P0423: Heated Catalyst Efficiency Below Threshold (Bank 1)

P0423 means the heated catalyst on Bank 1 is not converting emissions as efficiently as the ECU expects. A heated catalyst has an electric heating element that brings it to working temperature within seconds of a cold start. Such catalysts are still uncommon on production cars, so first check the manufacturer's definition of the code for your vehicle. On cars that do have one, the usual causes are a failed heater element, a fault in the heater's wiring or control, or a worn catalyst substrate.

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 shortly after a cold start
• Higher exhaust emissions during warm-up (not usually noticeable from the driver's seat)
• Possible faint sulfur smell during warm-up
• Failed emissions test or periodic technical inspection
• No change in how the car drives in most cases

Here are the most common causes of P0423:

Generally yes, for short-term driving. Safe for short-term driving. The code points to the catalyst's cold-start performance, so emissions are higher than they should be during warm-up, and a stored catalyst code with the check engine light on is the kind of defect that gets a car rejected at an emissions check or periodic technical inspection.

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 codes such as P0424 (heated catalyst temperature below threshold) or manufacturer-specific heater circuit codes, and read the manufacturer's definition of P0423 for your vehicle
3. Measure the resistance of the catalyst heater element and compare it with the manufacturer specification
4. Inspect the wiring harness and connector to the heater control module for corrosion, broken pins, or water ingress
5. Graph the Bank 1 downstream O2 sensor after a cold start and compare its behaviour with the manufacturer's data: a working catalyst settles the downstream signal quickly, while a failing one lets it follow the upstream sensor
6. Inspect the exhaust ahead of the downstream O2 sensor for leaks, which can skew its reading

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

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