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Trailer hitch wiring harness causing error messages?

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stevenericmost

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Discussion starter · #1 ·
Last June I had a trailer hitch wiring harness put on my 2021 RAV4 Prime by a company that does this sort of thing routinely. There were no issues with the car up until this February when I started getting messages on the instrument panel saying things like "Pre-Collision System Malfunction Visit Your Dealer" and Cruise Control Malfunction Visit Your Dealer and LTA Unavailable.
I took it into the dealer and after several failed attempts to come up with a diagnosis they finally said it was the aftermarket wiring harness.
Now the harness was on there for 9 months and I had zero issues with anything. I drove thousands of miles and everything worked great. If the wiring harness isn't attached to anything and isn't drawing any current how can it have any affect on anything? It's just a plug capped off under the bumper.
Could the dealer be trying to just get rid of me? Help please.
 
#2 ·
Most of those trailer wiring harnesses are plug and play, meaning they just plug in to existing connectors (the rear tail lights). Easy to unplug them, and then if the problem continues, that proves the wiring harness is not the problem.

The controller for the wiring harness is always powered whether or not you have a trailer attached. (The controller is directly attached to the 12 volt battery for power).

All those error codes are from the sensors that on the front of the vehicle, thus I suspect that is where the problem is, not at the back where the trailer wiring harness plugs in. It is possible, that the car got bumped which may have moved some of those sensors. There is an alignment procedure for the sensors that the dealer should do.

Also, if you have blind spot monitors, rear cross traffic warning or the parking assist, those sensors are at the back of the car and could have been bumped when the trailer wiring was installed. But the dealer should know how to diagnose those. There was even a T-SB on the blind spot monitor problems that is well known.
 
#3 ·
4 wire? Or 7 wire with brake controller perhaps? If 7 wire, this is a known issue, the brake light signal can backfeed voltage where it shouldn’t go and throw these exact errors. There is a fix with a diode. Before diving in there, what specific harness was installed? Can you share more details?
 
#4 ·
There was also a report, of the installer removing the kick open sensor for the hitch. The open kick sensor connector corroded over time, shorting out contacts and causing malfunction of the Body Control Module and power window functions. This is a time-delayed failure. Something to think about.
 
Discussion starter · #5 ·
chuckles70 said:
4 wire? Or 7 wire with brake controller perhaps? If 7 wire, this is a known issue, the brake light signal can backfeed voltage where it shouldn’t go and throw these exact errors. There is a fix with a diode. Before diving in there, what specific harness was installed? Can you share more details?
Click to expand...
FKHeath said:
Most of those trailer wiring harnesses are plug and play, meaning they just plug in to existing connectors (the rear tail lights). Easy to unplug them, and then if the problem continues, that proves the wiring harness is not the problem.

The controller for the wiring harness is always powered whether or not you have a trailer attached. (The controller is directly attached to the 12 volt battery for power).

All those error codes are from the sensors that on the front of the vehicle, thus I suspect that is where the problem is, not at the back where the trailer wiring harness plugs in. It is possible, that the car got bumped which may have moved some of those sensors. There is an alignment procedure for the sensors that the dealer should do.

Also, if you have blind spot monitors, rear cross traffic warning or the parking assist, those sensors are at the back of the car and could have been bumped when the trailer wiring was installed. But the dealer should know how to diagnose those. There was even a T-SB on the blind spot monitor problems that is well known.
Click to expand...
An odd thing is that I have at times hit the back arrow on the steering wheel to clear the warnings and have driven a good while without them coming back. The behavior had been kinda intermittent, although the frequency of recurrence has increased substantially lately. Why would it be intermittent?
I'm going to take it into the hitch installer Monday and find out if its a plug and play harness and if it is I'll have them unplug it. I have to wonder though if the error messages will go away or will some kind of computer reset be necessary.
Another thing. A guy backed into the RAV last summer very slightly damaging the rear bumper, but I drove a couple thousand miles without any issues before I had the bumper replaced. More driving after the bumper replacement and no messages until February when in snow country the messages started coming up for the first time. Could the bumper bang have anything to do with it? But why the big delay?
You know the dealer-installed harness is an $1800 job where they run wire up to the front of the car and do the connection up there, maybe under the dash. A very big job. Why would this be the case instead of what I had done, and it would seem thousands of other RAV owners have done?
Thanks for your input!
 
#6 ·
stevenericmost said:
Another thing. A guy backed into the RAV last summer very slightly damaging the rear bumper, but I drove a couple thousand miles without any issues before I had the bumper replaced. More driving after the bumper replacement and no messages until February when in snow country the messages started coming up for the first time. Could the bumper bang have anything to do with it? But why the big delay?
Click to expand...
Did it create any point for water intrusion into the electronics?
 
