sql server error EXEC -- OUTPUT

I have a c# app. This is a piece of code out of a stored proc. it is erroring: Procedure or function getTopParentDealerFromChildDealer has too many arguments
OR
@dealerID is not a parameter for procedure getTopParentDealerFromChildDealer.(if I put ",@dealerID=@parentID)

I have tried all combinations "@dealerID",@dealerID=@parentID" etc.

BEGIN
--get the top parent dealerID
DECLARE @parentID INT
SET @parentID = 0
EXEC getTopParentDealerFromChildDealer @dealerID, @parentID OUTPUT
IF (@parentID>0)
BEGIN

------------------------------------------------------
here is the getTopParentDealerFromChildDealer as called
------------------------------------------------------
ALTER PROCEDURE getTopParentDealerFromChildDealer @childDealerID INT
AS

SET NOCOUNT ON
DECLARE @dealerID INT
DECLARE @parentID INT
SET @dealerID = 0
SELECT @dealerID = dealerParentID from dealerRelations where dealerChildID = @childDealerID

WHILE @dealerID <> 0
BEGIN
declare @temp INT
set @temp = @dealerID
IF (SELECT count(dealerParentID) FROM dealerRelations WHERE dealerChildID = @temp)>=1
BEGIN
SELECT @dealerID = dealerParentID
FROM dealerRelations where dealerChildID = @temp
END
ELSE
BEGIN
SET @dealerID=0
set @parentID = @temp
END
END

if (@parentID IS NULL)
BEGIN
set @parentID = 0
--set @parentID = @dealerID
END

return @parentID

I don't usually use stored procedures but the job I have taken over previously used them. Any help would be much appreciated.

Thanks

SQL server does not agree with that syntax(parentID already declared in procedure) but it could be the fn call. What is the correct syntax at the function call.

Fixed the syntax and it worked thanks but had to create another almost identical proc to handle the single parameter call. Why would they do it? Why?

Thanks