I work at a "stealership" and for the most part, DRL/headlight issues are fairly straightforward to diagnose.
The item commonly replaced on Hondas is the CONTROL UNIT, not the RELAY as it is commonly called as Honda's have BOTH. The DRL relay is usually found in a relay cluster under the driver's dash which may include the low-beam cut relay that activates to open the low-beam circuit when the high-beams are used. The DRL control unit is what controls the operation of the daytime running lights as well as having some say in the operation of the high beams. It is usually (but not always) located under the driver's dash and easily noticeable by it's gray box and blue connector end. Some have them on the passenger side while others which shall remain nameless *COUGH* ACURA *COUGH* like to play hide and seek and tuck them way up under the passenger side dash behind the glovebox where you need long extensions, x-ray vision and contortionist abilities to access
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DRL control units fail because of solders on the circuit board that crack and cause poor connections or intermittent open circuits. Most people simply re-solder the cracked connections (they are easily discernible by their brownish tan and hairline cracks) and to be quite honest, if a good solder is used with careful technique, the re-soldered board will last a long time. I re-soldered my uncle's DRL control unit five years ago when I started at the dealership and it still works today.
If your DRL control unit is in fact going for a ****, usually one headlight in highbeam or DRL mode will not work and the "DRL" light on the cluster will illuminate. However, I've seen good techs get dupped by a failed headlight bulb in a hurried situation and since the DRL system controls power to the high-beams, a bad high-beam filament can trigger the DRL light as well because of the open in the circuit, leading to a mis-diagnosis.
One step to isolate the bulb itself as the cause is easy (well, depending on your comfort level with working on a vehicle, it may not be easy, however I'm going to assume for the sake of this post that you don't tremble in fear when you pull the hood release :lol: ) where you simply take the suspected non-working bulb out and install it into the opposite side. If the bulb works in the opposite side, suspect the DRL control unit. If the bulb still doen't work in the other side, suspect the bulb itself.
The next step you can try which sounds a bit mickey mouse but has worked for me 95% of the time to isolate a DRL control unit that is failing intermittently is simply tapping the DRL box with the handle-end of a screwdriver. You can also flick it firmly with your finger (for some reason this method seems to work awesome on '98-'02 Accord DRL control units). Many times this vibration is enough to temporarily close the connection and re-illuminate the DRL circuit. Again, you are gently tapping the DRL control unit box, not beating it like it owes you money
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Again, this is a flat-rate time-saving way to check it but it shouldn't be relied on solely to diagnose the concern, it is simply a quick and easy way to check. From my short 5-year experience though, tapping the box has proven handy in eighty to ninety percent of the DRL issues I've worked on.