#7 ·
stevenericmost said:
An odd thing is that I have at times hit the back arrow on the steering wheel to clear the warnings and have driven a good while without them coming back. The behavior had been kinda intermittent, although the frequency of recurrence has increased substantially lately. Why would it be intermittent?
I'm going to take it into the hitch installer Monday and find out if its a plug and play harness and if it is I'll have them unplug it. I have to wonder though if the error messages will go away or will some kind of computer reset be necessary.
Another thing. A guy backed into the RAV last summer very slightly damaging the rear bumper, but I drove a couple thousand miles without any issues before I had the bumper replaced. More driving after the bumper replacement and no messages until February when in snow country the messages started coming up for the first time. Could the bumper bang have anything to do with it? But why the big delay?
You know the dealer-installed harness is an $1800 job where they run wire up to the front of the car and do the connection up there, maybe under the dash. A very big job. Why would this be the case instead of what I had done, and it would seem thousands of other RAV owners have done?
Thanks for your input!
Click to expand...
You have a Prime, so the 12 volt battery is in the back where the power for the trailer wire controller is connected. All OEM (and most after market) wiring is done in the back. No need to run wires to the front or the dash. The OEM trailer wire harness installation takes, at most a couple hours, not $1800. It is not a big job. (See the instruction PDF file for the installation instructions.) The OEM wire harness installs the same way most after market harness do.

Be sure you keep the front radar sensor (Toyota emblem) clean. When in a snow storm, my radar cruise control would shut down when that sensor got slush on it. Like wise the windshield where the photo sensor is located, up by the rear view mirror.
 
Discussion starter · #9 ·
FKHeath said:
You have a Prime, so the 12 volt battery is in the back where the power for the trailer wire controller is connected. All OEM (and most after market) wiring is done in the back. No need to run wires to the front or the dash. The OEM trailer wire harness installation takes, at most a couple hours, not $1800. It is not a big job. (See the instruction PDF file for the installation instructions.) The OEM wire harness installs the same way most after market harness do.

Be sure you keep the front radar sensor (Toyota emblem) clean. When in a snow storm, my radar cruise control would shut down when that sensor got slush on it. Like wise the windshield where the photo sensor is located, up by the rear view mirror.
Click to expand...
Thanks, I considered the same thing but the car is now spotless and same problem. Any thoughts if disconnecting the harness will clear the messages? I will just live without it if that solves the issue.
 
#10 ·
stevenericmost said:
The messages started, as I mentioned, in snow country so I suppose water could have set it off. But why now that everything is dry? And much high speed freeway driving in dry conditions with the same problem.
Click to expand...
Water in places when it does not belong causes corrosion and usually leads latent failures.
 
#11 ·
Just to give some color on why brake lights or an accessory in the back can mess with these systems…. These are driver aids that can cause the car to autonomously apply the brakes. However, if the car senses anything out of the ordinary with the brake light system, these automations shut down, because Toyota doesn’t want to be legally exposed to one of their cars that slams on the brakes without lights coming on and a subsequent rear-end accident.

There is all kinds of instrumentation on the brake light system. If it’s one of the common harnesses (Tekonsha, etc) corrosion on the plug outside the car shouldn’t cause this. The way these are built, the light circuit on the trailer harness side is isolated from the car light circuit and any ground fault will blow the fuse.

I am guessing the harness itself is failing (it’s electronic) and feeding enough noise into the light circuit that the ECU shuts things down.

these harnesses are like $50. I’d just swap it. You’re in that much labor just pulling the back panels tounplug it.

Error codes may or may not clear on their own. I’ve read stories of both, probably depends on the type of failure detected.

And as much as I think dealers are experts at blame shifting and BS, I think they’re on to something g here.
 
#12 ·
chuckles70 said:
Just to give some color on why brake lights or an accessory in the back can mess with these systems…. These are driver aids that can cause the car to autonomously apply the brakes. However, if the car senses anything out of the ordinary with the brake light system, these automations shut down, because Toyota doesn’t want to be legally exposed to one of their cars that slams on the brakes without lights coming on and a subsequent rear-end accident.

There is all kinds of instrumentation on the brake light system. If it’s one of the common harnesses (Tekonsha, etc) corrosion on the plug outside the car shouldn’t cause this. The way these are built, the light circuit on the trailer harness side is isolated from the car light circuit and any ground fault will blow the fuse.

I am guessing the harness itself is failing (it’s electronic) and feeding enough noise into the light circuit that the ECU shuts things down.

these harnesses are like $50. I’d just swap it. You’re in that much labor just pulling the back panels tounplug it.

Error codes may or may not clear on their own. I’ve read stories of both, probably depends on the type of failure detected.

And as much as I think dealers are experts at blame shifting and BS, I think they’re on to something g here.
Click to expand...
Disagree. But not vehemently. I would remove the aftermarket stuff. Entirely. If the errors stop, the fine, you have your culprit. But stealerships are famous for blaming all sorts of anything installed by anyone except themselves (and even them sometimes) for whatever is wrong with your vehicle. You will NEVER get the dealer to actually figure out the problem, so long as the aftermarket stuff is in there.

