P1421 — Catalytic converter 1 temperature fault

Causes

• Faulty catalytic converter temperature sensor (open, shorted, or out-of-spec)
• Damaged wiring or poor connector contact to the temperature sensor
• Failed sensor heater circuit (where fitted)
• Exhaust leak upstream or near the sensor causing false readings
• Severely degraded, plugged or damaged catalytic converter causing abnormal temperatures
• ECU fault or incorrect calibration (less common)

Symptoms

• Malfunction Indicator Lamp (MIL) illuminated
• Failed emissions test or increased exhaust emissions
• Reduced engine performance under some conditions
• Poor fuel economy
• Possible exhaust odor (sulfur/rotten egg) or unusual exhaust temperatures/heat near underbody
• Possible stored related codes for O2/heater/catalyst efficiency

What to check

• Read and record freeze-frame and related codes with a scan tool; note operating conditions when code set
• Check live data for catalyst temperature sensors (pre- and post-catalyst) and oxygen sensors
• Visually inspect sensor body, connector and wiring for damage, chafing, corrosion or heat damage
• Check for exhaust leaks around manifold, flange, gasket and sensor mounting area
• Verify sensor power/ground and heater supply (if applicable) at the connector with key on/engine off
• Measure sensor resistance and compare to manufacturer specification

Signal parameters

• Catalyst temperature sensors are usually thermistors producing a voltage correlated to temperature; typical reported range to ECU is within the 0.1–5 V analog span depending on engine wiring
• Expected behaviour: low temperature reading on cold start rising as engine warms; post-catalyst sensor should normally be lower than pre-catalyst when catalyst is functioning
• Under load, an efficient catalyst often shows a significant temperature rise across the converter (commonly tens to a few hundred °C depending on load and engine), whereas a damaged/plugged catalyst or faulty sensor may show abnormal absolute values or no sensible delta
• Heater circuit (if fitted): current present to warm the sensor until operating temperature is reached; open heater or no current is a fault

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