# P0420: Catalyst System Efficiency Below Threshold (Bank 1) **Verdict:** Drive to a shop this week Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $150-$2,800 - **Urgency:** This week - **Most likely:** Worn catalytic converter or rear O2 sensor - **DIY?:** Steps 1-2 ## Quick answer P0420 means the engine computer has decided the catalytic converter on bank 1 is no longer cleaning the exhaust as well as it should. It judges this by comparing the oxygen sensors before and after the converter. A worn-out converter is the classic cause, but an exhaust leak, a lazy rear oxygen sensor, or an engine problem like misfires can also set it. ## What does P0420 mean? The catalytic converter uses precious metals such as platinum, palladium, and rhodium to turn harmful exhaust gases into water vapor, carbon dioxide, and nitrogen. A healthy converter also stores oxygen and releases it as needed, which smooths out the oxygen content of the exhaust leaving it. Bank 1 is the side of the engine with cylinder 1. A four-cylinder engine has only one bank, so P0420 is the converter code it would use. The engine computer judges the converter with two oxygen sensors. Sensor 1 sits ahead of the converter and switches rapidly as the fuel mixture swings rich and lean. Sensor 2 sits after it and should stay fairly steady, because a working converter evens out the oxygen. As the converter wears, sensor 2 starts copying sensor 1. The catalyst test usually runs once per trip with a fully warm engine, often at steady cruise. If efficiency falls below the maker's threshold, a pending code sets, and on most cars the light comes on after a second failed trip. ## What are the symptoms of P0420? - Check engine light on, car usually drives normally - Failed emissions test - Rotten-egg (sulfur) smell from the exhaust on some cars - Sluggish acceleration if the converter is partly clogged ## What causes P0420? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Worn-out catalytic converter** (most common). The precious-metal coating inside the converter gradually loses its ability to store oxygen and convert gases, usually over many years and high mileage. Short trips, heat, and small engine problems speed this up. On a scan tool, the rear oxygen sensor starts switching almost as fast as the front one, which is the classic sign. 2. **Exhaust leak near the oxygen sensors** (common). A leak at a manifold gasket, flex pipe, cracked manifold, or loose clamp can pull outside air into the exhaust between pulses. That extra oxygen fools the oxygen sensors and can make a good converter look weak. Leaks are more common on older cars in road-salt states and often tick or puff on a cold start. 3. **Failing rear (downstream) oxygen sensor** (common). An aging or contaminated downstream oxygen sensor can respond slowly or read wrong, making the converter look bad when it is not. Oil or coolant getting into the exhaust shortens sensor life. Because a sensor costs far less than a converter, a careful shop tests its response and wiring before recommending a new converter. 4. **Engine problem damaging the converter** (less common). Misfires, a rich fuel mixture, or burning oil or coolant send unburned fuel or contaminants into the converter, where they overheat it or coat the precious metals. This can melt or clog the inside and will ruin a new converter too. Check for misfire, fuel trim, and coolant loss problems before replacing anything. ## How do you diagnose P0420? 1. Check for other codes, especially misfire or fuel trim codes, and fix those first. 2. Look and listen for exhaust leaks ahead of and around the rear oxygen sensor. 3. Compare front and rear oxygen sensor readings on a scan tool at steady cruise. A healthy rear sensor stays fairly steady. 4. Check the rear sensor's response and the converter's temperature difference from inlet to outlet if needed. ## How do you fix P0420, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Repair exhaust leak | $100-$400 | Gaskets, flex pipes, or clamps near the sensors. | | Replace rear oxygen sensor | $150-$400 | Worth doing if the sensor is old or tests slow. | | Replace catalytic converter | $900-$2,800 | Aftermarket direct-fit parts cost less. CARB-compliant converters required in California and some other states cost more. | Full price breakdowns: [catalytic converter replacement cost](/cost/catalytic-converter-replacement), [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement). ## Is it safe to drive with P0420? > **Drive to a shop this week.** It is usually safe to drive for a few weeks while you sort out the cause. If the car loses power, smells strongly of sulfur, or the light starts flashing from a misfire, get it checked right away, because a clogged or overheating converter can cause further damage. ## Will P0420 fail an emissions test? P0420 with the check engine light on will fail an OBD-based emissions test in most states. Clearing it does not help, because the catalyst monitor resets to not ready and can take days of driving to complete. In California and states that follow its rules, a replacement converter must be CARB-approved for your exact vehicle to pass. ## After the repair: clearing P0420 After the repair, clear the code and drive normally, including some steady highway cruising once the engine is fully warm. The catalyst monitor often needs several trips to finish. The repair worked when the monitor shows complete with no pending code, and on a scan tool the rear oxygen sensor stays fairly steady while the front sensor switches. ## How to prevent P0420 - Fix misfires right away, and never drive far with a flashing check engine light. - Repair oil burning and coolant leaks early, since both can coat and poison the converter. - Keep up with spark plugs and ignition parts so unburned fuel does not reach the converter. - Fix exhaust leaks near the oxygen sensors before they cause false readings or a failed test. - Investigate rich-running codes such as P0172 promptly, because extra fuel overheats the converter. ## Codes