P0420 Code: Meaning, Symptoms, Causes & Fixes
Reviewed by the Wrenchit.ai editorial team · Last reviewed · Editorial standards
Causes ranked by likelihood
| Cause | Likelihood | DIY difficulty |
|---|---|---|
| Failing catalytic converter — Most common on vehicles with 100k+ miles, especially after upstream misfires. | Most likely | 3/5 |
| Faulty downstream O2 sensor — The rear sensor reports cat efficiency. A lazy sensor mimics a bad cat. | Possible | 3/5 |
| Exhaust leak before the cat — Leaks throw off the O2 reading and trigger P0420 falsely. | Less likely | 3/5 |
| Engine misfire damaging cat — Long-term P03xx misfires destroy cats from the inside. | Less likely | 3/5 |
Symptoms of P0420
- Check engine light on
- Reduced fuel economy
- Sulfur (rotten egg) smell
- Sluggish acceleration
How to diagnose P0420
- Confirm the code. Read P0420 with an OBD-II scanner and note the freeze-frame data (speed, temperature, load) recorded when it set.
- Check for companion codes. Look for other stored or pending codes such as P0430, P0171, P0300 — they often point to one shared root cause.
- Check: Exhaust leak before the cat. Leaks throw off the O2 reading and trigger P0420 falsely.
- Check: Faulty downstream O2 sensor. The rear sensor reports cat efficiency. A lazy sensor mimics a bad cat.
- Check: Failing catalytic converter. Most common on vehicles with 100k+ miles, especially after upstream misfires.
- Check: Engine misfire damaging cat. Long-term P03xx misfires destroy cats from the inside.
- Clear and verify. After the repair, clear the code and complete a full drive cycle to confirm P0420 does not return.
Can I drive with P0420?
Yes, you can usually drive several hundred miles with P0420, but you'll fail emissions testing and a damaged cat can cause power loss or further sensor damage.
Related codes
Related symptoms
P0420 by vehicle
- P0420 on the Ford F-150
- P0420 on the Ford Escape
- P0420 on the Ford Explorer
- P0420 on the Ford Focus
- P0420 on the Ford Mustang
- P0420 on the Chevrolet Silverado
- P0420 on the Chevrolet Equinox
- P0420 on the Chevrolet Malibu
- P0420 on the Chevrolet Tahoe
- P0420 on the Chevrolet Traverse
- P0420 on the Chevrolet Cruze
- P0420 on the Toyota Camry
- P0420 on the Toyota Corolla
- P0420 on the Toyota RAV4
- P0420 on the Toyota Highlander
- P0420 on the Toyota Tacoma
- P0420 on the Toyota Prius
- P0420 on the Honda Civic
- P0420 on the Honda Accord
- P0420 on the Honda CR-V
- P0420 on the Honda Pilot
- P0420 on the Honda Odyssey
- P0420 on the Nissan Altima
- P0420 on the Nissan Rogue
- P0420 on the Nissan Sentra
- P0420 on the Jeep Grand Cherokee
- P0420 on the Jeep Wrangler
- P0420 on the Jeep Cherokee
- P0420 on the Hyundai Elantra
- P0420 on the Hyundai Sonata
- P0420 on the Hyundai Santa Fe
- P0420 on the Hyundai Tucson
- P0420 on the Kia Forte
- P0420 on the Kia Sorento
- P0420 on the Kia Sportage
- P0420 on the Kia Telluride
- P0420 on the Kia Optima
- P0420 on the Subaru Outback
- P0420 on the Subaru Forester
- P0420 on the GMC Sierra
- P0420 on the GMC Acadia
- P0420 on the Ram 1500
- P0420 on the Dodge Charger
- P0420 on the Volkswagen Jetta
- P0420 on the Mazda CX-5
- P0420 on the BMW 3 Series
- P0420 on the BMW X5
- P0420 on the Mercedes-Benz C-Class
Frequently asked questions
What does P0420 mean?
P0420 means catalyst system efficiency below threshold (bank 1). P0420 means your catalytic converter on Bank 1 is no longer cleaning exhaust gases efficiently. The most common cause is a worn catalytic converter, but failing oxygen sensors and exhaust leaks trigger it too.
Can I drive with code P0420?
Yes, you can usually drive several hundred miles with P0420, but you'll fail emissions testing and a damaged cat can cause power loss or further sensor damage.
What is the most common cause of P0420?
Failing catalytic converter — Most common on vehicles with 100k+ miles, especially after upstream misfires.
What are the symptoms of P0420?
Check engine light on, Reduced fuel economy, Sulfur (rotten egg) smell, Sluggish acceleration.
How much does it cost to fix P0420?
A typical shop repair range is $200–$2,500, depending on which cause it turns out to be. Cheaper causes such as engine misfire damaging cat should be ruled out first.