In version 1.6.1 the Application_top.php has an incorrect SQL Statement.
In Application_top.php find:
$model_query = tep_db_query("select pd.products_model from " . TABLE_PRODUCTS . " p left join " . TABLE_PRODUCTS_TO_CATEGORIES . " p2c on p.products_id = p2c.products_id left join " . TABLE_CATEGORIES . " c on p2c.categories_id = c.categories_id, " . TABLE_PRODUCTS_DESCRIPTION . " pd where c.categories_status = '1' and p.products_id = '" . (int)$HTTP_GET_VARS['products_id'] . "' and p.products_id = pd.products_id");
Replace with:
$model_query = tep_db_query("select p.products_model from " . TABLE_PRODUCTS . " p left join " . TABLE_PRODUCTS_TO_CATEGORIES . " p2c on p.products_id = p2c.products_id left join " . TABLE_CATEGORIES . " c on p2c.categories_id = c.categories_id, " . TABLE_PRODUCTS_DESCRIPTION . " pd where c.categories_status = '1' and p.products_id = '" . (int)$HTTP_GET_VARS['products_id'] . "' and p.products_id = pd.products_id");
Thats it,
Al