Emissions

P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

Severity: medium Safe to drive

What P0420 means

The ECU compares the oxygen sensor signals before and after the catalytic converter. With a healthy catalyst, the rear sensor signal stays fairly steady while the front one switches rapidly. If the rear signal starts mimicking the front, the ECU concludes the Bank 1 catalyst is not working efficiently and sets P0420. The cause can be the catalyst itself, but also sensors, exhaust leaks or engine problems.

Symptoms

  • Check Engine light on
  • Failed emissions or smog test
  • Rotten-egg (sulfur) smell from the exhaust
  • Slightly worse fuel economy (sometimes)
  • Mild loss of power (rare)

Possible causes

  1. Worn out or damaged catalytic converter
  2. Faulty downstream oxygen sensor
  3. Exhaust leak ahead of the rear sensor
  4. Engine misfires or a rich/lean mixture
  5. Damaged sensor wiring or connector
  6. Oil or coolant contaminating the catalyst

How to diagnose

  1. Read all codes and freeze-frame data; fix any misfire or fuel mixture codes first
  2. Inspect the exhaust for cracks, bad gaskets and leaks
  3. Check wiring and connectors of both oxygen sensors
  4. Compare upstream and downstream oxygen sensor signals on a scan tool
  5. Review short and long term fuel trims at operating temperature
  6. Look for oil or coolant consumption that could poison the catalyst
  7. Check temperature difference across the catalyst or exhaust backpressure
  8. Condemn the catalyst only after the steps above

Common fixes

  • Repair exhaust leaks
  • Replace a faulty oxygen sensor
  • Fix the cause of misfires or mixture problems
  • Replace the catalytic converter if it is confirmed worn out
  • Repair damaged sensor wiring or connector

P0420 on different brands

Ford · manuals
On Ford, P0420:00 means the OBD monitor found Bank 1 (or Y-pipe) catalyst efficiency below the threshold; it identifies the monitor condition, not a confirmed failed catalyst.

Questions and answers

Can I drive with a P0420 code?

Usually yes for a short time, as the car typically drives normally. Still, find the cause soon, because misfires or a rich mixture can ruin the catalytic converter.

Does P0420 always mean I need a new catalytic converter?

No. A bad rear oxygen sensor, an exhaust leak or a mixture problem can set it too. Diagnose first and replace the catalyst only if it is proven faulty.

Will P0420 go away by itself?

Sometimes the light turns off after several passing monitor cycles, but if the cause remains the code returns. Clearing it without a repair is not a fix.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.