Can police search car if you say no?

Yes an officer can still search your car. The officer could claim there is probable cause to search your car, because you just denied the search, which the officer would say, looks like a sign of guilt.

Another View: NOT exactly! If you do not voluntarily consent to a vehicle search, the officer can still partially search it, but he MUST confine his search to the immediate area around, and within the reach of, the driver.

To carry the search any further he could call for a drug dog or need some probable cause to place you under arrest and impound your car. Of course if he observes anything suspicious while within the car (the butt of a gun - 'roaches' in the ash tray - drugs, pills, or other contraband on the floor mats, etc. then you're pretty much 'toast.'