You don't specifically say, but are you an independent contractor or a W-2 employee?
If you are an independent contractor, the source of your income is determined (for CA purposes) by where your customers live. If you have no CA customers, then temporarily working from CA does not subject you to CA income tax.
However, if you are a W-2 worker, the source of your income under CA tax law is determined by where you are living or working when you perform the work. See here. https://www.ftb.ca.gov/about-ftb/newsroom/covid-19/help-with-covid-19.html#Residency-and-income-sour...
Although this is a FAQ for COVID, I believe the principles are a permanent part of CA law.
Generally, you owe income tax to the place where you perform the work. You also owe income tax to the place where your permanent residence is located (but not if you live in TX). In this case, you would file a non-resident return for CA to only report the portion of your income earned while living or working in CA.
You might get away with it, if the FTB doesn't know you are there. But the law seems to be that you owe CA income tax.
Income sourcing for nonresidents temporarily relocated to California, and filing and paying California income taxes during COVID-19
Scenario 1: You work for an out-of-state employer and receive a W-2 from them. You temporarily relocate to California. Do you need to file a California return and pay California income tax?
Answer: Yes. As a nonresident who relocates to California for any portion of the year, you will have California source income during the period of time you performed services in California. You will need to file a California Nonresident or Part-Year Resident Income Tax Return (Form 540NR) return to report the California sourced portion of your compensation. One way to calculate the portion of your income that is California sourced is to multiply your total amount of income for the year by a ratio of your total number of days performing services in California over your total number of days performing services worldwide.
