All other lights work, front and back. Checked fuse, bulbs, and relays ...all good. Neither high nor low beams work, and when put high beams on high beam indicator doesn't light up on dash either. No power at the lights, tested fuse and looks like no power there either. Put neg lead of multimeter on ground and tested both sockets of both left and right headlight fuses and there was no power in either. Light Relays 1 and 2 had power.

May or may not be related: A few days before the headlights went out, check engine light came on. Codes "p1298 Electric Load Detector Circuit High Input" and "p0141 Oxygen sensor heater circuit bank 1 sensor 2 " came up. The heater motor started making a lot of noise and then the heater totally stopped working the next day. 2 days later the headlights went out.

 

Look at your 2nd paragraph ^^^ - seems issues that could be caused by non-standard output from the alternator.
So go to Harbor Freight and get the (idiot light) battery/alternator tester (<$10) that plugs into the electrical outlet on your dash.

An alternator will set you back $110 for the part; based on NAPA part for my '92 Accord
Expect someone else with direct experience will chip in

EDIT: welcome to forum

 

Its also good for us to know exactly what kind of car we're talking about. At least with an OBD-II error code we know it's 1996 or newer... ??

 

My apologies, I had it in there then browser froze when I submitted. Looks like I forgot when I rewrote the post.
It's a 2001 Accord EX 4cyl 2.3.
I pulled the #6 ECU/PCM fuse today to reset the computer and light came right back on. Going to try and figure out how to test the PCM this afternoon, see if that might be faulty. Will be heading into town this eve, I'll pick up the alternator tester.

 

I used to have a 1998 and the PCM reset fuse is #13 in the fusebox at the right-end of the dashboard (open the passengers door to see it). Pretty sure that's the same for your 2001. If the error codes come right back, that simply means the problem isn't fixed yet.

The O2 sensor heater is possibly a bad O2 sensor; the rear one that's in the catalytic converter under the car. It might be a circuit problem, so you can check... (got a multi-meter?)
When you unplug that O2 sensor, the pigtail on the sensor has 2 wires the same color (black?). Resistance between them should be 10 to 40 ohms (a failed heater is usually open-circuit). If the resistance is OK let us know for further troubleshooting. You can also follow the wiring harness as much as you can see; looking for broken wires or pinched or insulation cut open or stuff like that.