Troubleshooting Pre-Collision System Malfunction with Primus IQ Installed on a 2017 Highlander
Updated 05/21/2025 | Published 02/19/2018
Question:
I got the Tekonsha Primus IQ brake controller for my 2017 Toyota Highlander. When I apply just the trailer brake with the slide switch, I get an error message on the Toyota Pre-Collision System Malfunction.
asked by: Mike H
Expert Reply:
I spoke with my contact at Tekonsha who knew exactly what you were talking about.
He said that the best solution for installing a brake controller like the Primus IQ # TK90160 in your 2017 Toyota Highlander is to install a diode on the red wire of the wiring harness for the Primus IQ. This is the wire that connects to the cold side of the stop light switch.
To prevent the 12V from backfeeding into your Pre-Collision System he recommended using a diode rated for 1 amp. These are not very common so you may have to do a little searching to find one.
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Zachariah
5/8/2025
I found a youtube video where someone had a similar issue with a RAV 4. In the comments, someone said "Orient the diode so that the anode (the end with the white band) is toward the controller". From my personal knowledge, that would force the signal (voltage) to go only FROM the brake controller (kinda like a one-way road), as Jon has stated. On top of just the pre collision warning error, I had a 370mA parasitic draw. Installing the diode fixed both issues. The diodes that I purchased: pack of (5) 1N4001 diodes, which are rated for 1A, 50V. Hardware from etrailer that I installed: Tekonsha OEM Replacement Vehicle Wiring Harness w Brake Controller Adapter - 7 Way Trailer Connector 22119 Redarc Tow-Pro Liberty Brake Controller w/ Universal Wiring Harness - Dash Knob - Proportional RED52VR My vehicle: 2017 Toyota Highlander Hybrid I hope this can help someone. I spent HOURS troubleshooting and researching (for the parasitic draw).