Ask Andy AIP0420 Catalyst System Efficiency Below Threshold (Bank 1)
What it means, what causes it, and what it costs to fix.
What P0420 means
Your engine has an oxygen sensor before the catalytic converter and one after it. On a healthy converter, the rear sensor reads steady while the front one switches back and forth. P0420 sets when the rear sensor starts mirroring the front, which means the converter on bank 1 isn't storing oxygen and cleaning the exhaust like it should.
Can I drive with P0420?
Severity: Medium. Fix soon so it doesn't turn into something bigger.
Usually safe to drive short term, but you'll fail an emissions inspection, and the underlying cause (like a misfire) can get worse. Fix misfires first, because they kill new converters too.
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Ask Andy about P0420Symptoms
- Check engine light, often with no drivability change
- Failed emissions or smog test
- Rotten-egg (sulfur) smell from the exhaust
- Sluggish acceleration if the converter is clogged
Common causes
Most likely first.
- Worn-out catalytic converter (common past 100k miles)
- Faulty downstream (rear) oxygen sensor reading wrong
- Exhaust leak ahead of or near the rear O2 sensor
- Engine misfires or rich running that overheated the converter
- Oil or coolant burning in the engine and coating the converter
How to diagnose P0420
- Check for other codes. Fix any misfire, lean, or rich codes before replacing the converter.
- Look and listen for exhaust leaks around the manifold, flex pipe, and O2 sensor bungs.
- Watch live data: the rear O2 should hold a fairly steady voltage while the front one switches. A rear sensor that switches like the front one means a weak converter.
- Check the vehicle for a manufacturer TSB or software update, and check whether an extended emissions warranty covers it (often 8 years / 80,000 miles in the US).
Tip: most US auto parts stores will read your codes for free.
Repair costs
| Rear oxygen sensor | $150–$350 installed |
| Exhaust leak repair | $100–$400 |
| Catalytic converter (aftermarket) | $600–$1,500 installed |
| Catalytic converter (OEM) | $1,200–$3,000+ installed |
Typical US prices. Your vehicle, region, and shop rates will change these.
P0420 FAQ
What does P0420 mean?
P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)." Your engine has an oxygen sensor before the catalytic converter and one after it. On a healthy converter, the rear sensor reads steady while the front one switches back and forth. P0420 sets when the rear sensor starts mirroring the front, which means the converter on bank 1 isn't storing oxygen and cleaning the exhaust like it should.
Can I drive with a P0420 code?
Usually safe to drive short term, but you'll fail an emissions inspection, and the underlying cause (like a misfire) can get worse. Fix misfires first, because they kill new converters too.
What is the most common cause of P0420?
The most common cause is: Worn-out catalytic converter. Other causes include faulty downstream (rear) oxygen sensor reading wrong and exhaust leak ahead of or near the rear o2 sensor.
How much does it cost to fix P0420?
Typical repairs run about $100 to $3,000 in the US, depending on the cause. Rear oxygen sensor: $150–$350 installed.
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