P0423: Heated Catalyst Efficiency Below Threshold — Bank 1
What to do
Driveable, will fail an emissions test. Ask whether the converter's heater circuit was tested — a heater fault and a worn converter set the same code but cost very differently.
What this code means
An electrically heated converter on bank 1 is below its efficiency threshold. Heated converters carry a resistive element that warms the substrate before or during a cold start, cutting the light-off delay further than a close-coupled position alone can. They are uncommon on ordinary production vehicles and more usual on hybrids and low-emission variants.
Because the heater is part of the assembly, an efficiency fault here may be a heater fault rather than a catalyst fault: a converter that never reaches temperature in time will fail its monitor while the substrate itself is undamaged. Check the heater circuit before the converter.
Likely causes
Start here
- Failed catalyst heater element · specialist
- Failed catalytic converter · specialist
Less common
- Aged or lazy downstream oxygen sensor · easy
- Exhaust leak before the sensor · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
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