So i've been putting off doing a proper mobile install for months and finally did it last weekend. Got a Yaesu FTM-400 in the dash and ran the power cables direct to the battery with a fuse near the battery, pretty standard stuff. The antenna is a NMO mount i drilled into the roof, running a Diamond NR770H.

Thing is when i do an SWR check on 146.520 it comes back fine, like 1.2:1 or something close, but on the 70cm side around 446 it's reading closer to 2.5 or 3:1 which seems too high to me. I checked the connector and it looks fine visually but maybe i did something wrong with the PL-259? Or is this normal for a dual band whip and im just overthinking it. The radio works and i can hit the local repeaters on both bands but still bugs me.

Anyone dealt with this before or know what to check first

That SWR on 70cm isn't unusual honestly, a lot of those dual band whips are kind of optimized for 2m and the UHF side is more of a compromise. That said 3:1 is a little on the high side and worth looking at. First thing i'd do is check how much of the antenna base is actually in contact with the roof — if theres a gap or the NMO mount isnt fully torqued down you can get weird readings. Also look at the coax connection at the radio end, sometimes the center pin on a PL-259 doesn't seat right in the SO-239 and it reads fine at low power but the SWR looks off on the meter.

If you made the PL-259 yourself with solder that's another place to look. I've botched more connectors than i care to admit. Just resolder it or use one of the crimp type connectors if you have the tools.

I had almost the exact same thing with my FTM-300 and a similar Diamond antenna when i first installed it. Turned out the issue was the feedline had a little kink where it went through the headliner and down the A-pillar, not a sharp bend but enough apparently. Rerouted it and the 70cm SWR dropped quite a bit. Might not be your issue but worth checking the whole run of coax especially if you tucked it tight anywhere.