C1246 Master Cylinder Pressure Sensor

Master cylinder may be by-passing fluid. This is caused by corrosion and/or rust and dirt in the cylinder bore. You can usually confirm this by pressing moderately firm on the pedal then reducing the pressure on the pedal slightly (not completely). If the pedal drops to the floor you most likely need a new master cylinder.

 

The master cylinder on the Prius is not you run of the mill part. It's a complex and very expensive assembly ($1600-$2000). It would be advisable to take it to a dealer or authorized Prius shop for further diagnostics. Another alternative might be to get a used part.

 

I am having a similar problem the ABS, VS (!) and the ebrake lights are on. I scan with Launch tool and DTC error code C1246 come up "Master cylinder pressure sensor"
I cleared the code on the scan tool, but when I drive again and apply the brake with much force and hold it there for a second or two I feel like fluid pushing against my foot and hearing it as well and the brake pedal is coming up to me, then and only all the lights comes back on.
I have changed the pedal light switch and the brake pedal sensor, but still not working.
I did not do any brake work or remove any wheel.
Any help is much appreciated on how to solve this problem.

 

Is the brake fluid reservoir full?

 

Yes not up to the max mark but I can still add some if that is what it take s to solve it.

 

This is an interesting problem. It would feel the same against your foot whether pressurized fluid from the accumulator is somehow backing up into the forward chambers of the master cylinder and pushing back at you, as you describe it, or the pressure is bleeding off from the boost chamber (which is on your side of the master cylinder piston and assists your effort, so bleeding off of pressure from there will also feel as if the system is pushing back harder against you).

This can't be very much other than a valving/sealing defect within the Gen 3 assembly that contains the master cylinder with booster, actuator, and ECU. (Not to be confused with the other separate assembly that contains the accumulator and booster pump, even though it is awfully easy to mix up 'actuator' with 'accumulator' or 'booster' with 'booster pump'.)

Regrettably, that master-cylinder/actuator assembly is rather expensive for Gen 3.

The diagrams of the system (in the Gen 3 New Car Features manual) might be very helpful in seeing just what possible defects could be leading to this sort of behavior. Given the price of the assembly, it might be worth a try to see what can be done short of replacing it.

The most promising case would be if there is some small piece of grot that has lodged in one of the actuator valves; if you have a laptop with Techstream and a J2534 device to connect it to the car, Techstream has commands to open and close all the actuator valves independently, and also read the values from all of the internal pressure sensors to see what might be going on. Also, it has a few different routines for fluid bleeding and flushing. If you are very lucky, it's conceivable a little piece of grot could be dislodged from wherever it is stuck, by some combination of working the valves and going through the flush procedures.

Otherwise, you are probably looking at replacement of that assembly. For less money, you can look for used ones, but be careful: Toyota has very strong advice about handling the parts of the brake system (for example, no use of cloths or fabric gloves, anything that could shed small fibers that could get in the system, and would lead to problems very much like what you're now having!). So, if buying a used part, you might not be sure how carefully those warnings were heeded by the person who removed and packaged the part.

-Chap

 

obd2 with abs from harbor freight. Diagnose. Then order used part ebay or car-part.com

 

Looks like autoz already used a scantool to get the brake code shown in post #5.

Is there Harbor Freight in Trinidad?

-Chap

 

Online

 

Harborfreight.com

 

Thanks Chap for posting valuable info, I don't have the techstream and there's no Harbor Freight, I
will try and bleed the system, anyone has a repair manual or can point me to where I can get one.

 

If techinfo.toyota.com will work in your location, you're all set for manuals. And the manuals do explain which things you can do with jumpers if you don't have Techstream.

However, if you have or can get an old beater Windows laptop, there are lots of threads here about inexpensive J2534 dongles and ways to get Techstream set up, which will allow you to get a lot more information from the car, and also to access the bleed and flush procedures as well as individual valve tests. This problem sounds like it would be a challenge to diagnose without that, unless you just wanted to throw in the towel and replace the master cylinder/actuator (and even if you did that, you'd need the Techstream bleed procedure upon installing the replacement).

That could make you a lot more self-sufficient in general, especially if you don't have a reliable Toyota dealership nearby (do you?).

-Chap

 

Thanks again Chap for your very useful information,
I am not sure if tech info will work in my area.
We have a Toyota dealership in Trinidad I will call them on Monday not sure they support the Hybrids though.
Anyway this may be more interesting I have another Aqua and I did change out the pedal light switch and the brake pedal sensor, put them in the trouble car and it did not work or resolve the lights that are on, however after returning them to the original car, that car is now displaying the same problem!!!
Hard to believe the 2 cars now have problem with the master cylinder.
I think I should buy new switch and try them.
Any thoughts I appreciate in solving this problem.

 

When you say the "brake pedal sensor", do you mean the stroke sensor (rotary thing, up near the pivot of the brake pedal)?

I believe the calibration of that thing is tricky. It comes from the factory with a shearable pin holding it in a fixed position, and you're supposed to install it exactly that way, then step on the brake to shear the pin; that's how it ends up being correctly calibrated. I don't know that you would easily get it correct if you just pull a used one off one car and slap it on another.

I don't have any obvious theory to connect that with the hydraulic behavior you are seeing, but I suppose it could be something like the ECU saying "hey, I shouldn't have this master cylinder pressure reading with the pedal in that position" and holding a valve open to try to correct the pressure. If you had Techstream you could watch the pressure readings, stroke sensor reading, and valve positions in real time and see if something like that was happening.

Was there a specific reason you tried changing that switch and that sensor in the first place, or did those just seem like easy things to try?

You might end up having to buy a new, not-yet-sheared stroke sensor and install it the official way, just to get your puzzle back to the way it was before you swapped those.

I don't remember if there's any way to properly recalibrate an already-sheared stroke sensor, but if there is, it might require Techstream.

-Chap

 

Chap Sir you are so correct, you are spot on.
The reason why all this was happening is because the stroke sensor was not engaging by the brake, I reposition it and it is working, not seeing those pesky lights anymore, however I am now getting error code C 1445 " Vehicle driven with steering Angle sensor not initialized"
I am sure some one can help me out outlining the procedure.
By the way the next car that was good is behaving awkard no more lights as well but when I am running the car it is stalling, pressing the gas and the car won't move, this happened twice, however it moved slowely but I must press the accelerator.
again any help much appreciated. I thank you all who have helped so far.

 

Have you tried techinfo.toyota.com to make sure it doesn't work in your location?

I'm sure the procedures for doing that initialization will be in there, and there might even be a way to do it with a jumper and no Techstream. It's not in my notes, though.

-Chap

 

I will try techinfo Toyota as you suggest and get the procedures thanks again sir will let you know the outcome.

 

hey all I was able to do the " steering Angle sensor not initialized" with my launch scan tool and all is well vehicle is working.
Thanks for all your invaluable contributions.