How do you fix fault code P0423?

STEP 1: Inspect for exhaust leaks between the manifold and the catalyst. Seal any leaks. STEP 2: Monitor oxygen sensor data using a scan tool. During operation, the downstream sensor should show a stable voltage (approx 0.5V-0.7V). If it oscillates in sync with the upstream sensor (between 0.1V and 0.9V), the catalyst is worn out. STEP 3: Check the catalyst heating circuit. Measure resistance in the heating element (should be approx 2-10 Ohms depending on OEM spec) and verify 12V supply voltage via relay/wiring. STEP 4: If no leaks or sensor faults are found, replace the catalytic converter (Bank 1). Reset adaptation values after replacement.

Powertrain (Engine & Gearbox)Standard EOBD

P0423 - Heated Catalyst Efficiency Below Threshold (Bank 1)

Q: What are the symptoms of fault code P0423?

• Check engine light (MIL) illuminated • Failed vehicle emission test • Increased fuel consumption • Slight rotten egg (sulfur) smell from the exhaust • Reduced engine performance under load

Q: What causes fault code P0423?

• Most likely: Defective catalytic converter (Bank 1) • Likely: Faulty downstream oxygen sensor • Possible: Defective catalyst heating element or heating circuit fault • Possible: Exhaust leak upstream of or near the catalyst • Rare: Faulty catalyst temperature sensor

Last updated:1 February 2026

Related:Catalytic Converter, Syresensor (Lambda), Manifold, Catalyst Heating Element, Temperaturgivare avgas

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0400

Exhaust Gas Recirculation Flow Malfunction

P0401

Exhaust Gas Recirculation (EGR) Flow Insufficient Detected

P0402

Exhaust Gas Recirculation Flow Excessive Detected

P0403

Exhaust Gas Recirculation (EGR) Control Circuit

Technical Description

The Engine Control Module (ECM/PCM) monitors the efficiency of the catalytic converter by comparing signals from the upstream (regulating) and downstream (diagnosing) oxygen sensors. P0423 specifically refers to systems equipped with a Heated Catalyst (EHC), which uses an electrical heating element to reach operating temperature faster. The code is triggered when the downstream oxygen sensor signal mimics the upstream sensor too closely, indicating the catalyst's inability to store oxygen or convert emissions effectively. This signifies that the catalyst's capacity to reduce Hydrocarbons (HC), Carbon Monoxide (CO), and Nitrogen Oxides (NOx) has fallen below a calibrated threshold.

Possible Causes

Defective catalytic converter (Bank 1)

Faulty downstream oxygen sensor

Defective catalyst heating element or heating circuit fault

Exhaust leak upstream of or near the catalyst

Faulty catalyst temperature sensor

Symptoms

Check engine light (MIL) illuminated

Failed vehicle emission test

Increased fuel consumption

Slight rotten egg (sulfur) smell from the exhaust

Reduced engine performance under load

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P0423 - Heated Catalyst Efficiency Below Threshold (Bank 1)

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P0423 - Heated Catalyst Efficiency Below Threshold (Bank 1)

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