If in employee is allowed to use a company vehicle to travel to and from work and for limited personal use, can the employer put conditions on the use? example. Coming to work on a Saturday without pay to clean vehicle and do maintenance or vehicle use will be withdrawn.
The employee has no contract stating that the vehicle is part of the benefit package, it has just been an accepted practice since start of hire. The employer is located in CA.

Thanks

Yes, an employer can put conditions on the use of a company vehicle, such as keeping it clean. But if they require you to come to put in additional time to wash and perform maintenance on the vehicle outside of your regular work hours, they must pay you for that time. This assumes you are a non-exempt (hourly paid) employee; if you are in an exempt position they have no requirement other than paying you your regular weekly salary no matter how many hours you work.

Can an employer threaten to take the personal use of a company vehicle away if employee doesn't come in on weekends to do maintenance?

Yes.

As Beth3 mentioned, you may be entitled to be paid for those hours.