Do not think they mean the engine computer, perhaps the sensor that checks for burned out light bulbs, etc. If a problem a little icon lights up on the dash. Check the owner’s manual to determine if there is anything about trailers and trailer wiring.
Go to a website www.etrailer.com that has lots of great info on wiring trailers. Suggest if you want absolutely no chance of interfering with the cars electrical system to install one of the trailer light modules that has its own power source. The website covers this.
I have a 2001 Sienna and installed a PWP (professional wiring products) T-connector for this model. It installed very quickly and have no issues with any lights on the van. The T-connector does not have a separate power source.
My utility trailer has basic 2 lights (2 filaments) lighting and works perfectly on another vehicle. It does not on my Sienna. I get a voltage reading at the harness, but does not light up the signal lights on the trailer independently, i.e. when I put the van's flasher on, both flash (very dimly) on the trailer. When I put the van's headlights on, there's nothing at the trailer.
Out of curiosity, since the van's lights are workign with the T-connector installed, I spliced in another tail light converter by Hopkins (hoppy) before the PWP convertor. Same result.
It seems strange that many different companies sell this harness for the Sienna, but no mention of any special requirements for this vehicle.
Once again, we're talking about a T-connector, so there's no mistake on how the plugs go in and all my lights are working properly....I can only guess that I perhaps should have purchased one with a separate power source....my trial and error efforts are getting expensive.
The PWP convertor was defective and the trailer lights were not grounded properly, likely due to corrosion.
I purchased new trailer lights and wiring and wired everything up (but off the trailer). The PWP product only worked on 1 of 2 lights, but the two other convertors (Hopkins or Hoppy) and the other I do not recall, worked fine. So you do not appear to need a convertor with an independant line to the battery.
Not all was lost with the PWP product as the T-connectors it came with were ok, so I cut loose the PWP convertor and spliced in the other convertor.