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From your owner's manual:
Stability Traction Control (STC)
The STC system is designed to help reduce wheel spin by limiting power to the drive wheels if they begin to lose traction. At speeds over 25 mph (40 km/h), the STC system monitors and compares all four wheels. If one of the drive (front) wheels shows any tendency to slip, such as when driving on slippery roads, the difference in speed is immediately detected. This triggers a signal to the engine management system, which will reduce engine torque (by reducing fuel) until the differential is corrected. This torque reduction is handled in stages, and reaction time is extremely fast. The system can be switched on or off by pressing the button on the center console for at least half a second (see page 29). The indicator light ( ) in the instrument panel will be ON when you have switched the system off. The STC system comes on each time you start your car.
The STC indicator light () will come on for approximately 2 seconds when the engine is started as the system performs a self-diagnostic test. This system should be switched off if you, for any reason, temporarily have to drive with tires of different dimensions (e.g., spare tire).
The symbol will flash when STC is actively regulating power to the drive wheels. Normal power may be reduced at this time. This is normal as power is momentarily reduced to help keep the drive wheels from losing traction and spinning.
The STC indicator light () will come on for approximately 2 seconds when the engine is started as the system performs a self-diagnostic test.
If the STC system has been switched on using the button in the center console (the indicator light will normally go off) and the indicator light remains on, there is a fault in the system. In this case:
Stop the car in a suitable place and switch off the engine.
Restart the engine.
If the indicator light goes off, no further action is required.
If the light remains on, have the STC system inspected by an authorized Volvo retailer.
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Are all 4 of your tires the same age, size, and brand? If not, make it so.
The 'clunking' could be a rear brake pad that is not OEM and moving slightly, or a broken front pad.
Klaus
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There is no present time, just the past and future
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