If a male cat is neutered, why is he still marking around walls?

my mom's male cat is neutered, but keeps marking/urinating on walls. he has an adequate litter box which he uses, but he keeps marking everywhere on the walls. he is a very sweet cat, other than this. we are even trying the plug in fermone stuff, but w/ no good results. any good advise please. we don't... show more my mom's male cat is neutered, but keeps marking/urinating on walls. he has an adequate litter box which he uses, but he keeps marking everywhere on the walls. he is a very sweet cat, other than this. we are even trying the plug in fermone stuff, but w/ no good results. any good advise please. we don't want to get rid of him, but i don't want him pissing all over my house either. thanks.
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