Code
P0424
HUMMER
P — Powertrain
Heated Catalyst Temperature Below Threshold (Bank 1)
AI status
Completed
Completed
100%
Causes
- Failed or degraded heated catalytic converter assembly (Bank 1)
- Faulty heated catalyst temperature (HCT) sensor
- Open, shorted or corroded wiring/connectors in the HCT sensor or heater circuit
- Blown fuse or faulty relay supplying the heater circuit
- Poor ground or low supply voltage to heater circuit (battery/charging issues)
- Exhaust leak upstream of the HCT sensor or catalytic converter
Symptoms
- Malfunction Indicator Lamp (MIL) / Check Engine Light illuminated
- Possible failed emissions test or readiness monitor incomplete
- Reduced cold-start emissions control (higher HC/CO until warmed up)
- Possible reduced fuel economy or driveability issues if related engine faults are present
- Catalyst warm-up takes longer than normal (observable with scan tool)
What to check
- Retrieve stored codes and freeze-frame data with a scan tool; note operating conditions when code set
- Check pending codes and related P0420/P0430 family codes
- Verify battery voltage and charging system are within spec
- Visually inspect HCT sensor and heated-catalyst assembly connectors and wiring for damage, corrosion, or poor connections
- Check for exhaust leaks upstream of Bank 1 catalyst and sensor locations
- Use a scan tool to monitor HCT sensor temperature reading and compare to ambient and engine/exhaust temps during cold start and warm-up
Signal parameters
- HCT sensor temperature reading (°C or °F) — should rise during cold start and reach manufacturer threshold within specified warm-up time (compare to freeze frame)
- Heater supply voltage (approx. battery voltage when commanded on)
- Heater circuit current or resistance (ohms) for the heater element — compare to service spec
- PCM control status for heater circuit (on/off command) as reported by scan tool
- Related O2 sensor readings and catalyst efficiency parameters during warm-up
Diagnostic algorithm
- Read codes and freeze-frame; record HCT temperature, engine load, coolant temp, and battery voltage at fault. 2) Verify battery/charging system is healthy; low voltage can prevent heater operation. 3) Visually inspect HCT/heater wiring and connector for corrosion, damage, or poor pins; repair as needed. 4) Check for exhaust leaks upstream of the sensor or catalyst and repair any leaks. 5) With a scan tool, monitor HCT temperature during a cold start; confirm the heater is being commanded on by the PCM. 6) Measure heater element resistance at the sensor connector (engine off) and compare to spec. If open/shorted, replace heated catalyst assembly or sensor as required. 7) Measure heater supply voltage and ground when heater is commanded; if no supply, check fuse/relay and PCM outputs. 8) If heater circuit has voltage but heater element is out of spec, replace the heated catalyst assembly or the integrated HCT sensor per service information. 9) After repairs, clear codes, perform required drive cycle or readiness checks and verify code does not return. 10) If fault persists after wiring and component replacement, consult manufacturer service information for PCM tests or software updates and consider professional diagnostic support.
Likely causes
- HCT sensor failed or producing a low temperature reading
- Heater element in the heated catalyst assembly not powered or open
- Damaged wiring/connector to the HCT sensor or heater circuit
- Exhaust leak or engine misfire preventing normal catalyst warm-up
Fault status
Status
Heated catalyst temperature for Bank 1 is below the required threshold. Inspect the heated catalyst, HCT sensor and heater circuit, wiring/connectors, and upstream exhaust/engine conditions.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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Repair manuals
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138
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