Powertrain, generic · Auxiliary emission controls

P0420 Catalyst System Efficiency Below Threshold Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

The engine module checks whether the catalyst of bank 1 still converts enough. On petrol engines it estimates the oxygen storage capacity by comparing the sensors before and after the catalyst – a good catalyst smooths the rear signal. GM uses a test on overrun (ratio below 0.35 sets the code on the first trip), Ford an index ratio above 0.75 or fuel integration after fuel cut-off. On diesels P0420 concerns the oxidation catalyst, checked by its heat release during filter regeneration (GM Duramax LML, Ford below 45 % of the expected heat). Ford names oil-borne poisoning as the most common cause.

Likely causes

Catalyst aged or poisoned (oil consumption, silicone, coolant)
Catalyst overheated by misfire or rich running
Rear oxygen sensor sluggish, dirty or unsuitable
Exhaust leak ahead of or at the rear sensor
Diesel: oxidation catalyst sooted or poisoned, post-injection faulty

Symptoms

  • Check engine light on, otherwise often no change in driving
  • Emission test failed (higher HC, CO, NOx)
  • Rattling or loss of power if the catalyst has broken up

Repair estimate

DifficultyMediumDIY possible
Parts and labour€80–1,800
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Fix any misfire or fuel trim problem first
  2. Check for exhaust leaks
  3. Compare upstream and downstream O2 signals
  4. Replace the converter only after other causes are ruled out

Parts involved

catalytic converterdownstream oxygen sensorupstream oxygen sensorengine control module

Related codes

P0430P0421P0422P0136P2270P0300

References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk, fordservicecontent.com.