* Blend Door Actuator: This is the most likely culprit. The blend door actuator is a small motor that controls the flow of hot and cold air to the vents. If it's malfunctioning or broken, it might not be able to direct hot air to the passenger side. This often results in cold air, even with the heat on. The actuator itself might be broken, or the cable connected to it could be damaged.
* Vacuum Leaks: Many climate control systems in older cars rely on vacuum lines to operate the blend doors and other components. A leak in these lines can prevent the correct operation of the system, leading to cold air on one side. Check all vacuum lines for cracks or disconnections.
* Blend Door Cable (if applicable): Some models use cables instead of actuators. These cables can fray, break, or become disconnected, preventing proper blend door operation.
* Clogged Heater Core: While less likely to cause only passenger-side cold air, a partially clogged heater core can restrict hot air flow. If the core is partially blocked, the passenger side might receive less hot air than the driver's side.
* Low Coolant: If your coolant level is low, the heater core won't have enough fluid to heat effectively. Check your coolant reservoir and add coolant if necessary. However, a low coolant level usually affects both sides.
* Heater Core Failure: A completely failed heater core would result in no heat at all, but a partially clogged or damaged one could cause this issue.
Troubleshooting Steps:
1. Check the coolant level: This is the easiest and quickest check.
2. Listen carefully: When you turn on the heat, listen for any unusual sounds from the dashboard area. A clicking or whirring sound (or lack thereof) from the blend door actuator area could point to a problem.
3. Check vacuum lines: Visually inspect all vacuum lines connected to the HVAC system for cracks, leaks, or disconnections.
4. Test the temperature on both sides: See if the driver's side works correctly; this helps determine if the issue is isolated to the passenger side.
5. Manual Blend Door Test (if accessible): Some Buick Regals might allow access to manually operate the blend door. This requires opening the dash and is more complex, but could help diagnose a blend door or cable issue.
If you're not comfortable performing these checks yourself, taking your Buick Regal to a qualified mechanic is recommended. They have the tools and expertise to diagnose and repair the problem efficiently. Remember to specify that only the passenger side is affected.
