P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter on bank 1 is not cleaning exhaust as well as the computer expects.
Severity: 2 of 4
Usually safe to drive for a short time if the light is steady and the car runs normally. Fix the cause before it gets worse, because a converter that is overheating from misfires can become a fire risk and will fail completely. If the light flashes, or you smell strong sulfur or see a glowing red converter, park it.
Estimate range covers the repairs listed below; see the cost breakdown. A flashing check engine light, a hot temperature gauge, or brakes that feel different mean stop driving and get help.
What P0420 means
Your engine has two oxygen sensors around the catalytic converter: one before it and one after it. A healthy converter stores and uses oxygen, so the rear sensor reads a steady signal while the front one switches rapidly. When the rear sensor starts copying the front sensor, the computer concludes the converter is no longer doing its job and sets P0420.
Bank 1 is the side of the engine with cylinder number 1. On a four cylinder engine there is only one bank, so bank 1 simply means the engine.
The code does not always mean the converter is dead. A lazy rear oxygen sensor, an exhaust leak near the sensors, or an engine problem that sends unburned fuel into the converter can all trigger it.
Common causes (cheapest likely first)
- Exhaust leak before or near the rear oxygen sensor. A cracked flex pipe, loose flange, or failed gasket lets outside air in and fools the sensor. Often a cheap gasket or clamp.
- Aging or slow rear (downstream) oxygen sensor. A tired sensor can report a misleading signal. Worth testing before buying a converter.
- Engine problem damaging the converter. Misfires, oil burning, or a coolant leak into the cylinders poisons or overheats the catalyst. Fix these first or a new converter will fail too.
- Worn front oxygen sensor. If the upstream sensor is slow, fuel control suffers and the converter test can fail.
- Failed catalytic converter. The most expensive cause and usually the last one to confirm. Common on high mileage vehicles.
Symptoms
- Check engine light, usually steady
- Often no drivability change at all
- Rotten egg or sulfur smell from the exhaust
- Reduced power or a rattling heat shield if the converter is breaking apart inside
- Failed emissions inspection
DIY difficulty: Moderate
Checking for exhaust leaks and swapping an oxygen sensor is a reasonable driveway job. Replacing a converter is often bolt on but rusted hardware can make it a shop job, and in some states you must buy a converter certified for your vehicle.
Tools: OBD-II scanner with live data, Oxygen sensor socket, Penetrating oil, Jack and jack stands, Anti seize compound
Estimated repair cost
This is an estimate, built from typical parts price ranges plus labor hours times an assumed shop rate of $110 to $190 per hour. Actual prices vary by vehicle, region, and shop. Diagnosis fees are often extra.
| Repair | Parts | Labor | Estimate (low / high) |
|---|---|---|---|
| Rear oxygen sensor | $30 to $120 | 0.3 to 1.0 h | $30 + 0.3 × $110 = $63 $120 + 1.0 × $190 = $310 |
| Exhaust gasket or flex pipe repair | $15 to $150 | 0.5 to 1.5 h | $15 + 0.5 × $110 = $70 $150 + 1.5 × $190 = $435 |
| Catalytic converter (aftermarket, direct fit) | $250 to $1,400 | 1.0 to 2.5 h | $250 + 1.0 × $110 = $360 $1,400 + 2.5 × $190 = $1,875 |
Watch a repair video
P0420 How To Diagnose A Bad Catalytic Converter (EricTheCarGuy). Third-party video, loads from YouTube (privacy-enhanced mode) only when you press play. Procedures vary by vehicle. Search YouTube for “P0420 fix” videos.
P0420 by make
Brand specific notes: Toyota, Honda, Ford, Chevrolet, Nissan, Hyundai, Kia, Subaru, Mazda, Lexus. Other makes: the code works the same way; see all makes.
Repair manuals
Step-by-step service manuals with torque specs and wiring diagrams:
Haynes manuals Chilton manuals
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Related codes
Check for recalls
Before paying for a repair, enter your VIN in the NHTSA recall lookup. Emissions parts may also be covered by a federal emissions warranty; check your warranty booklet.
FAQ
Can a P0420 code go away by itself?
It can turn off after several drive cycles if the converter test passes, but the underlying weakness usually returns. Clearing the code without a fix will not get you through an inspection because the readiness monitor will show incomplete.
Will a catalytic converter cleaner fix P0420?
Additives rarely fix a converter that is truly worn out. They are cheap, so some people try one, but do not expect a lasting repair.
Should I replace the oxygen sensor or the converter first?
Check for exhaust leaks and engine faults first, then look at the rear sensor with live data. Replace the converter only when the other causes are ruled out.
Is an aftermarket converter good enough?
A quality direct fit converter is usually fine, but some states such as California require a specific approved part. Check your local rules before you buy.