P04F2 — Catalyst Heater Supply Voltage Bank 1

Causes

• Blown fuse or faulted relay in heater supply circuit
• Open or shorted wiring (short to ground or to battery) to the heater element
• Corroded, loose or damaged connector at heater or PCM
• Failed catalyst/heater element (open or shorted windings)
• High resistance connection or poor ground
• PCM or driver circuit fault

Symptoms

• Check Engine MIL illuminated (fault stored)
• Reduced or disabled catalyst heating function; longer warm-up times
• Possible failed emissions test (increased emissions)
• DPF regeneration problems or illumination of related warnings
• Possible limp-mode or reduced performance on some vehicles

What to check

• Retrieve freeze-frame and freeze data and note battery voltage and operating conditions when the code set
• Visually inspect wiring, connectors, and harness routing to the catalyst heater and PCM for corrosion, damage, or water ingress
• Check fuses and relays for the heater supply; replace if blown or faulty
• Measure battery voltage and verify charging system health before testing
• Backprobe heater supply connector and measure voltage with key ON and engine RUN or during commanded heater on
• Measure heater element resistance (with battery disconnected) and compare to specification

Signal parameters

• Expected supply voltage: ~11–14.5 V (near battery voltage) when heater supply is active or key ON depending on design
• Abnormal: 0 V (open), >16 V (overvoltage) or significant voltage drop under load (>0.5–1.0 V below battery)
• Typical heater element resistance: low ohms to tens of ohms (vehicle-specific). A very high or infinite resistance indicates open heater
• Expected current draw: up to several amps when heater active (vehicle-specific); large deviation suggests short or open

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