Hi and welcome to the site. 🙂
As mentioned above, if cleaning doesn't resolve the issue replacement is the only real fix, unless you buy a blanking plate and cheater, but allegedly the ones available from ebay are not too reliable. This site used to sell them but demand dwindled as the cars & engines got older.
The genuine GM part # is:
12569552 - Valve,Assy.,Exhaust Gas Recirculation (appx £200 from Vauxhall dealers)
PIerburg OEM part (as mentioned above) - £129.42 from AutoVaux
https://www.autovaux.co.uk/catalogsearch/result/?q=12569552
Hope that helps. (y)