Problem w/"P" lock mechanism = $ 4777! Need advice (2009 Prius)

This is a fairly common message if the 12V battery is low. It would not hurt to check your 12V battery voltage after the car has been off for a while and verify it is above 12V, preferrably in the 12.5V range. If your battery is low you may want to replace the battery and see if this problem goes away.

The DTC codes are worrisome, though. The P0851 is "Park/Neutral Switch Input Circuit Low" and then P0852 is "Park/Neutral Switch Input Circuit High". The wire harness or connector is the first item in the troubleshooting list, then P position switch and then Power Management ECU.

Good luck. Let us know how this goes.

 

If the OP doesn't have a voltmeter to measure at the terminals, then follow Weird stuff happening? MPGs dropping? Test The Battery | PriusChat after the car has sat overnight.

 

1. Replace the 12V battery now, to eliminate that as an issue.
2. Has your car ever been in a front-end accident or flood, causing wiring harness damage?
3. If your car continues to run well (other than the warning lights) then I would definitely defer any repair costing well into four-digits. Since you can make the Prius IG-OFF and the car goes into P, the very minor problem does not justify the large expenditure.

 

If you could fill in the details of where you are located, maybe there is an independent Prius shop nearby that would be better able to help.

 

If only the location field were mandatory...

 

A low 12V battery could trigger the DTC's, along with a lot of other random warning lights and messages. There have been numerous reports of dealer tech's not diagnosing the 12V battery correctly.

If it were my car, I would check the battery and replace it if it were questionable before I would spend the money to replace the wiring harness. $200-300 is a whole lot less than the wiring harness.

Good luck and keep us posted.

 

Checking the battery yourself would be best, as many battery testers give misleading results on Prius batteries. If that isn't possible then replacing the battery is reasonable. Even at a dealer that should be no more than $300, or as little as $200 if they're golden.

You might let these guys replace the 12V battery if they don't try to gouge you, and that may fix the problem, but whether or not it does, in future go to another dealer. I've had acceptable experience with T. of Richardson and T. of Plano. Sadly, most dealer service writers will try to "upsell" you on stuff that isn't needed, so you can't just put yourself in anyone's hands and tell them to do what's necessary.

 

If the replacement of the 12v battery does not clear the problem the route I would take is to change out these three items with salvage units in this order.

(1) Park motor relay. (shown centre top in diagram below)
(2) Transmission control ECU (centre bottom in diag)
(3) Shift control actuator (bottom left in diag)

The first two items are very easy to change the relay being in the engine compartment, the ECU above the passengers toes behind the glove box lid.

The third item is not so easy as it is on the back transmission in front of the drivers feet.

This eliminates most of the components apart from the harness in this system and should not cost much. RunnerCNY (see post where is transmission ECU located) purchased an ECU for $55 and took him 15 mins to change. Hope this helps.

John.

 

From the posts I have read through, it seems that when the dealer wants to replace the entire wire harness, it means that they are unable to troubleshoot the problem to a specific point in a short amount of time, and would rather not work on your problem. A harness can be damaged by road debris, accident, rodents, or very rarely by some manufacturing defect. The work it takes to expose *everything* to replace the *entire* wire harness would surely be greater than the work to expose just the suspect portion . But finding the bad spot, if it really exists, can take some creative detective work, so it is not the kind of thing that can be detailed in the service manual. Hence the "replace entire harness" method, which should be saved as a last, desperate measure, after the above suggestions.

When I have done repairs to car harnesses, it has been obvious (chewing, heat, rubbing) once access to the area can be had. But that access is the hard part.

 

I believe replacing the wire harness is a red herring as the likely places for problems are the connectors, and the most suspect one would be the connector to the transmission lock motor. Looking at the wiring diagram there are only 9 wires going from the motor assembly to the ECU and only one other that goes to the relay supplying 12v to the star point of the motor, surely testing those out or replacing those wires directly would not constitute anything like $4800.

 

The route for the wires can be seen on the 2diags below. The harness runs from S1 on the trans under the inverter (the most likely place for damage and the only place the harness cannot be seen) round the engine through the fire wall and to the ECU at T4 View attachment 41026

 

On checking the shift control actuator is easy to change on a lift and is available via Ebay for around $100.

 

I wonder did this fix it or did they buy another car? Also gen 2 or gen 3?