Heated Catalyst Efficiency Below Threshold (Bank 1)
What does P0423 mean?
The P0423 code means the vehicle's onboard diagnostic system has detected: Heated Catalyst Efficiency Below Threshold (Bank 1). This is a moderate severity code.
Common Symptoms
- Check Engine Light illuminated
- Sulfur smell from exhaust
- Slight decrease in fuel economy
- Failed emissions test
Probable Causes (Ranked by Likelihood)
- Degraded heated catalytic converter Very High
- Catalyst heater circuit failure preventing warm-up High
- Engine misfire contaminating the catalyst Moderate
- Faulty downstream O2 sensor Low
Step-by-Step Diagnostic Procedure
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Check for catalyst heater circuit codes (P0030-P0064) that may accompany P0423
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Compare upstream and downstream O2 sensor activity on Bank 1
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Verify catalyst heater operation by measuring current draw during cold start
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Check for exhaust leaks and misfire codes
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If heater circuit is functioning and no other faults exist, the heated catalyst needs replacement
Frequently Asked Questions
Can I drive with code P0423?
Yes. Driving is safe with this code. The heated catalyst is underperforming, increasing emissions output but not affecting engine operation.
What is a heated catalyst?
A heated catalyst has an electric heating element that warms it to operating temperature faster than exhaust heat alone. This is common on hybrid vehicles where the engine may not run continuously.
Why is P0423 common on hybrid vehicles?
Hybrid engines cycle on and off frequently, so the catalyst cools down between engine-on periods. The heater compensates, but if it fails, the catalyst cannot maintain efficiency during frequent cycling.