How to set a service account password in PowerShell

Are you getting an error when calling .Change? If not, the password is set correctly, and you might need to assign the Log on as a Service privilege to the account used.

There's no error. The Log on as a Service privilege would be assigned when the service is restarted, but it won't restart due to an incorrect password (or so it thinks). That's why I have to reset the password and manually restart it myself.

I cannot see how the privilege should get assigned. Do you have other code for doing so, or are you using a group policy to do? "Would be assigned when the service is restarted" is not true without taking other actions.
I can't recall exactly, but the effects from this privilege missing might be the same as if you set a wrong password.
If it is the privilege, changing to a wrong password should report a script failure. Changing it back should succeed, and the password being set properly.

Here's what I run first to create the account and password:

$computername = $env:computername   # place computername here for remote access
$username = 'anAccount'
$password = 'StrongPassword'
$desc = 'Backup service account'


$computer = [ADSI]"WinNT://$computername,computer"
$user = $computer.Create("user", $username)
$user.SetPassword($password)
$user.Setinfo()
$user.description = $desc
$user.setinfo()
$user.UserFlags = 65536
$user.SetInfo()
$group = [ADSI]("WinNT://$computername/administrators,group")
$group.add("WinNT://$username,user")

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$password = ConvertTo-SecureString 'StrongPassword' -AsPlainText -Force

Really though you should be storing your passwords in an encrypted fashion if they're being used in a script:

https://blog.kloud.com.au/2016/04/21/using-saved-credentials-securely-in-powershell-scripts/

$localaccount = ".\anAccount"
$newpassword = ConvertTo-SecureString "strongPassword" -AsPlainText -Force
$strCompName = ""
$filter = "avbackup"

$service = Get-WmiObject win32_Service -Filter "Name='$filter'"
$service.Change($null,$null,$null,$null,$null,$null,$localaccount,$newpassword)
$service.StopService()
  while ($service.Started){
    sleep 2
    $service = Get-WMIObject -ComputerName $strCompName -namespace "root\cimv2" -class Win32_Service -Filter $filter
  }
  $service.StartService()

Same error--The service did not start due to a logon failure.

I think you have to stop the service before attempting to change the password.  Try this:

$localaccount = ".\anAccount"
$newpassword = ConvertTo-SecureString "strongPassword" -AsPlainText -Force
$strCompName = ""
$filter = "avbackup"

$service = Get-WmiObject win32_Service -Filter "Name='$filter'"
$stop = $service.StopService()
if ($stop.returnvalue -eq 0) {
  $change = $service.Change($null,$null,$null,$null,$null,$null,$localaccount,$newpassword,$null,$null,$null)
  if ($change.returnvalue -eq 0) {
    Write-Host "The $filter service credentials were changed successfully" -ForegroundColor Green
  }
  else { 
    Write-Host "There was an error attempting to change the $filter service account credentials.  The error code was $($change.returnvalue)" -ForegroundColor Red
    break
  }
}
else {
  Write-Host "There was an error attempting to stop the $filter service.  The error code was $($stop.returnvalue)" -ForegroundColor Red
}

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Says here that you cannot use a secure string for change method.,   Also It looks like there are 11 parameters.  Not sure you need $null identifying them all.   Meaning adding 3 more $nulls after the eighth. (which is for password.

https://mcpmag.com/articles/2015/01/22/password-for-a-service-account-in-powershell.aspx


$localaccount = ".\anAccount"
 $newpassword = ConvertTo-SecureString "strongPassword" -AsPlainText -Force
  $BSTR = [system.runtime.interopservices.marshal]::SecureStringToBSTR($newpassword)
  $newpassword = [system.runtime.interopservices.marshal]::PtrToStringAuto($BSTR)



 $strCompName = ""
 $filter = "avbackup"

 $service = Get-WmiObject win32_Service -Filter "Name='$filter'"
 $service.Change($null,$null,$null,$null,$null,$null,$localaccount,$newpassword)
 $service.StopService()
   while ($service.Started){
     sleep 2
     $service = Get-WMIObject -ComputerName $strCompName -namespace "root\cimv2" -class Win32_Service -Filter $filter
   }
   $service.StartService()

Man it looks like we're trying to reinvent the wheel.  There is already a script for this in the Gallery:

https://gallery.technet.microsoft.com/Powershell-How-to-change-be88ce7e

Impressive work as always Qlemo

Thank you. I've used the carbon.dll to remotely restart the account from another server, but I hadn't tried locally, so this will be the best solution. Thanks to everyone who contributed.