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Pre Collision System Fail

3523 Views 3 Replies 3 Participants Last post by  mechatony11
So this was the second time the Pre Collision System freaked out. The system thought I wasn't going to brake enough to not hit the car in front of me. It slammed on the brakes and dropped the car's speed from 75 MPH to about 35 MPH. ABS was kicking off. I tried to get going again, but the stupid system kept the brakes applied. Meanwhile, a dump truck was barreling down on me. Fortunately, the brakes released and I was able to get back up to speed.

I stopped off for gas and when I started the car to leave, the dash lit up with all sorts of warnings and fault messages. Kept saying there was a ECU issue and the Pre Collision System was malfunctioning. The engine was running rough and the EPB wouldn't release. I got the parking brake to release and drove home.

I scanned the car for any fault codes. Here is the list:

Engine and ECT
P2646 (3 times)

C1201 (2 times)


None of these codes made sense and all triggered from the stupid Pre Collision System freaking out. I decided to clear the codes and see how the car would run. Clearing the codes restored everything to normal. The engine is now running smoothly and the EPB is working as normal. I've driven the car for a couple of days since the incident and all seems normal.

I really wish I can permanently disable this stupid Pre Collision System. But if anyone runs into the same or similar situation, try clearing out the DTCs and see how the car runs afterwards.
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while you can't disable this system you can change the default settings to off
here are the Toyota service bulletin
these are for another model but it shows how the tech can change the default settings to off for any of the auto features
Here's an update. I think I figured out why this happened along with another check engine light for a slow to start code or something to that effect. Well, I wanted to check the health of my battery with the electrical system tester module I have for my scan tool. When I went to connect the negative alligator clip to the negative battery cable, the terminal moved with very little effort. Found out the terminal was not tightened down to the battery post and was only barely making contact. This explains all the weird electrical/computer issues I experienced.

The reason the battery terminal was loose was because the car was in the body shop for repairs after hitting a deer. So the shop failed to tighten down the battery cable.
I apologize for bringing up this old thread. I had a similar issue with my car, and it turned out that I had damaged wiring in the harness. I had open wiring in some areas, and it was caused by rodents. I did a lot of searches on this issue and came across an in-depth article that may be worth reading. Search for "Toyota pre-collision system malfunction causes on YOUCANIC" and you will see an article that goes over common causes and possible fixes. You can read about it yourself. I hope this information can assist someone else with a similar issue.
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