P0423: Heated Catalyst Efficiency Below Threshold (Bank 1)

P0423 means the electrically heated catalyst on Bank 1 (an e-cat with an internal resistive heating element used on newer Euro 6 and 6d vehicles to reach light-off temperature within seconds of start) is not converting emissions efficiently. It typically points to either heater element failure, control module wiring issues, or substrate degradation.

P0423 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0423 and illuminates the check engine light.

If your vehicle has triggered P0423, you may notice one or more of these symptoms:

• Check engine light illuminated, often within the first minute after start
• Elevated cold-start emissions, visible at idle on a cold morning
• Slight reduction in fuel economy
• Possible faint sulfur smell during warm-up
• Failed emissions test or periodic technical inspection

Here are the most common causes of P0423, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Safe for short-term driving. Heated catalyst codes typically appear on newer Euro 6 and 6d vehicles equipped with electrically heated catalysts (e-cats). Cold-start emissions will be elevated and emissions or periodic technical inspection will not pass with this code active.

Follow these steps to pinpoint the root cause of P0423:

1. Read freeze frame data and confirm the code triggered shortly after cold start, which is typical of heated catalyst faults
2. Check for companion heater circuit codes such as P0420, P0422, or manufacturer-specific e-cat heater codes
3. Measure the resistance of the e-cat heater element using the manufacturer specification (typical values are 0.3 to 2.0 ohms depending on platform)
4. Inspect the wiring harness and connector to the heater control module for corrosion, broken pins, or water ingress
5. Graph Bank 1 downstream O2 sensor during cold start. A healthy heated cat stabilises within 20 to 40 seconds

Repair costs for P0423 vary depending on the root cause and your vehicle. Here are typical estimates:

P0423 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0423 or indicate related issues: