With more than 750 parts, the John Deere 6420 from the 6020 series is well covered here. These include trailer couplings, electrics and lighting.
Before ordering a belt for a John Deere 6420, check one thing: does your tractor have air conditioning? There are multi-rib belts and belt kits for both versions, and because the belt runs a different route, one will not stand in for the other. Look for an air conditioning compressor in the belt drive before you order. This page carries more than 750 John Deere 6420 parts, so details like that matter.
A thermostat should never go in without a fresh regulator housing gasket, which you will find here too. If the coolant looks brown or cloudy, flush the system while you are at it. There are also hoses, engine sensors, O-rings such as the one for the tank sender, and seals. Starter motors and alternators take care of starting and charging.
Many a John Deere 6420 spends its days in front of a trailer, and the hitch shows it. For that there are drawbars, including automatic ones from ROCKINGER, drawbar frames and forced steering connections, with Sauermann among the drawbar brands. Lift arms cover the linkage, and air brake parts come from Wabco.
Lift arms wear where they pivot. Play at those points leaves an implement hanging crooked or swaying from side to side. When you grease the linkage, check that both lift arms sit level, and replace a bent or worn arm before the play spreads into the rest of the hitch.
HELLA work lamps cover work after dark. A flickering lamp is often down to a poor earth or a corroded plug, so check those before replacing the lamp itself. A loose earth lead is quickly put right with a new crimp terminal and a smear of grease.
The serial number on the type plate is your starting point. For belts, gaskets and thermostats the reference number on the old part is a quick check. With a belt, count the ribs on the old one as well.
The 6420 belongs to the John Deere 6020 series, SE versions included. The other types in the series each have a page of their own, so you can see per tractor what fits:
The John Deere 6020 and 6420S are part of this series too. For those and every other model, see John Deere parts.