Error code C1391

Just a dumb question here. Does anyone ever clear a code such as this, to see if it returns?

 

After reading PriusChat for a while, it seems like nearly everybody wants to first find out if clearing the code will fix the problem, and only then want to find out what the problem is.

C1391 is not one of the codes where that's likely to work for long, though.

Not in Gen 3, it turns out: the booster and the master cylinder are the same assembly (whose full official name is some mouthful like SUB-ASSEMBLY, BRAKE BOOSTER WITH MASTER CYLINDER, and if that's not enough, that same assembly also contains the ABS actuator and the ECU).

Meanwhile, just for extra confusion, there is a separately-sold different assembly just below it, whose name is the brake booster pump subassembly (just one little word different) with accumulator. (And of course nobody ever mixes up the words accumulator and actuator.)

What you're seeing in the sketch you attached is a feature of how the Toyota parts catalogs work: you're looking at one part (the "brake booster assembly with master cylinder), but the page will show you links to all diagrams in the catalog where that part is shown, and it is shown on two different diagrams, one called master cylinder (possibly in the hydraulics section) and one called brake booster (maybe in electrical). Just one part, shown in two places.

Now, back to your "what part(s) to buy" problem, it's not pinned down for certain just by the C1391 code. That means pressure is not being held somewhere, but that somewhere could be in the "brake booster assembly with master cylinder", or in the "brake booster pump assembly with accumulator".

Somebody recently posted an issue where the pump is having to run every four seconds. With a leak that fast, my guess is they will be able to touch both assemblies with a mechanic's stethoscope and straight-up hear the leak in one assembly or the other (during the four seconds when the pump isn't running).

If your leak isn't that fast, pinning it down might not be as easy. Using Techstream, it is possible to read all the pressure sensors in the actuator in (near) real time, and also jigger the individual valves in it, which might be a way to gather more info.

 

I got the code. I had my local guy clear it and we marked the brake fluid level.

It came back on when I let the car sit at idle for about 20 minutes (Florida heat). It now flips on and off at random, sometimes with weeks in between, but most frequently after a long idle.

The key is that in five months he brake fluid level hasn’t dropped st all. I am absolutely convinced this is a sensor fault, not an actual leak or problem with the pump.

But neither I nor my mechanic can find WHICH sensor it is that we would replace. Can anybody help with that?

(2012 Prius, ~150,000 miles)

 

Anybody got an answer for this?

 

did you find an answer? I have same car with very similar issue.

 

Yes

 

My 2010 Trim level V just popped the C1391 code last week. I did a lot of reading before approaching my local Toyota dealer.

When I first called I explained I had taken my car to AutoZone and it was showing C1391 error code for brake actuator failure. I asked if there were any current recalls for my car. The lady said there are 3 current recalls for your car and all have been fixed.

The next thing she said is where I was glad I was paying attention. She said all of the software is up to date as a followup to "all recalls have been fixed".

I said yes I understand that, but are there any recalls for "hardware issues" concerning this since I have read that there was a voluntary recall last year concerning my code.

She came back after putting me on hold for a while and said yes there were and it expired Nov 30th of last year but that there was a secondary recall that my car might fall under. She asked how many miles were on my car (said wasn't applicable if over 150k). I told her I just bought the car last March from a low mileage 1 owner and it has 80k miles on it.

She had me bring it in for them to look it over and after about 2 hours sitting in the waiting room they told me that it would take about 4 business days for the parts to come in and that they would honor the voluntary secondary recall and fix the car for no cost.

Just a bit of info in case you are in the same situation, it is possible to still get this fixed by the dealer if you push back a bit and ask the right questions.

Paul

 

Hmm, I didn't see this post when it was first made. It overlooks the matter of an internal leak, which is often what's going on. The accumulator is supposed to hold fluid under pressure, but because of some valve in the system not sealing completely, the pressurized fluid escapes through the system back to the reservoir, until the pump has to run again and pump it back into the accumulator. Nothing leaks out of the system. You won't see any damp spots or puddles. You won't see the level dropping. You don't have a sensor problem.

 

Then the only fix is to replace the actuator? i have this problem, 4 lights on, ABS, brake skid control, and (!). I think the vehicle can still be used like this, the only thing that annoys me is that when that code comes the regenerative braking is disabled, shame on that. Now i do not have the money to buy the actuator which is around $902 or something like that, but when i do get it i will install it myself in my 2012 Prius C.

 

If the c is like the Gen 3 liftback, there is both an actuator assembly and an accumulator assembly, and an internal leak leading to pressure loss could be in either one, so to avoid frustration it may be worthwhile to try to pin down which one has the problem.

Some people post with their pump cycling so frequently that the leak is probably large enough to hear with a mechanic's stethoscope touching one assembly or the other.

 

Yeah, mine has an internal leak, i can see the brake fluid change when the actuator motor runs, then gets back to normal, it just runs constantly, every 3 or 4 seconds. When i get the new actuator i will try to upload pictures of the fix process, i have never seen anyone do this on a Prius C.

 

Ok good, if it is leaking that fast, you should easily be able to hear the leak with a mechanic's stethoscope touched to both the accumulator assembly and to the actuator assembly, and that way you will know which of those two different parts you need to purchase and replace.