P0420

What P0420 means

P0420 compares upstream and downstream O2 sensor signals to judge catalyst efficiency. If the downstream sensor's signal looks too much like the upstream (meaning the cat isn't doing its job), P0420 sets. A failing catalyst is the headline cause, but an aged downstream O2 sensor throws this code constantly, so start there.

Symptoms

• Check engine light
• Slight loss of power or fuel economy
• Rotten-egg smell (severe cases)
• Emissions test fail

Likely causes (most common first)

1. Worn / aged downstream O2 sensor (start here — it's cheaper and often the real cause)
2. Failing catalytic converter
3. Exhaust leak upstream of or near the catalyst
4. Oil or coolant burn-off contaminating the cat
5. Long-term rich running (misfires, leaking injectors)
6. Wrong fuel (leaded gas in a cat-equipped car)

Can I keep driving?

Yes, usually for a long time. But if an emissions test is coming up, fix it. And if it's paired with misfire codes, address those first to avoid killing a still-good catalyst.

Typical repair cost

DIY fixes can start near $40 (parts only). Shop repairs for P0420 typically run up to $2500 depending on the underlying cause and vehicle. Start with the most common cause — Worn / aged downstream O2 sensor (start here — it's cheaper and often the real cause) — before throwing parts at it.

Related codes

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