2018 Mitsubishi Outlander — metallic rattle for a few seconds after cold start

Start with the test that can prove you wrong. Move a coil or plug on the 2018 Mitsubishi Outlander if it's cylinder-specific and see whether the code follows. You'll know quickly if you're on the wrong tree. Just my two cents from doing this recently. I'll poke at it this weekend.

I'd want a measurement, not a hunch. Get a real battery/charging test on the 2018 Mitsubishi Outlander, not just resting voltage, before blaming a module. If the cheap test changes it, stop and reassess. This is from a 2018 Mitsubishi Outlander in Connecticut with 98,500 miles. Take it or leave it — that's what I'd do. I'll update if anything new shows up.