It could be anything, starting from a space character in one of your text boxes on up.
Please don't do it like that - you leave yourself wide open to an SQL Injection attack as well as to problems like this. Use a parametrized query instead:
SqlCommand cmd2 = new SqlCommand("Update diesel set SNO = @SNO,Date = @DAT,...", con);
cmd2.Parameters.AddWithValue("@SNO", cb_sno.Text);
cmd2.Parameters.AddWithValue("@DAT", dateTimePicker1.Value);
...
cmd2.ExecuteNonQuery();
It also makes the code a whole lot more readable.