Could be one of several things.

• Cooling fans not operating correctly.
• Air pocket in the coolant system.
• Combustion gases in the coolant.

What tools do you have for testing and are you experienced in checking these things out?

After making sure the cold coolant level is okay start the engine and turn the a/c on. The fans should come on when the a/c pressures start coming up. If they do turn the ignition and the a/c off and restart the engine. Monitor the engine temperature with the scan tool as it warms up from cold to operating temperature. If the fans do not come on by the time the engine temp reaches 240 F turn the engine off.  Let the engine cool fully and observe the coolant level in the reservoir to see if it drops. If it does it may take a few heat cycles to purge any air from the system.

Make sure the reservoir is at the full cold level. Install the cap. Run the engine until the gauge goes up to just above normal operating temperature. Do not let it over heat. Pay attention to the coolant level in the reservoir. If it comes up that is okay just don’t let it overflow or boil.  Once the engine has gotten slightly over normal operating temperature and the reservoir has not overflowed turn the engine off and observe the coolant level over several hours as the engine cools. If it does and drops below the full cold line that will mean that some air has been removed and the coolant will need to be topped off again and the engine run through another heat cycle. It may take 3 or 4 heat cycles to purge the air. Assuming that is the issue.

The following is from GM service information.

Cooling System Draining and Filling

Draining Procedure

Caution: As long as there is pressure in the cooling system, the temperature can be considerably higher than the boiling temperature of the solution in the radiator without causing the solution to boil. Removal of the pressure cap while the engine is hot and pressure is high will cause the solution to boil instantaneously — possibly with explosive force — spewing the solution over the engine, fenders and the person removing the cap.

Caution: An electric fan under the hood can start up even when the engine is not running and can injure you. Keep hands, clothing and tools away from any underhood electric fan.

1. 1. Remove the surge tank pressure cap.

1. 2. Remove the front air deflector. Refer to Front Air Deflector Replacement in Body Front End.
   3. Open the radiator drain at the bottom of the radiator.
   4. Completely drain the cooling system.
   5. If the coolant is dirty, or if there are deposits in the radiator, flush the cooling system before refilling. Refer to Flushing.
   6. Remove the surge tank. Refer to Radiator Surge Tank Replacement.

2. Install the surge tank. Refer to Radiator Surge Tank Replacement.
3. Close the radiator drain.
4. Install the front air deflector. Refer to Front Air Deflector Replacement in Body Front End.

Filling Procedure

Notice:  Do not add cold water to the cooling system with the engine at or above operating temperature. Adding cold water causes rapid cooling, resulting in possible engine damage.

NOTICE:  When adding coolant, it is important that you use GM Goodwrench DEX-COOL® or HAVOLINE® DEX-COOL® coolant. If Coolant other than DEX-COOL® or HAVOLINE® DEX-COOL® is added to the system the engine coolant will require change sooner; at 50 000 km (30,000 mi) or 24 months.

Notice:  Do not use a solution stronger than 70 percent antifreeze. Pure antifreeze can freeze at -22°C (-8°F).

Important:  This engine is not equipped with block drains, 2.5 L (2 Qt) of residual coolant remains in the engine block.

1. Refill the cooling system. In order to ensure sufficient engine cooling, freezing and corrosion protection, maintain the protection level at -37°C (-34°F) or lower.
2. Start the engine.
3. Place the heater and A/C control in any A/C mode except Max and the temperature in the highest setting.
4. Allow the engine to continue idling until the lower radiator to water pump hose is hot.
5. Turn OFF the engine.
6. Allow the engine to cool to outside temperature. Ensure the coolant level in the surge tank is at the proper level (1).

The number one thing to do at this point is to double check that all wiring was correctly reinstalled during the leak repair. Be sure to check for blown fuses also. If no problems are found the easiest thing is to find a quality shop with the correct type of scan tool to access data and bidirectional controls.

If that is not an option and we need to use what you have I would suggest locating and disconnecting each of these components one at a time and then check to see if a code will set.

Engine coolant temperature sensor. Disconnecting the coolant temperature sensor may force the ECM to turn on the fans.

Cooling fan relays

A/C high pressure sensor.

The “white” wire should actually be yellow in color and is most likely faded. It should be in the B position and the black wire should be in the A position.

The way that the fans are controlled is fairly complicated. I have supplied the OE descriptions below. From your description of how only one fan operated when the sensor was disconnected I am thinking that either the second fan motor is faulty and/or one of the three cooling fan relays is faulty.

Cooling System Description and Operation

Cooling Fan Control

The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block. The ground path is provided at G104.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage through the cooling fan motor supply voltage circuit to the right cooling fan. The ground path for the right cooling fan is through the cooling fan s/p relay and the left cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cooling fan 2 relay and the cooling fan s/p relay through the high speed cooling fan relay control circuit. This energizes the cooling fan s/p relay coil, closes the relay contacts, and provides a ground path for the right cooling fan. At the same time the cooling fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage on the cooling fan motor supply voltage circuit to the left cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The PCM commands low speed fans ON under the following conditions:

• Engine coolant temperature exceeds approximately 106°C (223°F).
• The transmission fluid temperature exceeds 150°C (302°F).
• A/C operation is requested.

