P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream oxygen sensor is detecting too many unburned gases, which means the cat is worn out or there's another exhaust issue.
Symptoms
- Check engine light on
- Possible slight fuel economy decrease
- Possible rotten egg smell from exhaust
- May fail emissions test
Causes (Ranked by Likelihood)
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DIY Fix Steps
⚠ Safety first: work on a cold engine, chock wheels, use jack stands - never rely on a floor jack alone.
Clear the code and drive 2-3 full warm-up cycles. If it comes back, the issue is real.
With engine warm, listen for ticking or hissing near exhaust manifold and cat. An exhaust leak can fool the sensor.
Use a live data scanner to watch Bank 1 Sensor 2. It should read ~0.6-0.7V steadily. If it oscillates like the upstream sensor, the cat is dead.
Spray penetrating oil on the O2 sensors and cat flange bolts 24h before. Remove in this order: downstream O2 sensor, cat flange bolts, cat inlet connection.
Reconnect all O2 sensors. Clear the DTC and run 2 drive cycles to confirm the fix.