This guide provides a detailed walkthrough for replacing the thermostat on a 04 Audi A4 1.8t. The original thermostat part number is 06B-121-111-D, often replaced by 06B-121-111-G or the latest revision. This DIY guide is intended for informational purposes only and does not substitute for professional mechanical advice. Proceed with caution and consult a qualified mechanic if you are unsure about any step.
Draining the Coolant
Begin by preparing the vehicle. Park on a level surface, engage the parking brake, and allow the engine to cool completely. Open the hood and locate the coolant reservoir tank. Remove the reservoir cap and the plastic clip securing the coolant temperature sensor at the lower radiator hose. Position a drain pan beneath the sensor and carefully remove it to drain the coolant.
Accessing the Thermostat
Step 1: Upper Radiator Hose Removal. Disconnect the small hose connecting the coolant reservoir to the upper radiator hose. Then, detach the larger hose from the hard pipe along the intake manifold and the radiator. You’ll need to release the metal clip securing the hose to the radiator before pulling it back.
Step 2: Intake Hose Removal. For easier access, remove the driver’s side headlight (optional). Loosen the clamps at both ends of the intake hose (connecting the throttle body to the intercooler) and remove it.
Step 3: Throttle Body Removal. Disconnect the electrical connector and remove the four 5mm hex bolts securing the throttle body.
Step 4: Alternator and Support Removal. Loosen the serpentine belt tensioner and carefully slip the belt off the alternator pulley. Remove the four bolts holding the alternator to its bracket. Disconnect the electrical connector and grounding cable from the alternator (cover the grounding strap with tape to prevent shorts). Remove the two 6mm hex bolts securing the vertical support piece connecting the intake manifold to the engine block.
Step 5: Detaching the Coolant Hard Pipe. Remove the bolt securing the metal coolant pipe to the thermostat housing and the bolt from the top of the oil cooler. This will allow you to move the hard pipe away from the thermostat for easier removal.
Thermostat Replacement
Disconnect the upper and lower hoses from the thermostat housing and remove the electrical connector on the back. Remove the two bolts holding the thermostat to the engine block. Install the new thermostat with a new O-ring (lubricate with coolant for a proper seal), ensuring the old O-ring is removed. Reassemble in reverse order.
Completion
After reassembling all components, refill the cooling system with the correct coolant type and bleed the system according to your vehicle’s specifications. Start the engine and check for leaks. Monitor the engine temperature to ensure the new thermostat is functioning correctly.