The engine cooling fans will switch from low to OFF when the coolant temperature drops below 102°C (216°F).

The PCM commands High Speed Fans on under the following conditions:

• Engine coolant temperature reaches 112°C (234°F).
• The transmission fluid temperature is more than 151°C (304°F).
• When certain DTCs set.

The engine cooling fans will switch from high to low, except when DTCs set, when the coolant temperature drops below 106°C (223°F).

Engine Coolant Indicators

Engine Coolant

The IPC illuminates the engine coolant indicator and sends a class 2 message in order to activate an audible warning when the following occurs:

• The IPC determines that the coolant temperature is greater than 128°C (262°F). The IPC receives a class 2 message from the PCM indicating the coolant temperature.
• The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.

The IPC turns OFF the engine coolant indicator when the engine coolant falls below 125°C (257°F).

CHECK COOLANT LEVEL – 2

The IPC illuminates the CHECK COOLANT LEVEL – 2 indicator in the DIC when the IPC detects that the engine coolant level is below the normal operating range (signal circuit is high). The IPC receives a discrete input from the engine coolant level switch.

ENGINE COOLANT HOT/IDLE ENGINE – 44

The IPC illuminates the ENGINE COOLANT HOT/IDLE ENGINE – 44 indicator in the DIC and sends a class 2 message in order to activate an audible warning when the IPC determines that the coolant temperature is greater than 128°C (262°F). The IPC receives a class 2 message from the PCM indicating the coolant temperature.

The IPC turns OFF the indicator when the engine coolant falls below 125°C (257°F).

ENGINE HOT – AC OFF – 16

The IPC illuminates the ENGINE HOT – AC OFF – 16 indicator in the DIC when the dash integration module (DIM) detects that the engine temperature is above the normal operating range and has turned off the air conditioning in order to allow the engine to cool down. The IPC receives a class 2 message from the DIM requesting illumination.

ENGINE OVERHEATED/STOP ENGINE – 42

The IPC illuminates the ENGINE OVERHEATED/STOP ENGINE – 42 indicator in the DIC and sends a class 2 message in order to activate an audible warning when the powertrain control module (PCM) detects that the engine temperature is above the normal operating range. The IPC receives a class 2 message from the PCM requesting illumination.

Coolant Level Control

The engine cooling system contains an engine coolant level switch to alert the driver in the event of a coolant loss. When the engine coolant level switch reads a low coolant level in the surge tank, the switch opens. This sends a coolant loss signal to the IPC by the coolant level switch signal circuit. Ground is provided by G201 for the coolant level control.

Coolant Heater

The optional engine coolant heater (RPO K05) consists of 2 electrical heating elements fastened to the left and right sides of the engine block. These heating blocks are precisely machined to fit the engine block area. Each heating element is rated at 300 watts and supplies 1025 btu/hr. The engine coolant heater operates using 110-volt AC external power and is designed to warm the coolant in the engine block area for improved starting in very cold weather -29°C (-20°F). The coolant heater helps reduce fuel consumption when a cold engine is warming up. The units are equipped with a detachable AC power cord. A weather shield on the cord is provided to protect the plug when not in use.

Cooling System

The cooling system’s function is to maintain an efficient engine operating temperature during all engine speeds and operating conditions. The cooling system is designed to remove approximately one-third of the heat produced by the burning of the air-fuel mixture. When the engine is cold, the system cools slowly or not at all. This allows the engine to warm quickly.

Cooling Cycle

Coolant is drawn from the radiator outlet to the thermostat. The flow of coolant will either be stopped at the thermostat until the engine is warmed, or it will flow through the thermostat and into the water pump inlet by the water pump. Some coolant will then be pumped from the water pump, to the heater core, then back to the water pump. This provides the passenger compartment with heat and defrost.

Coolant is also pumped through the water pump outlet and into the engine block. In the engine block, the coolant circulates through the water jackets surrounding the cylinders where it absorbs heat.

The coolant is then forced through the cylinder head gasket openings and into the cylinder heads. In the cylinder heads, the coolant flows through the water jackets surrounding the combustion chambers and valve seats, where it absorbs additional heat.

From the cylinder heads, the coolant is then forced into the radiator where it is cooled and the coolant cycle is completed.

Operation of the cooling system requires proper functioning of all cooling system components. The cooling system consists of the following components:

Coolant

The engine coolant is a solution made up of a 50/50 mixture of DEX-COOL and clean drinkable water. The coolant solution carries excess heat away from the engine to the radiator, where the heat is dissipated to the atmosphere.

Radiator

The radiator is a heat exchanger. It consists of a core and 2 tanks. The aluminum core is a crossflow tube and fin design. This is a series of tubes that extend side to side from the inlet tank to the outlet tank. Fins are placed around the outside of the tubes to improve heat transfer from the coolant to the atmosphere. The inlet and outlet tanks are molded with a high temperature, nylon reinforced plastic. A high temperature rubber gasket seals the tank flange edge. The tanks are clamped to the core with clinch tabs. The tabs are part of the aluminum header at each end of the core. The radiator also has a drain cock which is located in the bottom of the left hand tank. The drain cock includes the drain cock and drain cock seal.

