I can't scan DTV channels on my vizio 52''

Have you tried doing the channel search? If that does not help replace the main board

the freeview board has failed easy to change it is just clipped in on to the board. that also could be the fault it may have come out of its conector.

REPLACE main board!

I had the same problem. Change your input to dtv even if you are using coaxial input. Then go to the menu screen and select tuner settings. Dtv setting should appear.

Do you have the antenna aimed correctly and the correct antenna for the distance between your location and the stations? Check antennaweb.org and tvfool.com for the directions to the stations you should be able to receive. Antennaweb assumes an external antenna and is very conservative in the station list. TVfool gives the stations you should get with various different types of antennas. Tall trees and buildings will block signal strength so consider the environment around you.

With digital signals, if you do not have enough signal strength for a given tuner, the channel will not come in. An amplifier may help you get the signal, particularly if the signal is shared to several devices. Failure of an amplifier or lightning surge protection can stop the signal from reaching the TV. A surge through the antenna can damage the tuner.

A damaged cable can also be a source of a problem. Check the outside portion of the wire as well as the interior wires. Replace any that have damaged insulation or ends.

For this TV, if you have done a channel scan with the TV set to the Input TV/DTV, and the Signal Source set to Antenna. Turn the antenna and then set the Additional Scan to On. (Menu > TV > Channels > Additional Scan.) Then do another channel scan.

If you need a copy of the manual, see: http://www.vizio.com/m220va.html#support .

go to setting and factory reset when it comes up it will ask you to scan channels analog digital or both click both

Check for proper cable connection on back of TV if there is more than one cable connector, and that you are scanning for proper source like AIR for the DTV tunner if no DTV tunner in TV you will need a Box, and if you are a cable user make sure you are scanning for cable channels.

Reset the TV to the factory default settings then reconnect the DTV, select correct input then perform auto scan.

Your TV probably does not have a digital tuner connected to the DTV/ANT input. Mine didn't. I had to connect the digital antenna to a digital converter box THEN go to the DTV/ANT input. Only then would it activate the DTV menu and allow me to scan for channels. If your TV worked fine with Cable TV connected to the input, it was because your signal was coming from the digital tuner in your DVR, or your cable provider was sending you an unscrambled digital signal (not likely).
I used a digital converter box I had purchased with the government $40 coupon back in 2009 but the resolution is only 480 and looks like a Monet or Renoir. Now I am shopping for a high quality tuner.

Run a channel scan on the DTV and/or TV input depending on what one your not getting channels on. This will solve your issue.

Change the input on the unit to the DTV or TV input. Press the menu button on the remote. Navigate to the tuner DTV or TV tuner setup menu. Go down to auto search using the down arrow. go right when with auto search selected. This may take a few minutes but you should have all the channels back after the scan.