MinorAffected: 2003-201360,000–120,000 miles

Audi A3 8P 2.0 TDI 140bhp PD — Engine Thermostat Sticking Open

The engine coolant thermostat fails in the 'open' position. The engine never reaches its optimal operating temperature, killing fuel economy.

Severity

Minor

DIY Difficulty

Moderate

Est. Cost

£100–£250

OBD Codes

P0128

Component Diagram

Cooling

Typical layout and location for the Cooling assembly.

Reference Illustration

Symptoms

Temperature gauge drops when driving on the motorway
Heater blows lukewarm air in winter
MPG drops significantly
Engine takes a very long time to warm up

Root Cause

The wax element inside the thermostat degrades, or the return spring weakens, preventing the valve from closing off coolant flow to the radiator.

How To Fix

Scan for P0128 (Coolant Thermostat - Coolant Temp Below Regulating Temp).
Replace the thermostat housing unit and gasket.
Bleed the cooling system of air.
Cost: £100-250

Estimated Repair Costs

Repair Option	Est. Cost (GBP)
DIY (parts only)	£40–£100
Independent Garage	£100–£250
Main Dealership	£160–£400

Other Faults on This Variant

Flywheel / DMF Shudder

Moderate

2003-201380,000–160,000 miles£700–£1000

Like the Golf Mk5, the A3 2.0 TDI consumes Dual Mass Flywheels. Worn DMFs cause significant vibration and require complete replacement.

High Pressure Fuel Pump Cam Follower Wear

Critical

2003-201320,000–100,000 miles£15–£1500

The same critical cam follower issue as the Golf Mk5 — the HPFP cam follower wears through to the camshaft. A £15 inspection and part replacement prevents a £2,500 camshaft repair.

Porous Cylinder Head (BKD Engine)

Critical

2003-200850,000–120,000 miles£1200–£2000

Early 2.0 TDI 140bhp engines (engine code BKD) suffered from porous cylinder heads, causing coolant to leak directly into the combustion chambers or oil ways.

Disclaimer: Repair cost estimates are indicative and based on community data. Always get a quote from a qualified mechanic before proceeding with any repair.