P0420 code is one of the most common OBD-II trouble codes, and it points to an issue with your catalytic converter’s efficiency.
Sometimes it’s a minor problem you can fix for under $50. Other times, it means a $1,000+ repair. The trick with these types of code is knowing how to diagnose it properly so you don’t waste money replacing the wrong part.
What is P0420 Code
P0420 is the “Catalyst System Efficiency Below Threshold (Bank 1)” code.
In simpler terms, the oxygen sensor after the catalytic converter is reporting similar readings to the one before it, which tells your car’s computer the converter isn’t cleaning exhaust gases as well as it should.
- Bank 1 refers to the side of the engine that contains cylinder number one.
- The code can be triggered by problems with the catalytic converter itself or by upstream issues in the engine or exhaust system.
What Are the Symptoms of a P0420 Code?
While some drivers notice little change, here are the most reported symptoms:
- Check Engine Light: Often the only sign.
- Reduced Power: Acceleration may feel sluggish/delayed.
- Poor Fuel Economy: Extra fuel is burned to compensate.
- Sulfur or Rotten Egg Smell: From unburned fuel in the exhaust.
- Failed Emissions Test: High hydrocarbon or NOx levels.
Most Common Causes of P0420 Code
Here are some of the most common causes of P0420 engine code and the average cost to fix it:
| Cause | Typical Repair Cost (USD) |
| Failing catalytic converter | $500-$2,500 |
| Faulty oxygen sensor (upstream or downstream) | $150-$400 |
| Exhaust leaks (manifold, pipes) | $100-$600 |
| Engine misfire (spark plugs, coils) | $100-$500 |
| Fuel injector leak or high fuel pressure | $150-$800 |
| Coolant temp sensor malfunction | $80-$250 |
| Incorrect fuel type (leaded gas in an unleaded engine) | Flush and refill, $50-$150 |
Source: Data collected from online discussion forums based on car owner’s expereince
How to Diagnose P0420
Tools you might need:
- OBD-II scanner
- Basic socket set
- Smoke tester (for exhaust leaks)
- Infrared thermometer
- Safety gloves and glasses
DIY/Professional Steps to Fix P0420 Code:
- Check for Other Codes First: Misfire codes (P0300-P0306) or fuel system codes should be fixed before tackling P0420.
- Inspect for Exhaust Leaks: Look for cracks, holes, or loose connections before the catalytic converter.
- Test Oxygen Sensors: Using live data, compare the upstream and downstream readings. If they’re too similar, efficiency is low.
- Check Engine Performance: Misfires, poor combustion, or rich/lean conditions can cause premature converter failure.
- Measure Converter Temperature: Outlet should be hotter than the inlet by at least 100°F after warm-up.
How to Fix P0420 Code
Start with the simplest, lowest-cost options:
- Clear the Code and Road Test: Sometimes, a one-off condition sets the code.
- Replace the Faulty Oxygen Sensor: If testing shows it’s not reading correctly.
- Repair Exhaust Leaks: Seal cracks or replace damaged sections.
- Address Engine Problems: Fix misfires, replace spark plugs, and repair fuel issues.
- Replace Catalytic Converter: Only after confirming all other causes. Use OEM or high-quality aftermarket parts.
Can I Still Drive With Code P0420?
Technically, yes, especially if the car runs normally. But driving too long with the P0420 code active can:
- Damage the converter beyond repair
- Cause your car to fail emissions testing
- Lead to poor performance and higher fuel costs
If the check engine light is flashing, stop driving and get the vehicle inspected immediately. That’s a sign of active misfire, which can destroy a converter in minutes.
You Might Also Be interested In: How to Check A Used Car’s History
Preventing P0420
- Follow your vehicle’s maintenance schedule
- Use high-quality fuel and the correct octane
- Fix engine problems early (especially misfires)
People Also Ask
What is Bank 1?
Bank 1 is the side of the engine with cylinder number one. On inline engines, there’s only one bank. On V-type engines, check your manual to locate it.
Will fuel additives fix P0420 code?
Some products may temporarily improve readings, but they won’t repair a physically damaged converter.
How much does it cost to fix P0420 code?
According to RepairPal, the average cost for a Catalytic Converter Replacement is between $2,123 and $2,459. However, the prices may vary depending on the location and other factors.
Final Thoughts
I’ve seen plenty of drivers replace a catalytic converter only to find the real problem was a misfire or a $200 oxygen sensor.
The P0420 code isn’t always bad news, but it’s rarely a code you can ignore. If you work through the simple checks first, you can save yourself a lot of money and guesswork. And if you’re not comfortable diagnosing it yourself, a good shop with the right tools and an experienced mechanic can pinpoint the issue before you spend anything on the wrong part.
Disclaimer: This content is for informational purposes only. Always consult a qualified mechanic for replacements or any serious concerns.
6 Responses
Also it says “HCAT 🚫”
And “AIR🚫”
What does this mean as well if you can please help me out on this one
HCAT🚫 refers to the heated catalytic converter monitor. The 🚫 usually means it has not passed or is not ready yet. Since you already have a P0420 code, this makes sense. The system is seeing that the converter isn’t performing properly, so it won’t mark it as passed.
AIR 🚫: This might be related to the A/C system like low refrigerant level, faulty compressor, or even system overheating. To get to the bottom of the issue, I recommend having the system inspected by a mechanic or an A/C specialist. They can check for any electrical or mechanical problems.
Hi my Camry 2014 hybrid has code p 04620 it show bellow threshold bank 1 which sensor need to be change is 1 or sencore 2
Hi Marc, P0420 on a 2014 Camry Hybrid doesn’t automatically mean a sensor needs replacing. If a sensor is bad, it’s usually the downstream O2 sensor (Sensor 2, after the catalytic converter). But make sure there are no exhaust leaks or misfire issues first, since replacing sensors alone often doesn’t fix this code.
My 2008 Honda Civic hybrid is showing the P0420 code on the obd11 scanner. And says
“Catalyst monitor b1-fail
Evap system leak test- fail. What does all this mean and what should I do? Anybody got any answers for me please
Hi Johnny,
The P0420 catalyst monitor B1 fail is saying the catalytic converter isn’t working as efficiently as it should. That could be the converter itself, but it can also be triggered by something else like a tired oxygen sensor, a small exhaust leak, or even an engine running slightly off.
The “EVAP system leak test fail” is a different problem. That’s your fuel vapor system, and it usually comes down to something simple like a loose or worn gas cap, a cracked hose, or a faulty purge valve.
I recommend checking gas cap and make sure it’s tight and not damaged. If it looks old, replacing it is cheap and worth trying.
Then scan again and see if any other codes are stored, especially misfire or fuel-related ones. If those are present, fix them first since they can trigger the P0420.
If no other codes show up, have the oxygen sensors and exhaust system checked before even thinking about replacing the catalytic converter. A lot of people jump straight to that and end up wasting money.