P0423 — - Catalyst performance is too low during warm-up (bank 1)

Causes

• Degraded or damaged catalytic converter (Bank 1)
• Faulty upstream (Bank1 Sensor1) or downstream (Bank1 Sensor2) oxygen sensor
• Exhaust leak upstream of the catalytic converter
• Engine running rich or lean during cold start (fuel control issue)
• Ignition or misfire condition increasing emissions
• O2 sensor heater failure delaying warm-up

Symptoms

• Check Engine Light (MIL) illuminated
• Failed emissions test or high tailpipe HC/CO readings on cold start
• Poor fuel economy or rough cold idle
• Sulfur/rotten-egg odor from exhaust
• Possible loss of low-speed drivability until warmed

What to check

• Read stored codes and freeze-frame data; note engine temp and conditions when code set
• Check readiness monitors and related codes (O2 sensors, misfire, fuel trim)
• Visually inspect exhaust for leaks, damaged pipes, or loose flanges upstream of the catalytic converter
• Inspect O2 sensor connectors and wiring for corrosion, breaks, or short circuits
• Measure O2 sensor heater resistance and supply voltage against manufacturer spec
• Use a scan tool to monitor upstream and downstream O2 sensor voltages and switching during cold start/warm-up

Signal parameters

• Upstream O2 (Bank1 Sensor1) voltage: ~0.1–0.9 V switching rapidly once warmed; switching frequency typically >1 Hz when closed-loop
• Downstream O2 (Bank1 Sensor2) voltage: should become more stable/less reactive once converter is effective (often ~0.4–0.6 V); during proper warm-up expected relationship to upstream measured by monitor
• O2 sensor heater resistance: typically low (manufacturer-specific, often a few ohms); heater circuit supply voltage ≈ battery voltage when key on
• Catalyst temperature: should rise quickly after start; slow or low temperature rise indicates poor warm-up
• Fuel trims: excessive positive/negative long-term trims or large short-term excursions during warm-up suggest fueling or leak issues

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