MinorAffected: 2012-201560,000–120,000 miles

Vauxhall Astra Astra J 1.6 CDTi 110bhp — 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

Timing Chain Rattle and Tensioner Failure

Critical

2012-201560,000–120,000 miles£800–£1500

The 1.6 CDTi A16DTH/DTJ engine is notorious for timing chain noise and failure, often from as low as 60,000 miles. This is the most serious known fault on the Astra J diesel.

Clutch Pedal Sticking Down

Severe

2009-201550,000–100,000 miles£150–£700

The clutch pedal fails to return to the top of its travel, often getting stuck halfway or hitting the floor completely.

DPF Blockage and Regeneration System Failure

Severe

2012-201540,000–90,000 miles£200–£1000

DPF blockage is extremely common on the Astra J 1.6 CDTi. The engine's tendency to get stuck in failed regen cycles results in diesel contaminating the engine oil — a serious secondary issue.

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