I bought a 04 civic over the weekend and got it home and realized the high beams and low beam on the headlight wasn’t working. The marker/turn signals work and all twilights work. Replaced the bulbs, fuses, relays, replaced the combination switch for the headlights/blinker and still nothing. Did some checking with the multi meter and not finding the culprit. Anyone else have a similar issue or any other ideas?

 

May possibly be due to a faulty Multiplex Control Unit (MCU), which is integrated as part of the under-dash fuse/relay box. If the MCU doesn't send a ground signal to the lighting relays, the relay coils won't get energize to allow power through the relays. Also, can be due to an open (break) somewhere in the wiring from the MCU to the under-hood fuse box, or a loose connector.

 

Under the dash you will find the daytime running light control unit. You have to remove it and open it up and resolder the centre pins.

 

The headlight bulb is an H4/9003 with high/low beams in one - just in case it is something as simple as the wrong bulbs.

There is a recall on the headlight wiring at the combination light switch harness for the 2002 and 2003 model years. Maybe this problem affects your vehicle?

 

2004 U.S. models do not have daytime running lights.