a or fuel trim problem. When an engine runs rich, the exhaust gas contains a higher amount of unburnt fuel that must get burned in the catalytic converter. This can raise the temperature in the catalytic converter enough to cause permanent damage. Catalytic converters can also fail if they become contaminated with engine oil or coolant from an engine in poor mechanical condition.

Malfunctioning oxygen (o2) or air/fuel (A/F) ratio sensor: The ECM uses the signals from the o2 and/or A/F ratio sensors to monitor the catalytic converters efficiency. If one of these sensors fails to operate correctly it will provide incorrect feedback to the ECM. This can result in the ECM falsely determining the catalytic converters efficiency and set a P0420 code.

Exhaust leaks: A leaking exhaust system causes two major problems with catalytic converters. One, it allows exhaust gas to leak out before being measured by the o2 and/or A/F ratio sensors and two, it allows outside air to be pulled into the exhaust. This skews the exhaust gas being read by the o2 and A/F ratio sensors making the exhaust gas measurement inaccurate. This results in the ECM not only mis-calculating fuel trims, but also the efficiency of the catalytic converter which can cause the ECM to set a P0420 code.

Inferior Catalytic Converter: Each catalytic converter is designed for a specific application. Catalytic converters must meet the size and oxygen storage capacity specifications that the exhaust system and P0420 test was designed for. Whether universal or direct fit, not all new catalytic convertors are created equal and may not meet the efficiency requirements resulting in a failed P0420 test.

How to diagnose a P0420 DTC?

Recommended tools

• Scan tool (Autel scan tools that show data)

• Flashlight

• Soapy water bottle

• Multimeter (Astro multimeter)

Diagnostic checks

Preliminary testing: Before starting catalytic converter diagnosis, make sure the engine is in good running order and is not misfiring. Also check the engine for an oil or coolant consumption issue. Repair any problems found before continuing P0420 diagnosis

Inspect exhaust: Visually inspect the exhaust components and catalytic converter for cracks or damage. Look for any black soot marks near manifolds and pipe connections as these are tell-tale signs of an exhaust leak. With the engine cold, start the vehicle and listen for any exhaust leaks. Spray suspect leaks with soapy water and look for bubbles to pin-point location.

Check oxygen (o2) or air/fuel (A/F) ratio sensor operation: Test the oxygen and/or A/F ratio sensors for correct signal feedback and range. With the vehicle at operating temperature, use a scan tool or multimeter to monitor the signal voltage of the sensors being tested. Then manually, or with a scan tool, force the engine rich and lean while watching the response of the o2 or A/F ratio sensors. Compare data with a known good reading or factory listed specifications, if any sensor is out-of-range continue diagnosing sensor issue.

For more detailed instruction see

Generic switch-ratio scan tool test: (Test not valid for vehicles using A/F ratio sensors) With the vehicle at operating temperature, monitor the upstream and downstream oxygen sensor signals. Observe the sensors switching rate from rich to lean and compare the cross-count difference. The upstream sensors should be much more active and switch from rich to lean at least twice as much as the downstream sensors.

For more detailed instructions to diagnose a P0420 code see

How do I repair a P0420 DTC?

While replacing the catalytic converter is often the fix for a P0420 code, it does not solve the root cause of why the catalytic converter failed and repeat failure is likely. If a bad catalytic converter is suspected, make sure to determine the root cause of failure before replacing it. This is the only way to guarantee an accurate repair and avoid a costly repeat catalytic converter failure

Can I still drive my car with a P0420 code?

It is not recommended to drive your vehicle with a P0420 code set. While your vehicle may seem to operate normally, driving your vehicle with a P0420 code can rob your engine of horsepower and fuel economy. It will also raise your vehicles emissions output and cause your vehicle to fail a state inspection or smog test.