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A/C-Heater Control Unit Does Not Maintain Set Temperature

WARNING: This page is about a different car, the 2001 Pontiac Bonneville and 2000 Pontiac Bonneville. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check. See DIAGNOSTIC SYSTEM CHECK  under TROUBLE SHOOTING and go to next step.
  2. Connect scan tool. Turn ignition on (engine off). Use scan tool to perform RECALIBRATE ALL MOTORS command in INSTRUMENT PANEL MODULE SPECIAL FUNCTIONS. Use scan tool to observe current DTC list in instrument panel module. If scan tool displays any DTCs, see DIAGNOSTIC TROUBLE CODE IDENTIFICATION  table under SELF-DIAGNOSTICS. If scan tool does not display any DTCs, go to next step.
  3. Inspect battery for low or discharged condition. See appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS and go to step  22 . If battery is okay, go to next step.
  4. Inspect HVAC controls for interference by associated trim, causing a sticking switch. If a problem was found and corrected, go to step  22 . If no problem was found, go to next step.
  5. Ensure customer is enabling personalization, using correct ignition key and is not inadvertently activating passenger HVAC controls. If a problem was found and corrected, go to step  22 . If problem was not found, go to next step.
  6. Connect scan tool. Cover sunload sensor. Start engine. Set temperature to 75°F (24°C) position. Set passenger's temperature switch to OFF position. Use scan tool to observe RIGHT and LEFT SOLAR SENSOR data parameters in INSTRUMENT PANEL MODULE OPEN/SHORT data list. If scan tool indicates RIGHT and LEFT SOLAR SENSOR data parameters are more than 224 counts, go to next step. If scan tool indicates RIGHT and LEFT SOLAR SENSOR data parameters are 224 counts or less, go to step  17 .
  7. Uncover sunload sensor. Direct a light source at right and then left sunload sensors. If counts change consistently and correspond to proper side when illuminated, go to next step. If counts do not change consistently and or do not correspond to proper side when illuminated, go to step  17 .
  8. Open all instrument panel outlets. Set temperature to 75°F (24°C) position. Set mode switch to panel position. Set blower speed to 2 bars on fan speed indicator. Set passenger's temperature switch to OFF position. Place a 2" (5 cm) square piece of paper over sensor inlet. If paper stays in place, go to next step. If paper does not stay in place, go to step  16 .
  9. Turn ignition off. Check right and left, upper and lower air temperature sensors for proper installation. See AIR TEMPERATURE SENSORS  under REMOVAL & INSTALLATION. Repair any problems found and go to step  22 . If all sensors were installed correctly, go to next step.
  10. Check for loose wires and or poor connections at left and right, upper and lower temperature sensors. Repair any problems found and go to step  22 . If all wiring and connectors are okay, go to next step.
  11. Check resistance of left and right, upper and lower air temperature sensors. See SENSOR RESISTANCE  table under A/C SYSTEM INSUFFICIENT COOLING. If all sensors measure near specified value, go to next step. If any sensors do not measure near specified values, go to step  21 .
  12. Check for loose wires and or poor connections at ambient air temperature sensor. Repair any problems found and go to step  22 . If wiring and connectors are okay, go to next step.
  13. Check outside air temperature sensor resistance. See SENSOR RESISTANCE  table under A/C SYSTEM INSUFFICIENT COOLING. If resistance is as specified, go to next step. If resistance is not as specified, go to step  19 .
  14. Check for loose wires and or poor connections at inside air temperature sensor. Repair any problems found and go to step  22 . If wiring and connectors are okay, go to next step.
  15. Check inside air temperature sensor resistance. See SENSOR RESISTANCE  table under A/C SYSTEM INSUFFICIENT COOLING. If resistance is as specified, system is okay at this time. If resistance is not as specified, go to step  20 .
  16. Check for a loose, blocked or otherwise damaged aspiration hose. Repair any problems found and go to step  22 .
  17. Check for loose wires and poor connections at sunload sensor. Repair any problems found and go to step  22 . If wiring and connectors are okay, go to next step.
  18. Replace sunload sensor assembly. See SUNLOAD SENSOR  under REMOVAL & INSTALLATION and go to step  22 .
  19. Replace ambient air temperature sensor. See AMBIENT AIR TEMPERATURE SENSOR  under REMOVAL & INSTALLATION and go to step  22 .
  20. Replace inside air temperature sensor. See INSIDE AIR TEMPERATURE SENSOR  under REMOVAL & INSTALLATION and go to step  22 .
  21. Replace suspect air temperature sensor. See AIR TEMPERATURE SENSORS  under REMOVAL & INSTALLATION and go to next step.
  22. Operate system to verify proper operation. If system is not okay, go back to step  2 .
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