Hummer H1 Service Manual page 728

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Condition
Inadequate or No Heat
Inadequate Defrost
NOTE: Improperly adjusted or binding mode door linkage can also cause low, poor or no airflow to any location.
05745159
Heating/Ventilation/Air Conditioning (HVAC)
Heater Diagnosis
Possible Cause
1. Heat control knob not set in heat mode, or
engine not warmed up.
2. Low engine coolant level.
3. Air in coolant system. Usually occurs after
coolant drain and refill.
4. Thermostat faulty.
5. Kinked or collapsed radiator hose or heater
hose.
6. Water pump failure.
7. Heater core plugged or leaking.
9. Blower motor fault.
11. Outside air leaking into heater core section
of plenum.
13. Water control valve or vacuum solenoid
fault.
15. Blend air door stuck or misadjusted.
1. Duct restricted or connecting hose is loose.
2. Defroster nozzle misaligned or damaged
(bent, cracked, partially closed).
3. Insufficient heat.
4. Blower motor fault.
Correction
1. Advise owner/driver.
2. Add coolant as needed and check for leaks.
3. Bleed air from cooling system.
4. Replace thermostat(s) if stuck closed or
remains open continuously.
5. Replace hose.
6. Replace pump.
8. Replace core.
10. Check circuit fuse and wiring. Replace
motor if foiled.
12. Seal plenum or realign as needed.
14. Replace valve if stuck closed or partially
open. Replace solenoid if closed or
shorted.
16. Replace main unit.
1. Repair as needed.
2. Repair as needed.
3. Check for low coolant level, coolant leaks,
air in system, faulty thermostat or water
valve, collapsed hose, plugged heater core,
binding air door in main unit.
4. Check circuit fuse and wiring. Replace
motor if open or shorted.
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