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We systematically analyze the full angular distribution in $D \to P_1 P_2 l^+l^-$ decays, where $P_{1,2}=\pi,K$, $l=e,\mu$. We identify several null testsof the standard model (SM). Notably, the angular coefficients $I_{5,6,7}$,driven by the leptons' axial-vector coupling $C_{10}^{(\prime)}$, vanish bymeans of a superior GIM-cancellation and are protected by parity invariancebelow the weak scale. CP-odd observables related to the angular coefficients$I_{5,6,8,9}$ allow to measure CP-asymmetries without $D$-tagging. Thecorresponding observables $A_{5,6,8,9}$ constitute null tests of the SM. Leptonuniversality in $|\Delta c| =|\Delta u|=1$ transitions can be tested bycomparing $D \to P_1 P_2 \mu^+ \mu^-$ to $D \to P_1 P_2 e^+ e^-$ decays. Datafor $P_1 P_2=\pi^+ \pi^-$ and $K^+ K^-$ on muon modes are available from LHCband on electron modes from BESIII. Corresponding ratios of dimuon to dielectronbranching fractions are at least about an order of magnitude away from probingthe SM. In the future electron and muon measurements should be made availablefor the same cuts as corresponding ratios $R_{P_1 P_2}^D$ provide null tests of$e$-$\mu$-universality. We work out beyond-SM signals model-independently andin SM extensions with leptoquarks.

Null tests from angular distributions in $D \to P_1 P_2 l^+l^-$, $l=e,\mu$ decays on and off peak