I found a slow leak in the drain valve area with tracing dye and a blacklight after noticing the expansion tank level was too low. I got an OEM replacement petcock and used silicone grease on the o-ring, type 3 gasket sealant (a syrupy, non-hardening liquid) on the threads, and even threw some JB-Weld around the end caps, thinking it might have a hairline crack somewhere. It reduced the loss, but not entirely, so I decided to replace the whole thing.

You shouldn't tighten the plastic drain valve any tighter than you would put on an oil cap. The seal is very basic and a lot of people advise against putting anything on the threads at all, saying the o-ring is what forms the seal (which is true, but not the only way to stop it from leaking). It's possible to avoid it entirely while replacing coolant and drain the radiator from the lower hose. You can see from the picture of the old, empty radiator that it's really nothing but a hole with an internal shelf for the o-ring to compress against. With the engine completely cold and the radiator cap on, I didn't actually lose very much coolant when I took the old one out to replace it (maybe a few ounces). I'm guessing it formed enough of a vacuum cooling down to hold the remaining fluid in place. It might still be worth a shot if you thought it was only leaking from the o-ring itself due to age and/or overtightening. It could of course, be leaking from more than one place simultaneously. The water pump is a common place for coolant leaks, which tracing dye would help identify. A head gasket leak might not be easy to see, but could be confirmed with a used oil analysis (automotive sample testing is < $50 for a mail-in kit) white smoke in the exhaust, or milky oil indicating mixing has occurred. Ethelene glycol is extremely toxic to both animals and humans even in small amounts and should be recycled properly. Most auto parts stores don't accept it like they do used oil however, so look for a household hazardous waste collection site.

If you can change the oil, you can probably replace the radiator on a second generation Fit. Far and away, the most difficult part for me was the fan and temperature sensor electrical connectors on the passenger's side. They're in a very difficult location. I broke one of the original clip brackets on the fan housing just looking at it; the plastic was about as thick as a potato chip! All the bolts and even hoses were no problem at all. Just be prepared for the buried, fidgety electrical clips and the rest is easy. I got the radiator (a little over $100) from the link in the YouTube video, which was very helpful, although they don't do a good job of identifying the problem connectors. The radiator itself seemed to be pretty good quality considering the price compared to an OEM radiator (at least several hundred dollars more). Blue coolant only, a no-spill funnel for burping air out of the system, and you're on your way unless you damage the wiring! I lost maybe two or three ounces of transmission fluid disconnecting the lines for it, so you may want to have at least a partial bottle on hand.