P0420: Catalyst System Efficiency Below Threshold (Bank 1)

What does P0420 mean?

What are the symptoms of P0420?

What causes P0420?

• Aged or failed catalytic converter, the substrate's coating has degraded so it can no longer convert exhaust gases efficiently

  : high

• Worn downstream (post-cat) oxygen sensor, a lazy Bank 1 Sensor 2 starts mirroring the upstream sensor and mimics a dead converter

  : medium

• Exhaust leak before or near the converter, drawn-in air skews the oxygen readings and makes catalyst efficiency look low

  : medium

• Engine misfire or rich-running condition, raw fuel reaches the converter, overheats it and destroys the substrate over time

  : medium

• Dirty or failing MAF sensor, incorrect fuel trims cause rich or lean running that the converter cannot clean up

  : low

• Oil or coolant burning into the exhaust, contaminants coat and poison the catalyst substrate

  : low

Is it safe to drive with P0420?

How do you diagnose P0420?

How much does P0420 cost to fix?

P0420 on specific vehicles

Related diagnostic codes

• P0430
• P0421
• P0422
• P0141
• P0136

Common questions about P0420

What does the P0420 code mean?

P0420 stands for Catalyst System Efficiency Below Threshold (Bank 1). The engine computer compares the oxygen sensors before and after the catalytic converter and has decided the converter on Bank 1 is no longer cleaning the exhaust efficiently enough. Bank 1 is the side of the engine that contains cylinder 1. The most common reason is a worn-out catalytic converter.

What are the symptoms of P0420?

Usually the only symptom is the check engine light, plus slightly worse fuel economy. Some drivers notice a faint sulfur or rotten-egg smell. The car normally drives the same, but at a periodic inspection the lit warning light can be recorded as a deficiency, and a badly clogged converter can cause sluggish acceleration.

What causes a P0420 code?

The leading cause is an aged catalytic converter whose internal coating has worn out. Other causes include a lazy downstream O2 sensor that starts reading like the upstream one, an exhaust leak before the converter that skews sensor readings, a misfire or rich-running condition that overheats and damages the converter, and a dirty MAF sensor throwing off fuel trims.

Is it safe to drive with a P0420 code?

Yes, for the short term it is generally safe because the car still runs normally. The catch is that if the underlying cause is a misfire or rich running, continued driving can finish off the converter. You should also expect it to count against you at an emissions inspection until the code is fixed, so do not leave it unresolved for long.

How do you fix and clear a P0420 code?

Fix the actual cause first. Test the downstream O2 sensor and check for exhaust leaks and misfires before assuming the converter is dead, since those are far cheaper. If the converter itself has failed, it must be replaced. After the repair the code can be cleared with a scan tool, then a full drive cycle lets the catalyst monitor re-run and confirm the fix held.

How much does it cost to fix P0420?

It depends on the cause. A downstream O2 sensor is usually 50 to 200 EUR fitted. Repairing an exhaust leak is often around 100 to 300 EUR. Replacing the catalytic converter is the expensive one, typically 400 to 1800 EUR or more, and higher again on cars where the converter is built into the exhaust manifold.

Read our detailed guides

• Next stepHow to Find Out What Is Wrong With Your Car for 15 EurosA 15 euro Bluetooth adapter and a free phone app read the exact fault code a shop charges 30 to 100 euros to pull. Here is how to do it yourself in 5 minutes.
• How Much Does P0420 Cost to Fix? Catalytic Converter GuideP0420 repair cost runs from 150 euros for an oxygen sensor to 2,500 euros or more for a catalytic converter. Here is how to work out which repair you need.
• Catalytic Converter: Symptoms, Causes, and What a Bad One CostsA failing catalytic converter throws P0420, smells of rotten eggs, and fails the MOT. Here is how to read the symptoms, codes, and real repair cost.
• Catalytic Converter Replacement Cost (And Whether You Need One)An aftermarket catalytic converter runs 350 to 1,200 euros and an OEM one 1,000 to 2,500 and up, but a P0420 code often means a 100 to 300 euro sensor instead.
• Car Smells Like Rotten Eggs: What the Sulphur Smell MeansA rotten egg smell means hydrogen sulphide is getting past the catalytic converter. Why the converter is usually the victim, and when the smell is urgent.
• Check Engine Light Flashing? Stop Driving and Read This FirstA flashing check engine light means the engine is misfiring right now. Pull over and read the code: the fix is usually $80 to $400, the delay costs $1,500 up.
• Car Dashboard Symbols and Meanings: Warning Lights ExplainedAn unfamiliar symbol just lit up and you need to know whether to stop. Read the colour first, then find your symbol in the table: every one carries its verdict.