Legal Question in Landlord & Tenant Law in California

Do not withhold any rent. Pay the rent and have the items fixed yourself after giving the LL 10 days to fix them. Make sure your demand is in writing. If you are served with a three-day notice to pay the rent you've been withholding, pay it up immediately and within the three days (not on the fourth day). You do not want an eviction action started against you.

After you've given the LL 10 days to make the repairs and they refuse, you can repair the items yourself and deduct the cost from the rent. You can only do this once every 12 months. Make sure you document everything. If the repairs cost more than a month's rent, you can only deduct a month's rent. If you are in a rent controlled apartment, call the Housing Authority and file a complaint. Good luck.

If the unit does not have adequate heat to maintain livable temperatures in cold weather, call the local code enforcement agency and have them issue a citation. If the problems with the two heaters do not prevent the property from being heated adequately to be "habitable," however, you are potentially in deep trouble. A leak in the garage and two screens are not habitability issues, nor is a problem with heaters unless, as I said, it prevents the property from being habitable in cold weather. By withholding rent you are in breach of your lease and may be evicted at any time unless the property is deemed not to be habitable, and the legal standard for "habitable" is quite low. Pretty much as long as there is heat, light, water, sanitation, and complete closure from the elements, without infestation by vermin or structural defects that make the property too dangerous to live in, it is habitable. There is no remedy for a cruddy landlord's failure to fix things unless they make the property not habitable, or if you are on a lease for a specific term they may give you grounds to cancel the lease and move if they are material breaches of the implied promise to keep the property in the condition it was in when you moved in. That's it.

Having read Iadevaia's answer I would agree with him about the repair and deduct remedy, but you need to be careful there. If the conditions were present when you rented the property and disclosed, you cannot use the repair and deduct option except for habitability issues or if the landlord promised to fix them at the time you rented the property and failed to do so.