Toyota Highlander PCS/ICS Error After Brake Controller Install Explained and Fixed

Updated 05/09/2026 | Published 05/07/2026

Products Featured in This Question

Curt Spectrum Brake Controller w/Universal Harness - Dash Mounted Knob - 1 to 4 Axles - Proportional

(425)

$269.95

Roadmaster Single Park Light Diode

(132)

$17.90

Question:

I have a 2021 Highlander Platinum and just recently had a Curt Spectrum brake controller installed.. With engine running, no travel trailer hooked up, control knob depressed momentarily activates the extreme LH and RH as Blue LEDs. When hooked up to trailer, with the same procedure a Red LH LED lights up. Pressing the knob for 2 seconds results in Toyota PCS and ICS malfunctions. I checked trailer ground, removed the screw, terminal and cleaned and re-bedded the connection to chassis. Ohmmeter indicates a good clean ground on the TT. I used the ohmmeter to check the TV to ground connection with this newly installed trailer receptacle and this also appears good. What should I do next?

asked by: Barry K

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Expert Reply:

Hi Barry, thank you for the detailed explanation, that really helps narrow things down.

What you’re seeing is actually a known issue on newer Toyota Highlanders, so you’re not doing anything wrong. When you use the manual override on the brake controller, it can send a small amount of power back through the brake signal wire. The vehicle sees that voltage without the brake pedal being physically pressed and flags it as a fault, which is why you’re getting those PCS and ICS warnings.

The most effective fix is to install a small 1 amp diode like part # RM-690 on the red brake signal wire between the controller and the vehicle harness. The diode acts like a one-way valve. It lets the controller see when you press the brake pedal, but blocks any power from feeding back into the vehicle’s system. The orientation matters, so the arrow or “out” side should point toward the brake controller.

Since you’re using a plug-in harness, I’d also double check that the connector is fully seated, just to rule out anything simple. Some people also notice the warning only happens when using the manual override, and if they lightly press the brake pedal at the same time, the message won’t show up.

I know it’s frustrating to see those warnings pop up, but this is a pretty common quirk with these vehicles and once that diode is in place it usually clears things up completely.

Let me know, if you need anymore help adding the diode, I'm still around.

expert reply by: Heather A