Trouble codes › P0420

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Emissions

P0420 means the catalytic converter on bank 1 isn't cleaning the exhaust well enough, usually a worn converter, an exhaust leak or a lazy oxygen sensor.

The engine computer compares the oxygen sensor before the catalytic converter with the one after it. A working converter smooths out the exhaust, so the rear sensor reads steady. When the rear sensor starts copying the front one, the computer decides the converter on bank 1 isn't cleaning the exhaust well enough and sets P0420.

Can I keep driving? Yes, for now.

Usually safe to drive for now, but you'll fail an emissions test. If the engine also misfires, runs rough or you smell rotten eggs, fix that first: unburned fuel can overheat and ruin the converter.

Common causes

  1. A worn-out catalytic converter (most common on higher-mileage engines)
  2. An exhaust leak near the oxygen sensors, which lets outside air skew the readings
  3. A lazy or failing rear (downstream) oxygen sensor
  4. An engine problem damaging the converter: misfires, burning oil or coolant, or running rich
  5. Aftermarket or worn-out replacement converters that don't meet the original spec

Symptoms

  • Check-engine light, often with no change in how the car drives
  • Failed emissions test
  • Sometimes a rotten-egg smell, a rattle from under the car, or sluggish acceleration if the converter is clogged

Common on: Ford F-150, Toyota Camry, Honda Civic, Honda Accord, Honda CR-V, Toyota RAV4, Toyota Corolla, Toyota Tacoma, Ram 1500, Nissan Altima, Chevrolet Equinox, Jeep Grand Cherokee, Ford Escape, Ford Explorer, GMC Sierra 1500, Nissan Rogue, Jeep Wrangler, Toyota Highlander, Chevrolet Malibu, Honda Odyssey, Hyundai Elantra, Chevrolet Tahoe, Dodge Grand Caravan, Chevrolet Traverse, GMC Acadia, Chevrolet Suburban, GMC Yukon, Ford Expedition, Honda Pilot, Nissan Sentra, Dodge Durango

What to check first

  1. Check for other codes, especially misfires (P0300-P0308) or fuel trim codes (P0171, P0172). Fix those first.
  2. Listen and feel for exhaust leaks between the engine and the rear oxygen sensor (ticking on cold start, soot marks at joints).
  3. With a scan tool, watch both oxygen sensors at a steady 2,000-2,500 rpm: the front should switch quickly, the rear should stay fairly steady. A rear sensor that switches like the front points to the converter.
  4. Look up service bulletins and extended emissions warranties for your vehicle: converters are often covered for 8 years or 80,000 miles in the US.
  5. If the converter is replaced, use one rated for your vehicle (CARB-approved in California and some other states).

What does P0420 mean on your vehicle?

The likely cause depends on your engine. iRepair researches your exact year, make, model and engine (service bulletins, recalls and owner reports) and tells you what to check first. Your first 3 questions are free.

Ask iRepair about P0420

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Related codes

General guidance for P0420 on most vehicles. Some makers use the same code a little differently; confirm in the service information for your vehicle, put safety first, and see a professional when in doubt.