Emission Control

P0420Catalyst System Efficiency Below Threshold (Bank 1)

Medium RiskDifficulty:Moderate1-3 hours
Is it safe to drive?
Yes, drivable

If the engine is healthy, it doesn't stop you driving in the short term. But if the cause is a misfire or rich mixture, fix that first, otherwise a new catalyst will die quickly too.

Will it pass inspection?
Risky

If catalyst efficiency is low, HC/CO values can exceed limits in the emissions test. A car with the catalyst removed or hollowed out fails the emissions test.

01 / 06

What does P0420 mean?

The rear oxygen sensor sees that the catalyst is no longer cleaning the exhaust well enough. But note: P0420 does not mean 'the catalyst is definitely dead'. A faulty rear lambda sensor, an exhaust leak before the catalyst or engine misfiring/oil burning also trigger this code. Rule out these three before paying for an expensive catalyst.

The ECU compares the signals of the oxygen sensors before and after the catalyst. Because a healthy catalyst stores oxygen, the rear sensor signal is flat and calm. When the rear sensor starts fluctuating like the front one, the catalyst's oxygen storage capacity has dropped and P0420 is stored.

02 / 06

How to fix P0420

Steps are ordered from cheapest to most expensive. If a step doesn't solve it, move to the next one, and don't let a garage replace parts blindly.

  1. 1

    Check for other codes

    DIY

    If there are codes like P0300, P0171 or P0172, fix those first. They are usually what kills the catalyst.

  2. 2

    Compare front and rear lambda signals

    DIY

    If the rear sensor fluctuates at the same rate as the front in live data, the catalyst is weak; if the rear sensor doesn't respond at all, suspect the sensor.

  3. 3

    Exhaust leak check

    Pro job

    Check the manifold, flex pipe and catalyst joints. A leak makes the rear sensor read incorrectly.

  4. 4

    Test / replace the rear oxygen sensor

    Pro job

    Much cheaper than a catalyst and fails often, especially on cars above 150,000 km.

  5. 5

    Catalyst replacement (type-approved)

    Pro job

    If every other step checks out, the catalyst gets replaced. Choose a type-approved unit for inspection and emissions; removing the catalyst is illegal and will make you fail inspection.

Don't replace these first

  • Catalyst (without proper diagnosis)
  • Front oxygen sensor

These parts are often replaced unnecessarily for P0420. Don't spend money on them before doing the checks above.

03 / 06

P0420 symptoms

  • Check engine light (often the only symptom)
  • Rotten egg (sulphur) smell from the exhaust
  • Slight loss of performance at a later stage
  • With a clogged catalyst, loss of power uphill and overheating on the exhaust side

04 / 06

What causes P0420?

  • Catalyst worn out / poisonedHigh
  • Faulty rear oxygen sensorMedium
  • Exhaust leak ahead of the catalystMedium
  • History of misfires, oil burning or rich mixtureMedium
  • Constant rich/lean swings due to a poor LPG tuneLow

05 / 06

Which cars commonly get P0420?

06 / 06

Frequently asked questions

What does fault code P0420 mean?

P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1)”. The rear oxygen sensor sees that the catalyst is no longer cleaning the exhaust well enough. But note: P0420 does not mean 'the catalyst is definitely dead'. A faulty rear lambda sensor, an exhaust leak before the catalyst or engine misfiring/oil burning also trigger this code. Rule out these three before paying for an expensive catalyst.

Can I drive with code P0420?

Yes, drivable. If the engine is healthy, it doesn't stop you driving in the short term. But if the cause is a misfire or rich mixture, fix that first, otherwise a new catalyst will die quickly too.

Will a car with code P0420 pass vehicle inspection?

Risky. If catalyst efficiency is low, HC/CO values can exceed limits in the emissions test. A car with the catalyst removed or hollowed out fails the emissions test.

Do catalyst cleaner additives work for P0420?

They can give a short-term improvement on a lightly fouled catalyst, but won't save one with a melted or poisoned substrate. Not a permanent fix.

Is it a problem if I have the catalyst removed?

Removing the catalyst is illegal, makes you fail the emissions test and requires ECU software tampering. Not recommended.

Related codes

P0300P0171P0172