Toyota Prius Forum banner

Master Warning Light

129K views 14 replies 9 participants last post by  av84doh 
#1 ·
Last night as I was making the 30-mile drive home from work, I had gone about one mile on surface streets, was slowing down to a red light before getting on the freeway, and the Master Warning Light (red triangle around "!") came on briefly, about 1/4 second, and then went back off. There was NO "check engine" or other warning, nor was there any message on the MFD, which is what the manual says should occur when the MWL comes on. There was no noticeable change in performance.

So I thought it must have been a fluke.

I drove about 22 miles on the freeway, all normal, and then after I got off the freeway, and was accelerating on a surface street, the MWL again came on briefly. No other warning lights, no "check engine," etc., no MFD message, no change in performance.

The fact that this happened twice concerns me.

Any thoughts?
 
#4 ·
The Prius is 2004 with 120,000 miles on it.

It happened again today driving TO work, on surface streets about 4 miles from home and 2 miles before getting on the freeway.
Going DOWN a downgrade about 50 miles per hour with foot on neither brake nor accelerator.
Master Warning Light flashed on briefly with no message on the MFD, no other warning lights and no change in performance. I drove the rest of the way including about 22 miles of freeway with no further incident.
Before this trip, I had driven several local errands today without incident.

The only other times I have ever seen the Master Warning Light flash is a couple of times when (at very slow speeds) I made the mistake of opening the driver door, but when that happened it was accompanied by an MFD message, which is what the manual says should happen, and I could associate it with a specific cause.
 
#6 ·
I suppose a door issue could possibly be a factor, however when the door is ajar or open, the Master Warning Light stays on and there is an MFD message.

Right now, the light just flashes on and back off quite fast.
In fact, there may have even been an MFD message but it was on and off so briefly that maybe I didn't see it???

I called my dealer to make arrangements to have it looked at -- he said with that symptom it seems most probable that it is a bit low on oil and, since it is about due for a routine oil change, I'll bring it in and have that done. But I have always been extremely religious about regular service, there is no indiciation of oil leakage, and this has never happened before, so we'll see.
 
#8 ·
Just my guess and that's you might be developing a slight leak in your hydraulic brake accumulator. As it starts to lose pressure you would get the light which will go off as soon as the electric hydraulic pump "kicks in."
Not to worry because a complete failure of either the accumulator or electric pump will give you a "steady" light. "Book" then says, "pull over" to side of road and call Toyota.
There are "Two" warning lights in the system. One for the pump failure, and the other for the accumulator, so with only one, you could drive safely home or to the dealer.
 
#10 ·
No, this one would have no indication of a fluid loss. The brakes are mainly pressurized by the accumulator. (Picture a ball with a membrane dividing the center.) On one side of the membrane is hydraulic brake fluid and the other an inert gas. The pressure is maintained by the membrane forcing the hydraulic pressure when the brakes are applied. There is enough pressure stored in the "ball" by the movement of the membrane for many brake applications. At a certain setting the electric hydraulic pump comes on to re-establish full system pressure. That's the noise you hear when the electric pump comes on. The brakes can be powered by either system and you get a separate warning light when the pressure is too low in either. The leak in the accumulator is usually in the "gas" side of the membrane. Your pump then runs more often. Then one or the other warning light illuminates. Until you get any one steady light you really have no problem. Same as just about any system in the Prius. They have always told you to ignore all unusual sounds and drive normally until you get a steady "idiot" light. The occassional one should be overlooked as worthless as even the Toyota dealer will be confused.
My guess is that sometime in the future you will have to have the accumulator replaced. It could be next month or next year. Good luck!
 
#14 ·
FutureGlyph - anything more come out of your Master Warning Light and "Problem"?


I have a 2007 with 126k and mine just did this recently. Each time the master warning light would come on momentarily and the message "Problem" would come on the multifunction display. No other lights or message came on. This happened 6 times today (I'll be calling my dealership service tomorrow). But all six times I was in the process of braking. It didn't happened every time I hit the brakes but that was the only times it happened. The car seems to run fine otherwise. I'm about 3800 miles into my 5000 mile oil change cycle.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top