often seen with P0420 - [P0430](/codes/p0430): Catalyst System Efficiency Below Threshold (Bank 2) - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0171](/codes/p0171): System Too Lean (Bank 1) - [P0172](/codes/p0172): System Too Rich (Bank 1) - [P0133](/codes/p0133): O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) - [P0141](/codes/p0141): O2 Sensor Heater Circuit (Bank 1 Sensor 2) ## Frequently asked questions ### Can I drive with a P0420 code? Yes, in most cases you can drive normally with P0420 for a while. You will fail an emissions inspection, and you should rule out engine problems such as misfires that could damage the converter further. Get it checked right away if power drops or the check engine light starts flashing. ### Does P0420 always mean I need a new catalytic converter? No, an exhaust leak or a bad rear oxygen sensor can cause P0420 on a car with a working converter. A shop should check those first, along with misfire and fuel trim problems. Ask how the shop confirmed the converter is bad before agreeing to a converter quote. ### Will a catalytic converter cleaner fix P0420? Additives rarely fix a worn converter, though they may help briefly if the problem is light deposits. If the converter's coating is used up, replacement is the lasting fix. Be wary of products promising a guaranteed pass, since the code often returns once any short-term effect wears off. ### Is the catalytic converter covered by warranty? Federal law requires the catalytic converter to be covered for 8 years or 80,000 miles on most cars sold in the US. California and some other states have longer coverage for certain vehicles. Check your warranty booklet, and keep service records, since damage from neglect such as ignored misfires may not be covered. ### How much does it cost to fix P0420? Most P0420 repairs cost $150 to $2,800 at an independent shop. Fixing an exhaust leak runs about $100 to $400 and a rear oxygen sensor $150 to $400. A new catalytic converter costs $900 to $2,800, with CARB-compliant parts and manifold-mounted converters at the top of that range. ### Can I fix P0420 myself? You can handle the first steps yourself, such as reading codes, looking for exhaust leaks, and replacing an easy-to-reach rear oxygen sensor with a sensor socket. Converter replacement is possible on some bolt-on systems, but rusted bolts and welded joints usually make it a shop job. Confirm the diagnosis before buying a converter. ### How long can I drive with P0420? Many cars run normally for months with P0420, but there is no safe fixed number. The risk is that an underlying problem, such as a misfire or oil burning, keeps damaging the converter until it clogs. A clogged converter causes power loss and overheating, so find the cause soon. ### What damages a catalytic converter? Overheating and contamination are the main things that damage a converter. Misfires and rich running overheat it, while burning oil or coolant coats the precious metals. Silicone sealants not labeled as sensor-safe can also contaminate the oxygen sensors and converter. Normal wear over many years and miles is the other common cause. ### Why did P0420 come back after replacing the catalytic converter? It usually comes back because the original cause was never fixed, such as an exhaust leak, a lazy rear sensor, or an engine problem. Some low-cost aftermarket converters also hold less precious metal and may not satisfy every engine computer. Ask for a direct-fit part approved for your vehicle and state. ### Will P0420 go away on its own? Sometimes the light goes off for a while if the converter passes its test on later trips, but a worn converter usually triggers the code again. The computer keeps a record of the code for a while after the light goes off. If it returns more than once, the converter or a sensor needs attention. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0402 Exhaust Gas Recirculation "A" Flow Excessive Detected](https://diagxcar.com/codes/p0402) - [P0403 Exhaust Gas Recirculation "A" Control Circuit](https://diagxcar.com/codes/p0403) - [P0404 Exhaust Gas Recirculation "A" Control Circuit Range/Performance](https://diagxcar.com/codes/p0404) - [P0405 Exhaust Gas Recirculation Sensor "A" Circuit Low](https://diagxcar.com/codes/p0405) - [P0406 Exhaust Gas Recirculation Sensor "A" Circuit High](https://diagxcar.com/codes/p0406) - [P0410 Secondary Air Injection System "A"](https://diagxcar.com/codes/p0410) - [P0411 Secondary Air Injection System "A" Incorrect Flow Detected](https://diagxcar.com/codes/p0411) - [P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0421) - [P0430 Catalyst System Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0430) - [P0431 Warm Up Catalyst Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0431) - [P0108 Manifold Absolute Pressure/Barometric Pressure Circuit High](https://diagxcar.com/codes/p0108) - [P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0133) - [P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0134) - [P0136 O2 Sensor Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0136) ## Symptoms - [Why is blue smoke coming from my exhaust?](https://diagxcar.com/symptoms/blue-smoke-from-exhaust) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car lose power when accelerating?](https://diagxcar.com/symptoms/car-loses-power-when-accelerating) - [Why does my car smell like rotten eggs?](https://diagxcar.com/symptoms/car-smells-like-rotten-eggs) ## Repair costs - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 ## Warning lights linked to this - [Check engine light](https://diagxcar.com/warning-lights/check-engine) ## Terms explained - [What is Catalytic converter?](https://diagxcar.com/glossary/catalytic-converter) - [What is Check engine light?](https://diagxcar.com/glossary/check-engine-light) - [What is DTC?](https://diagxcar.com/glossary/dtc) - [What is OBD-II?](https://diagxcar.com/glossary/obd-ii) --- Source page: https://diagxcar.com/codes/p0420 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.