SPN 420 - Catalyst Efficiency Below Threshold (Bank 1)
Catalytic converter efficiency issue on bank 1.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The engine computer is seeing that the catalytic converter is not cleaning up the exhaust gases efficiently; it's allowing more pollutants than it should.
What the computer is actually seeing
The Engine Control Module (ECM) compares the pre-catalytic converter oxygen sensor readings with the post-catalytic converter readings. If the oxygen variation shows less than a 5-15% difference, it flags a problem.
What a healthy reading looks like
Pre-catalytic converter O2 sensor switch should see fluctuations between 0.1V and 0.9V rapidly, while post-catalytic should show less fluctuation with stable readings around 0.5V within the same timeframe.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Scan for additional codes that may help identify a related issue (ex: misfire codes).
- 22. Inspect for exhaust leaks in the manifold and the exhaust system, repair as necessary.
- 33. Check the wiring and connectors to the O2 sensors for damage or corrosion; repair as necessary.
- 44. Test the pre-cat O2 sensor (Bank 1) with a multimeter; at idle, expect fluctuating voltage (0.1-0.9V). If not correct, replace the sensor.
- 55. Perform a similar test on the post-cat O2 sensor. If it's reading stable and within spec but the pre-cat varies correctly, suspect a failed catalytic converter.
- 66. If all sensors are functioning but code persists, conduct further tests to assess fuel trim values; adjust or repair fuel system issues where applicable.
Common causes
- Faulty catalytic converter
- Bad O2 sensors
- Exhaust leaks
- Fuel system issues (too rich or too lean)
- Poor engine performance (misfires, etc.)
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's drivable but not recommended for long periods; it can lead to more emissions and potential damage to the catalytic converter.
Will this come back after repair?
If fixed properly, it shouldn't return; if it's a symptom of a bigger issue, it might.
Why does it sometimes pair with SPN 522?
SPN 522 often indicates a related issue with O2 sensor performance, reflecting the catalytic converter's effectiveness.
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