[B00MER]

Edit customer/search.php:

	if ($store_language != $config["default_customer_language"] && $substring) {
	
		$substring_query = "AND (($sql_tbl[products].product like '%$substring%' or $sql_tbl[products].descr like '%$substring%' or $sql_tbl[products].fulldescr like '%$substring%' OR $sql_tbl[products].productcode like '%$substring%') OR ($sql_tbl[products_lng].code='$store_language' AND ($sql_tbl[products_lng].product LIKE '%$substring%' OR $sql_tbl[products_lng].descr LIKE '%$substring%' OR $sql_tbl[products_lng].full_descr LIKE '%$substring%' OR $sql_tbl[products].productcode like '%$substring%')))";

		$search_query_count = "select count(*) from $sql_tbl[pricing], $sql_tbl[categories], $sql_tbl[products] LEFT JOIN $sql_tbl[products_lng] ON $sql_tbl[products].productid=$sql_tbl[products_lng].productid where $sql_tbl[pricing].productid=$sql_tbl[products].productid and $sql_tbl[pricing].quantity=1 and $sql_tbl[products].categoryid=$sql_tbl[categories].categoryid $membership_condition and ($sql_tbl[pricing].membership='". $user_account['membership']."' or $sql_tbl[pricing].membership='') AND $category_condition and $sql_tbl[products].forsale='Y' and $sql_tbl[categories].avail='Y' $price_condition $substring_query group by $sql_tbl[products].productid";

	}
	else {
		$substring_query = "AND ($sql_tbl[products].product like '%$substring%' OR $sql_tbl[products].descr like '%$substring%' OR $sql_tbl[products].fulldescr like '%$substring%' OR $sql_tbl[products].productcode like '%$substring%')";

		$search_query_count = "select count(*) from $sql_tbl[products], $sql_tbl[pricing], $sql_tbl[categories] where $sql_tbl[pricing].productid=$sql_tbl[products].productid and $sql_tbl[pricing].quantity=1 and $sql_tbl[products].categoryid=$sql_tbl[categories].categoryid $membership_condition and ($sql_tbl[pricing].membership='". $user_account['membership']."' or $sql_tbl[pricing].membership='') and ($sql_tbl[products].product like '%$substring%' or $sql_tbl[products].descr like '%$substring%' OR $sql_tbl[products].productcode like '%$substring%') AND $category_condition and $sql_tbl[products].forsale='Y' and $sql_tbl[categories].avail='Y' $price_condition $substring_query group by $sql_tbl[products].productid";
	}

Note this was done with 3.4.5 and was line 81 to line 92.

This can also be extended and an optional $substring_sku could be added and the advanced search (or basic search) form could have a "product code/sku" field.