2016 Prius Interior Lights Not Working???

The interior lights are powered through the DOME fuse (7.5 A) and DOME CUT relay in the Instrument Panel Junction Block Assembly, but if this fuse were blown (open) or the relay were burned out, the lights wouldn’t come on at all, even for a moment.

Looking at the schematic diagram for the interior lighting system (see the Electrical Wiring Diagram or the Repair Manual, both available by subscription to techinfo.toyota.com), I wonder if one or more of the LED lamps might have shorted out, or nearly so, providing a low-resistance path to ground.

The coil of the DOME CUT relay is wired in parallel with the lights. One terminal of the coil is connected to the load side of the DOME CUT fuse, and the other to ground, via a switching transistor in the Main Body ECU. Normally, the current through the lights is low enough that there is enough voltage across the coil to keep the relay energized.

If a lamp were faulty, however, immediately after it’s turned on, the excessive current through the lamp would reduce the voltage across the coil of the DOME CUT relay, causing the relay contacts to open and all the lights to turn off. The lights might flash briefly, but if the contacts open before the time-current characteristic curve of the DOME fuse were exceeded, the fuse wouldn’t blow.

Try removing the LED lamps, one at a time, and replacing them with the original incandescent lamps. If you remove one, and then the other lights start working, it’s faulty. I suppose it could be a problem with the DOME CUT relay or the Main Body ECU, but I think a defective LED lamp is the more likely explanation.

 

LEDs are polarized - i.e. it is possible to insert them backwards - try reversing them? (…ooops, sorry I didn't see that they'd worked flawlessly for over 4 months! )

 

My experience with a few I used 4 or 6 years ago was that they weren't long life items. Household 240volt ones seem good, but the 12v weren't. Hopefully they've improved.

 

I replaced all the interiors and exterior markers with a 15pc kit from eBay (<$6) over a year ago, and (so far) I've not seen any failures! (I even got a couple more kits for spares (just in case!))