It sounds like you have a reason for cancelling the contract, are within your inspection contingency timeline, and just want to know whether you have to provide the actual reason for your cancellation to the Seller.
Given all of my assumption are correct, CAR Form RPA (Residential Purchase Agreement) Para 10 covers buyer investigations and Para 14 covers your inspection contingency. Furthermore, CAR Form CC (Cancelation of Contract/Release of Deposit) is the form used to cancel a contract. Para 1E of the CC provides the Buyer/Seller to list a section of the RPA for the reason for termination. I'm speculating Para 14 is the source of your desire to cancel.
You should discuss your desire to cancel the contract with your Agent before making a final decision.
Be aware RPA [Para 10B] states the Buyer, at no cost, must provide complete copies of all investigative reports obtained by Buyer. The obligation to do so even survives after the termination of the Agreement.
Additionally, via the RPA, Para 14F, the Seller typically has 30 days to sign escrow instructions to return your deposit:
"If Buyer or Seller gives written notice of cancellation â€¦.A Buyer or Seller may be subject to a civil penalty of up to $1,000 for refusal to sign such instructions if no good faith dispute exists as to who is entitled to the deposited funds (Civil Code Â§1057.3)." The Seller must release the deposit within 30 days per Civil Code Â§1057.3"
( Civil Code Â§1057.3 can be found at http://law.onecle.com/california/civil/1057.3.html
Do you need to provide a specific reason for cancelling? No, you could simply refer to an inspection report when filling out the CC form I imagine.
Would it be in your best interest to actually tell the Seller why you are cancelling? Probably. Being upfront with people is a sign of respect, and sharing the true reason(s) for wanting to cancel the contract might actually help you arrive at a mutual agreement that works with both Buyer and Seller.
Best of luck with your purchase!