If you have the key to the door, you can enter: the obligation is to give the tenant notice, but the tenant is not allowed to keep you out for maintenance or repairs when you provide notice. You can therefore ignore the tenant's protestations or refusal.If the tenant has changed the locks, put a deadbolt on, etc. or otherwise physically excluded you, you can serve the tenant with a Notice to Cease requiring him/her to stop preventing you from entering and to, at the tenant's own cost, put things back so you can enter; if the tenant will not do so, you could then evict the tenant for barring the landlord from entry and making unapproved modifications to the rental unit (the new locks or chains, etc.).