Car not heating up

Good morning,

This sounds like you have air in the system. Every time you return the car to your guys, they most likely top off the coolant level and then bleed the system.

https://www.2carpros.com/articles/car-heater-not-working

They need to pressure test the system to find where the coolant is leaking.

https://www.2carpros.com/articles/radiator-pressure-test

https://www.2carpros.com/articles/car-is-leaking-coolant

Roy

Notice: All entrapped air must be purged from the powertrain cooling system before the final coolant level can be determined. Proper coolant level is critical to avoid engine damage.

Important: The vehicle must be level when filling the cooling system.

Slowly add a mixture of 50/50 DEX-COOL antifreeze and clean water to the surge tank. Refer to Capacities - Approximate Fluid for cooling system capacity.
1 As the system begins to fill, watch for coolant at the water pump tower.
2 As coolant begins to seep from the air bleed, close the air bleed screw.
3 Once the bleed screw is closed, continue to add coolant until a minimum of 5 liters (5.28 quarts) is added to the cooling system. Start the engine and run at idle to add the remaining coolant.
5. Install the surge tank cap.
6. Run the engine and cycle the vehicle from idle to 3,000 RPM in 30 second intervals until the engine cooling fans come ON.
7. Return the engine to idle, idle for 30 seconds then turn the engine OFF.
8. Allow the vehicle to cool.
9. Add coolant to the surge tank until the level is approximately 25 mm (1 inch) above the top of the Cold Fill Range at the surge tank seam.
10. Install the surge tank cap.