P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
Category: Powertrain Severity: Medium
Updated:
DTC code P0420 means: Catalyst System Efficiency Below Threshold (Bank 1). It falls under the "Powertrain" category, with an estimated severity of Medium. Common causes include failed catalytic converter, faulty oxygen (O2) sensor and exhaust leak; it's worth confirming the diagnosis before replacing parts.
Possible causes
- Failed catalytic converter
- Faulty oxygen (O2) sensor
- Exhaust leak
- Engine misfire
- Faulty coolant temperature sensor
Symptoms
- Check engine light
- Rotten egg smell
- Loss of power
- Higher fuel consumption
How to diagnose it step by step
- Check for other codes (misfires, O2 sensors, fuel trim) and fix them first.
- Look for exhaust leaks between the manifold and the downstream sensor.
- Compare O2 signals: the upstream should switch and the downstream should stay steady.
- If the downstream mirrors the upstream, the catalyst has lost capacity.
- Replace the downstream sensor if it responds poorly; only then the catalytic converter.
How to fix it
Rule out leaks and O2 sensors before replacing the catalytic converter (the most expensive part).
Estimated repair cost
O2 sensor: $100–280. Aftermarket catalytic converter: $400–1,200; OEM: $1,000–2,500.
Frequently asked questions
What does DTC code P0420 mean?
Catalyst System Efficiency Below Threshold (Bank 1).
Is code P0420 serious?
Severity Medium. Not urgent, but worth diagnosing to avoid bigger damage.
Can I keep driving with code P0420?
Yes, you can drive: it isn't an urgent fault. But a badly damaged converter can clog and cause power loss, and you won't pass an emissions test with the light on.
Related codes
P0421 P0411 P0410 P0430 P0431 P0406 P0405 P0404 P0403 P0402
Refine by car make
- Audi P0420: Same as VW: O2 sensor or exhaust manifold leak.
- BMW P0420: Check the oxygen sensors and rule out exhaust leaks before the catalytic converter.
- Citroën P0420: Same as Peugeot: O2 sensors and exhaust leaks.
- Dacia P0420: Same as Renault: downstream O2 sensor.
- Fiat P0420: O2 sensor or catalytic converter; check the exhaust condition.
- Ford P0420: Downstream O2 sensor or PCM update (bulletin on several Focus/Fiesta models).
- Honda P0420: Secondary O2 sensor (after the catalytic converter) or catalytic converter.
- Hyundai P0420: O2 sensor or catalytic converter (Theta/Gamma engines).
- Kia P0420: O2 sensor or catalytic converter (Gamma/Nu engines).
- Mazda P0420: O2 sensor or catalytic converter; on SkyActiv, check the downstream sensor.
- Mercedes-Benz P0420: O2 sensors or catalytic converter; rule out exhaust leaks.
- Nissan P0420: Downstream O2 sensor or catalytic converter.
- Opel P0420: Downstream O2 sensor or catalytic converter.
- Peugeot P0420: O2 sensors and possible leaks in the exhaust line.
- Renault P0420: Downstream O2 sensor and catalytic converter condition.
- Seat P0420: Same as VW: O2 sensor or exhaust leak.
- Škoda P0420: Same as VW: O2 sensor or exhaust leak.
- Toyota P0420: Worn catalytic converter or downstream O2 sensor.
- Volkswagen P0420: O2 sensor or exhaust manifold gasket leak (common on TSI).
- Volvo P0420: O2 sensor or catalytic converter.
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