Can a lienholder repossess a car if it is not in default but in the impound?

As long as you continue to make the payments, they would have no reason the instigate a repossession. When a vehicle is financed or leased, the creditor has an interest in the vehicle and rights under the contract you signed. If you are in default of the contract either by default in payment or otherwise (failure to insure or other terms) the vehicle can be repossessed.