Tax Year Prior to 2020: If I have paid as least as much tax for 2016 as paid in 2015 to the Feds and State (Az & Ca) when filing for an extension, am I penalty proof?

[KarenL2]

Yes.  But not interest proof if you owe more.   Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and estimated tax payments, or if they paid at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller.

Also, please see the following link for more information about Underpayment Penalties.