The car condition bar is below your Power Unit and Tyre management bars during a race. As long as the car condition bar stays above 0%, the car runs normally.

Likewise to your Tyre management bar, your car condition deteriorates during a race.

Your selected Speed mode and your Reliability stat affect the rate of this decline: the faster the speed the faster your car condition declines and the higher your Reliability stat the slower your car condition decreases during a race.

If the bar drops to 0% the car breaks down and its speed is severely decreased until it gets repaired at the pit stop.

On the occasion of any pit stop, you can choose to service your car besides changing tyres. This will return your Car Condition back to 100%, but also add an extra service time to your overall pit stop. Choosing not to service the car at the pit stop doesn’t add any extra time but it doesn’t refill the car condition bar either.

The extra service time depends on the following factors:
The car’s condition - the lower your car condition bar the more time it takes to service the car.
The player's Reliability stat - the higher the Reliability stat the player has the less extra service time is added to pit stops.