You've been driving around for a while when suddenly your check engine light comes on. You scan the code and see P0420: your catalytic converter isn't performing up to standard. Don't panic about an expensive repair bill just yet—the culprit is often a worn oxygen sensor or a small exhaust leak, not necessarily the catalytic converter itself.
Quick answer: Error code P0420 indicates that the catalytic converter on bank 1 is underperforming. Your vehicle detects this using the oxygen sensor (O2 sensor) located downstream of the converter. The most common cause is a worn downstream oxygen sensor or an exhaust leak, not always the catalytic converter. You can keep driving, but you'll fail your emissions test.
What does error code P0420 mean?
There's an oxygen sensor both upstream and downstream of the catalytic converter. The upstream sensor measures the exhaust entering the converter, while the downstream sensor measures what comes out. When the converter is working properly, the downstream signal stays relatively flat. If the downstream signal looks too similar to the upstream signal, your vehicle's computer concludes the converter isn't cleaning the exhaust effectively and triggers P0420 (bank 1 = the side of the engine with cylinder 1).
Severity: warning light. Not an immediate engine problem—you can continue driving. However, the light will stay on, your emissions will increase, and you won't pass your vehicle inspection.
Symptoms
Check engine light is on. Often the only sign; the car drives normally otherwise.
Sometimes a sulfur or rotten egg smell. A sign the converter isn't functioning properly.
Slightly higher fuel consumption. Sometimes noticeable, often not.
Possible causes (from cheapest to most expensive)
Worn downstream oxygen sensor (€40 to €120). A sluggish sensor gives an inaccurate reading of the converter. Start here.
Exhaust leak at or before the converter (€20 to €100). Unmetered air disrupts the measurement.
Engine running too rich or too lean (varies). Another fuel mixture problem (see P0171) can overload the converter.
Worn catalytic converter (€200 to €800+). Only confirm this after ruling out the cheaper causes.
Step-by-step guide: how to find the cause yourself
- Scan and clear the code. Are there also fuel mixture or oxygen sensor codes present? Address those first.
- Read the oxygen sensors live. Use an app like OBDeleven to monitor the upstream and downstream sensors. The upstream should switch rapidly, while the downstream should stay relatively steady.
- Compare the two signals. If the downstream sensor is fluctuating as much as the upstream sensor, either the converter isn't doing its job or the sensor is failing.
- Inspect the exhaust for leaks. Listen and look for leaks between the engine and converter; unmetered air causes incorrect readings.
- Rule out the cheap fixes first. Replace the downstream oxygen sensor if needed and clear the code before considering an expensive converter replacement.
- Drive and recheck. If P0420 doesn't return after a few drives, it was the sensor or the leak.
What will it cost?
DIY: downstream oxygen sensor. $40 to $120, often just needing a wrench.
DIY: seal the exhaust leak. $20 to $100 depending on location.
Garage: new catalytic converter. $200 to $800 or more, so only as a last resort.
Fix it yourself or visit a garage?
The oxygen sensor and an exhaust leak are straightforward DIY jobs with basic tools, and they resolve most P0420 cases. If the converter itself is worn out, a garage is your best bet. Here's the key takeaway: never replace an expensive catalytic converter blindly without checking the live data first.
The right diagnostic tool for this code
With P0420, you'll want to compare both oxygen sensors in live data—this prevents unnecessary replacement of a pricey converter. A basic scanner reads and clears codes, OBDeleven displays sensor data in real time, and Launch offers detailed graphs and comprehensive testing.
View the Topdon CarPal →
View the OBDeleven 3 →
View the Launch CRP919X →
Related fault codes
If your vehicle has dual exhaust banks, you might also see P0430 (converter bank 2). A lean fuel mixture can wear out a converter faster; check out P0171 for more details. New to reading and clearing codes? Start with how to read and clear fault codes.
Frequently asked questions
Do I need to buy a new catalytic converter right away?
No. First check the downstream oxygen sensor and exhaust for leaks. These are much cheaper and often fix P0420.
Can I keep driving with P0420?
Yes, it's not an emergency. However, the warning light stays on and you'll fail your emissions test at inspection.
Will the light turn off on its own?
Once you fix the underlying issue, the vehicle usually clears the code after a few drive cycles. You can also clear it manually with a scanner.
Can premium fuel or additives help?
Rarely with a genuine P0420. The problem lies in the sensor, exhaust, or converter—not fuel quality.
Quick summary
• P0420 = bank 1 catalytic converter is underperforming.
• Start with the downstream oxygen sensor and exhaust leaks; these are usually the culprit.
• Compare oxygen sensors in live data before replacing an expensive converter.
• You can drive on, but won't pass emissions testing with the light on.