No Heat/Insufficient Heat at Low Speeds/Low Ambient Temperatures (Verify Heater Performance, Check Thermostat Operation, Flush Engine Cooling System)

This bulletin is being revised to update the information, remove the Aveo model, delete VIN breakpoint information and add a specific engine for the Optra. Please discard Corporate Bulletin Number 04-01-37-003A (Section 01 - HVAC).

Some customers may comment on no heat or insufficient heater performance in stop-and-go traffic or at low speeds, especially at low ambient temperatures (below about -20°C [-4°F]).

Verify the customer concern by measuring the heater performance in the shop at idle.

Then, if required, verify correct thermostat operation and flush the engine cooling system using the heavy duty coolant cleaning kit following the procedure below.

Verify the customer's concern

Caution: Use the appropriate exhaust ventilation equipment.

Important: The engine should be warmed up fully to an average operating temperature before measuring the heater outlet air temperature.

1. Start the engine and allow the engine to warm up for about 20 minutes in a workshop with a minimum ambient temperature of 17°C (63°F).
2. Measure the discharge air temperature at the driver's heater floor outlet using a thermometer. The blower motor should be on the maximum speed with the temperature switch set to full hot and the mode switch set for "floor."
3. Verify that the heater output as measured at the driver's heater floor outlet is at 59°C (138°F) or higher.

•	If the discharge temperature is below 59°C (138°F), proceed with the thermostat check and if the thermostat is OK, go to the flush procedure.

•	If the discharge temperature is above 59°C (138°F), the heater performance is normal and no further action is required.

Check the thermostat operation

Important: Engine should be warmed up fully to an average operating temperature before measuring the engine coolant temperature so this procedure should be done immediately after the initial heater performance evaluation.

1. Turn OFF the ignition switch and connect a scan tool.
2. With the engine idling, check the engine coolant temperature using the scan tool.
3. If the measured value meets the minimum of 83°C (181°F), the thermostat is OK. Proceed to the next step for draining and flushing the cooling system.

Drain the cooling system

Caution: Dispose of the used coolant into a used coolant holding tank for disposal. Never pour the used coolant down the drain. It can contaminate the local environment.

Caution: Do not remove the surge tank cap or open the cooling system drains while the engine is hot. Allow the system to cool.

1. Place a container below the vehicle to catch the draining coolant.
2. Remove the radiator lower hose or open the radiator drain cock.
3. Remove the surge tank cap.

Flush heater core and cooling system with H/D coolant chemical cleaner

Caution: Avoid spilling solution on the vehicle's finish. Do not mix with hydrofluoric acid or phosphoric acid. Do not swallow. Do not get in eyes. Do not get on skin or clothing.

1. Reinstall the lower radiator hose or radiator drain cock.
2. Mix the cleaner (from the upper part of can) in approximately 3L (3.17 qt) of water.
3. Pour the mixture into the surge tank.
4. Fill the remaining system with plain water.
5. Reinstall the surge tank cap.
6. Run the engine at fast idle for one hour.
7. Stop the engine and allow the engine to cool.
8. Drain the cooling system (refer to previous procedure titled "Draining the Cooling System").

Flush heater core and cooling system with neutralizer

1. Reinstall the lower radiator hose or radiator drain cock.
2. Mix the neutralizer (from the bottom part of can) in approximately 3L (3.17 qt) of water.
3. Pour the mixture into the surge tank.
4. Fill the remaining system with plain water.
5. Reinstall the surge tank cap.
6. Run the engine for 10 minutes.
7. Stop the engine and allow the engine to cool.
8. Drain the cooling system (refer to previous procedure titled "Draining the Cooling System").

Flush heater core and cooling system with water only

1. Reinstall the lower radiator hose or radiator drain cock.
2. Fill the system with plain water only.
3. Reinstall the surge tank cap.
4. Run the engine for 10 minutes.
5. Stop the engine and allow the engine to cool.
6. Drain the cooling system (refer to previous procedure titled "Draining the Cooling System").
7. Remove the heater hoses to ensure drainage of water. Reinstall the heater hoses once they are drained.

Check the heater performance after draining and flushing

1. Refill the cooling system with a 50/50 mixture of coolant.

Important: Engine should be warmed up fully to an average operating temperature before measuring the heater outlet air temperature.

2. Start the engine in the workshop (ambient temperature above 17°C [63°F]) and allow the engine to warm up for about 20 minutes.
3. Measure the discharge air temperature at the driver's heater floor outlet using a thermometer. The blower motor should be on the maximum speed with the temperature switch set to full hot and the mode switch set for "floor."
4. Verify that the heater output as measured at the driver's heater floor outlet is at 59°C (138°F) or higher.
5. If the discharge temperature is below 59°C (138°F), replace the heater core.

For vehicles repaired under warranty, use: