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