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The vehicle is a 2003 Chevrolet Trailblazer. The low beams aren't working. High beams are. The fuse is perfectly fine (tested and swapped out). The bulbs are fine too.

A mechanic told me it could be the "body modulator". I've never heard of this before. Could this be the problem? If so, how hard is it to change out for a shade tree mechanic?

If not, what else could this be?

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    As Brian and Nick said, I would check the stalk (switch). When this happened to me I found some loose screws in the switch; it was falling apart. The low beam contacts weren't connecting. Mind you this car was 26 years older than yours.
    – Igby Largeman
    Nov 28, 2011 at 17:04
  • I have a 2004 Trailblazer and the low beams werent working. I checked fuses, then replaced bulbs neither worked. I then checked the HDM relay as explained above and it worked!! Bought the relay and i now have low beams!! Thanks!!
    – user15206
    Feb 20, 2016 at 1:10

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If you can get a copy of the electrical diagrams, you can trace them through and start testing each piece involved in the circuit. I had a similar problem with a Mitsubishi once, and it ended up being a bad headlight stalk... The low beam portion had failed somehow without impairing the high beam or parking light functions!

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I'd get a multimeter and check though the circuit - as Brian says the point at which the current is switched is a common failure point - either in the switch or the relay if one is fitted. I've never heard of a body modulator and can't see how one would be fitted in a headlamp circuit!

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There is a relay in the fuse box under the hood on the driver's side. On a 2004, it's #46 and labeled 'HDM Relay'. YMMV. You can swap it with a known good relay (same type from the box for something you know is working). #45 should be the same (on a 2004 at least) and should be the engine fan clutch. If the headlights work after the swap, swap them back and get a new relay (you don't want to have a non-operational fan). This is from a writeup at Sparky's Answers.

Generally, start with the bulbs. Check the connectors with a multimeter and work your way back from there. You really need a wiring diagram. Based on some Youtube videos, it looks like there should be a 4 pin connector on the headlight module with 4 wires, Tan, Green, Black and Black. The blacks should be the grounds, tan should be low beam, green should be high beam. Each bulb also has a 2-wire connector you'll also need to check if the 4-wire connector checks out.

I suspect there is a dimmer switch somewhere, probably on the steering column. Hopefully you can get at the connector to check it without removing the lower dash or dropping the steering column. If you know what wires go where, you can jumper the appropriate connectors to simulate a working switch set to low beam.

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