EngineLightCodes
Powertrain code

P0420 Catalyst System Efficiency Below Threshold Bank 1

Severity
Fix soon
Typical repair cost
$90–$1,980
Do it yourself
Possible, medium
Shop time
0.5–3 hours

Your catalytic converter isn't cleaning the exhaust as well as it should on bank 1. Sometimes the converter is simply worn out, but an exhaust leak or a lazy oxygen sensor can trigger the same code for much less money.

How the engine computer detects it

The engine module checks whether the catalyst of bank 1 still converts enough. On gasoline 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.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

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

Common causes

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

How to diagnose P0420

  1. Fix any other codes first, especially misfires and fuel trim codes. They are a common reason converters fail.
  2. Check the exhaust for leaks in front of and around the rear oxygen sensor. A leak lets in air and fools the sensor.
  3. With a scanner, watch the rear oxygen sensor at a steady 2,500 rpm. A good converter keeps it steady; if it switches like the front sensor, the converter is weak.
  4. If the engine burns oil or coolant, find out why before replacing the converter, or the new one will fail too.
  5. Replace the converter with one approved for your state. California and some other states require CARB-certified parts.

Vehicles that report P0420 most

VehicleYearsReportsMedian miles
Kia Soul2012–202189119k
Nissan Altima2002–20051990k
Ford Focus2012–201716—
Honda Odyssey1999–20191291k
Hyundai Tucson2005–202012—
Honda Accord1998–201910107k
Honda Pilot2016–201910—
Ford F-1502006–20209—
Chevrolet Equinox2010–20159117k
Chevrolet Traverse2010–20219—

Counts of owner complaints to the NHTSA that mention P0420 (380 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.

Reading the code

P
Powertrain
0
Generic (SAE) code
4
Auxiliary emission controls
20
Specific fault

Questions drivers ask

Can I drive with code P0420?

Yes, the car usually drives normally. It will likely fail an emissions inspection in states that scan the OBD system, and if the converter is breaking apart you may hear rattling or lose power. Fix any misfire first, because misfires destroy converters.

Will my car pass an emissions inspection with P0420?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0420 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0420 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.

Often reported together

Codes that owners mention in the same NHTSA complaints as P0420. They can point to a shared cause, so it's worth checking them together.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.