P0128

Coolant thermostat temperature below the control range.

The engine did not reach the expected temperature under the controlled conditions. The cooling, measurement and conditions of use are still to be separated.

What should be checked

Thermal behaviour and circulation to be checked

The heating depends on the thermal control, the traffic and the operating conditions of the vehicle.

In plain language: The engine may be too late to warm up or losing heat before reaching its normal temperature.

How to check it: Comparison of actual heating to room temperature and the limits of the specific procedure.
Before correcting it: Do not open the hot cooling system or change the thermostat without testing it. The correction is decided only after locating and confirming the defect.
Related components: This path may involve the thermostat, housing, coolant, passages, radiator, or circulation components. None has been confirmed.
When to stop: Stop if overheated, steam, leakage or if the test requires the refrigerant circuit to be opened.

Temperature measurement to be checked

A temperature signal or its circuit can report a value other than the actual temperature.

In plain language: The computer may be seeing the wrong temperature even though the engine is normally heating up.

How to check it: Comparison of cold and heated reading with an independent reference and the authorised procedure.
Before correcting it: You don't replace the sensor until you confirm the difference and check its circuit. The correction is decided only after locating and confirming the defect.
Related components: This path may involve the engine coolant temperature sensor, its connector, wiring, power supply, or ground. None has been confirmed.
When to stop: Use professional help if you need to measure active wiring or access the heating system.

Thermal behaviour and circulation to be checked — The engine may be too late to warm up or losing heat before reaching its normal temperature.

Information needed for a more precise diagnosis

Exact brand, model, year, version and engine.
All the codes present and their OBD status.
Freeze frame and live data related to the system.
Symptoms, mileage and recent maintenance history.
Procedure and limits of an authorised source for that vehicle.

Where to find diagrams and manuals

Manufacturer repair information directory

Choose the make and open its official portal to find the manual, diagrams, cylinder arrangement, and bulletins for the exact vehicle.

Availability, registration, and cost depend on the manufacturer. The link does not grant permission to copy the manual.

Freedom to Fix: emissions service information

Understand which emissions-related service information manufacturers must make available for purchase.

Explains regulated access in the United States; it does not contain the vehicle-specific diagram or make the manual free.

Sources reviewed

Audi A3 Quick Reference Specification Book — Audi of America / archivo NHTSA. It supports the punctual definition of the code; It does not identify a cause, a part or a repair.
Frequently Asked Questions About On-Board Diagnostics — U.S. Environmental Protection Agency. Supports the safety difference between a fixed breakdown lamp and a flashing lamp; It doesn't diagnose the cause.