And then, since your vehicle is in their system, even if the problem persists, they will say you/your installer damaged something . Might need to remove and then find a different dealer and hope they don't share information.

I used a Tekonsha harness. Should ANTHING electronic happen to my RAV4, I will likely remove the wiring entirely before taking to a dealer. Dealers are just generally lazy SOB. Honestly, even with an OEM wiring harness, I'd expect a dealer to blame a different dealer if it was installed elsewhere.
 
#13 ·
cdherman said:
Disagree. But not vehemently. I would remove the aftermarket stuff. Entirely. If the errors stop, the fine, you have your culprit. But stealerships are famous for blaming all sorts of anything installed by anyone except themselves (and even them sometimes) for whatever is wrong with your vehicle. You will NEVER get the dealer to actually figure out the problem, so long as the aftermarket stuff is in there.

And then, since your vehicle is in their system, even if the problem persists, they will say you/your installer damaged something . Might need to remove and then find a different dealer and hope they don't share information.

I used a Tekonsha harness. Should ANTHING electronic happen to my RAV4, I will likely remove the wiring entirely before taking to a dealer. Dealers are just generally lazy SOB. Honestly, even with an OEM wiring harness, I'd expect a dealer to blame a different dealer if it was installed elsewhere.
Click to expand...
All fair points- although the SOB dealer will now probably blame it on the since removed harness in the off chance codes don’t clear after it’s out / disconnected.
 
#14 ·
At this point, rather than all this run around, just ask the dealer to re-calibrate the TSS sensors. That is a common procedure when the windshield is replaced, or the front bumper/grill is bumped. I think the dealer is just making excuses, blaming the problem on the trailer wiring. Besides, the factory does not always get the calibration right, and you could have been on the edge of the standards. If that is the case, the re-calibration should be free under warranty. Dealers don't always like doing warranty work for they get less money from Toyota, so they make excuses.
 
Discussion starter · #15 ·
FKHeath said:
At this point, rather than all this run around, just ask the dealer to re-calibrate the TSS sensors. That is a common procedure when the windshield is replaced, or the front bumper/grill is bumped. I think the dealer is just making excuses, blaming the problem on the trailer wiring. Besides, the factory does not always get the calibration right, and you could have been on the edge of the standards. If that is the case, the re-calibration should be free under warranty. Dealers don't always like doing warranty work for they get less money from Toyota, so they make excuses.
Click to expand...
This is an update on the trailer harness problem I was experiencing in my RAV4 Prime:
I had the shop that provided and installed the harness, remove it three days ago (cost-$175). Since then I have driven over 500 miles and have had no more of the on-screen warnings about the disabling of lane keeping, cruise control or reduced braking that I had previously.
So this tells me the shop installed an incompatible harness and I had to pay them to remove it. I am considering suing them in small claims court to recover the more than $400 I spent for nothing.
 
#16 ·
stevenericmost said:
This is an update on the trailer harness problem I was experiencing in my RAV4 Prime:
I had the shop that provided and installed the harness, remove it three days ago (cost-$175). Since then I have driven over 500 miles and have had no more of the on-screen warnings about the disabling of lane keeping, cruise control or reduced braking that I had previously.
So this tells me the shop installed an incompatible harness and I had to pay them to remove it. I am considering suing them in small claims court to recover the more than $400 I spent for nothing.
Click to expand...
I am sorry for you. Shoddy support and workmanship is why I do most of my stuff myself. But I also have acquired a lot of skill though the years, grew up in a different era, where DIY was more expected.

And I enjoy the challenge and the victory.

Dunno how small claims works in your local. Good luck. Its doing your civic duty almost to go after them. They must have done something wrong. But you will not know what they did wrong and a judge may find just your anecdotal "cessation of error codes" to be lacking before requiring them to refund your $$
 
#19 ·
2023 rav4 hybrid. Installed a CURT 56434 harness and all was well until I went to use it. After connecting a trailer it started showing dash warnings about braking system, LTA, blind side monitoring etc. Unplugging the trailer didn't help. Toyota spent a couple of hours on it (no charge) and told me to remove the harness. They had unplugged the fuse and it still threw error codes. Removed the harness and all is well. They believe it has to do with the led lights.
 
Discussion starter · #20 ·
89-300ce said:
2023 rav4 hybrid. Installed a CURT 56434 harness and all was well until I went to use it. After connecting a trailer it started showing dash warnings about braking system, LTA, blind side monitoring etc. Unplugging the trailer didn't help. Toyota spent a couple of hours on it (no charge) and told me to remove the harness. They had unplugged the fuse and it still threw error codes. Removed the harness and all is well. They believe it has to do with the led lights.
Click to expand...
Yes, that seems to be the cause as that is exactly what happened to me. My son is an electrical engineer and he suggested the harness may have a diode in the control module that conflicts with LED lighting.
The question for me now is, should I seek to recover the $400-plus that it cost me for the harness, installation, and removal by the shop I went with? A painful lesson for them, but in the end they are responsible for installing an incompatible product on a customer's vehicle.
 
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