The radiator removes heat from the coolant passing through it. The fins on the core absorb heat from the coolant passing through the tubes. As air passes between the fins, it absorbs heat and cools the coolant.

Surge Tank

The surge tank is a plastic tank that the pressure cap mounts onto. The tank is mounted at a point higher than all other coolant passages. The surge tank provides an air space in the cooling system. The air space allows the coolant to expand and contract. The surge tank also provides a coolant fill point and a central air bleed location.

During vehicle use, the coolant heats and expands. The coolant that is displaced by this expansion flows into the surge tank. As the coolant circulates, air is allowed to exit. This is an advantage to the cooling system. Coolant without bubbles absorbs heat much better than coolant with bubbles.

Pressure Cap

The pressure cap is a cap that seals and pressurizes the cooling system. It contains a blow off or pressure valve and a vacuum or atmospheric valve. The pressure valve is held against its seat by a spring of predetermined strength, which protects the radiator by relieving pressure if it exceeds 18 psi. The vacuum valve is held against its seat by a spring, which permits opening of the valve to relieve vacuum created in the cooling system as it cools off. The vacuum, if not relieved, might cause the radiator to collapse.

The pressure cap allows pressure in the cooling system to build up. As the pressure builds, the boiling point of the coolant goes up as well. Therefore, the coolant can be safely run at a temperature much higher than the boiling point of the coolant at atmospheric pressure. The hotter the coolant is, the faster the heat moves from the radiator to the cooler, passing air. The pressure in the cooling system can get too high, however. When the pressure exceeds the strength of the spring, it raises the pressure valve so that the excess pressure can escape. As the engine cools down, the temperature of the coolant drops and a vacuum is created in the cooling system. This vacuum causes the vacuum valve to open, allowing outside air into the cooling system. This equalizes the pressure in the cooling system with atmospheric pressure, preventing the radiator from collapsing.

Air Baffles and Seals

The cooling system uses deflectors, air baffles and air seals to increase system cooling. Deflectors are installed under the vehicle to redirect airflow beneath the vehicle to flow through the radiator and increase cooling. Air baffles are also used to direct airflow into the radiator and increase cooling. Air seals prevent air from bypassing the radiator and A/C condenser. Air seals also prevent recirculation of the air for better hot weather cooling and A/C condenser performance.

Water Pump

The water pump is a centrifugal vane impeller type pump. The pump consists of a retaining plate, pulley and an impeller. The impeller is a flat plate mounted on the pump shaft with a series of flat or curved blades or vanes. When the impeller rotates, the coolant between the vanes is thrown outward by centrifugal force. The impeller shaft is supported by one or more sealed bearings. These sealed bearings never need to be lubricated. With a sealed bearing, grease cannot leak out, and dirt and water cannot get in.

The purpose of the water pump is to circulate coolant throughout the cooling system. The water pump is driven by the camshaft via the drive belt.

Thermostat

The thermostat is a coolant flow control component. It’s purpose is to regulate the operating temperature of the engine. It utilizes a temperature sensitive wax-pellet element. The element connects to a valve through a piston. When the element is heated, it expands and exerts pressure against a rubber diaphragm. This pressure forces the valve to open. As the element is cooled, it contracts. This contraction allows a spring to push the valve closed.

When the coolant temperature is below 85°C (185°F), the thermostat valve remains closed. This prevents circulation of the coolant from the radiator and allows the engine to warm up quickly. After the coolant temperature reaches 85°C (185°F), the thermostat valve will open. The coolant is then allowed to circulate through the thermostat to the engine and then to the radiator where the engine heat is dissipated to the atmosphere. The thermostat also provides a restriction in the cooling system, even after it has opened. This restriction creates a pressure difference which prevents cavitation at the water pump and forces coolant to circulate through the engine block.

Engine Oil Cooler

The engine oil cooler is a heat exchanger. It is located inside the right side end tank of the radiator. The engine oil temperature is regulated by the temperature of the engine coolant that surrounds the oil cooler as the engine oil passes down through the cooler.

The engine oil pump, pumps the oil through the engine oil feed line to the oil cooler. The oil then flows down through the cooler while the engine coolant absorbs heat from the oil. The oil is then pumped through the oil return line, to the oil filter, then to the main engine oil passage.

Transmission Oil Cooler

The transmission oil cooler is a heat exchanger. It is located inside the left side end tank of the radiator. The transmission fluid temperature is regulated by the temperature of the engine coolant that surrounds the oil cooler as the transmission fluid passes down through the cooler.

The transmission oil pump, pumps the fluid through the transmission oil cooler feed line to the oil cooler. The fluid then flows down through the cooler while the engine coolant absorbs heat from the fluid. The fluid is then pumped through the transmission oil cooler return line, to the transmission.