Another Engine Light (P0401) question

What is it that was repaired by the dealer the first time around?
Are you in a state that is exempt from getting smog checks?

You should first start by cleaning out your throttle body and MAF sensor. Then pour in a nice bottle of Techron fuel injector cleaner. That's less than $20 and it

 

To check the EGR valve condition, you might try removing the pipe between it and the intake manifold. This is harder than it needs to be, due to an extra plastic clip blocking access to the pipe's connection at intake manifold end.

But doable, either by yourself or a mechanic.

The state of the inside of that pipe will help determine the EGR clogging amount. The system is known to clog as miles accumulate.

 

I have a similar problem. Engine light is on and tripping error code p0401. I have a 2010 Prius with 160k miles, that I just purchased a month ago.This is my first Prius, so I am just learning about it. The engine light came on several days after I purchased the car and produced this same error code.
I am not in a state that requires emission tests, but bought it in a state that requires emission testing.
I took it to the local dealer and the dealer read the same code and wanted $545 plus tax to replace egr valve assembly. Ouch.
I cleaned the EGR pipe from the egr valve to the intake manifold to determine it's condition and it was almost completely clogged with carbon, per the suggestion from Mendel Leisk in another post. Very helpful. Thanks.
I had the error code reset by a local mechanic, but later that same day, it came back on.
I am not very mechanically inclined, but decided to buy the egr valve online and put it on myself, thinking that if the pipe was that clogged, the valve was probably full too. So I purchased a new egr valve (genuine Toyota). I had a problem getting the valve disconnected from the egr cooler. But finally got it off and the new valve on the car.
Yesterday I had the error code deleted by a local mechanic and drove it. Later in the day the check engine light came back on again. Bummer. I called another mechanic that I know who is 3 hours away to get his input. I am going to try his suggestion next. He said it could be the gas cap or it could be the DPFE sensor. He suggested I try the gas cap first, since it doesn't cost much and then if that doesn't work, maybe the sensor. (I am not sure where the DPFE sensor is.)
He asked, "When you remove the gas cap, does it poof a little? I said, "No, it really doesn't." In other cars when I would remove the gas cap, some pressure would have built up and I could hear the pressure being released. In this vehicle in does not do that. Also, when the error light first started coming on, I began to notice it came on when I had driven about 100 miles. I would then removed the gas cap and put it back on making sure it clicked one time. After driving it two or three more times it would go off. I would then full up with gas, and sure enough about 100 miles after filling up the check engine light would come on.
So, I am wondering if my gas cap may be the culprit. I do believe my egr valve and pipe needed cleaning, and the car seems to run smoother and the engine doesn't seem to be strained. So, I am glad I had that fixed.
I will try to keep you posted on what I find out.

 

So, my P0401 error code seems to be the gas cap. I replaced it this morning. Drove about 50 miles in two drive cycles. Got back in the car a 3rd time and the check engine light did not come back on. I have driven it twice now with no check engine light. It was a $22 gas cap from the Toyota dealer. I don't know if that is your problem or not. The dealer service department person told me bluntly that this was not my problem and I was wasting my money buying this part. If the gas cap was the problem it would produce another a different error code. But, for my case, that doesn't appear true. I will keep driving it and we'll see.

 

I have been told conflicting information on the check engine light going off. A service tech at the Toyota dealer said that for the check engine light to go off, someone has to erase it with a scanner device. It will not go off by itself. I had a guy at the local auto parts store that if you fix the problem, like I seemed to with the gas cap, it will take 3 or 4 driving cycles. It will take this many cycles for the car computer to be convinced that the problem really is solved and will turn the check engine light off.
I don't know if it goes off in all situations, but the light went off the 3rd time I drove the car after two 20-30 minute drives.
Yes, for me too, the egr valve I believe was an issue. I would also make sure they clean out the egr pipe that goes from the egr valve to the intake manifold. Mine was almost full of carbon. I will try to post a picture so you can see, if interested.

 

Check engine light is back on. P0401. The gas cap didn't fix my problem.

 

From the ridiculous amount of clogging your picture shows, I wonder: how far did you go with the clean up: could it be in the intake manifold, or fouled injectors?

This seems crazy, btw. A poor design.

 

I have cleaned the egr pipe between the egr valve and intake manifold, and replaced egr valve with new one. I had the intake manifold cleaned. Mechanic thought there might be backflow of oil from around the seals coming back into the intake manifold. He said the intake manifold was real sticky. He changed my oil and added Lucas engine seal lubricant (Don't remember the precise name of the Lucas product.). It doesn't seem to be using as much oil now. Gone about 1400 miles since oil change and seems to have used about 1/4 quart. Before oil change it used about 1 quart in a 1000 miles. I don't know about the injectors. I have had the car about 2-3 months. I have put chevron concentrate plus in as an additive several times. Don't know if that is helping or not. I am thinking it is worth a try.

Mechanic wondered about a mass air flow sensor may be a problem giving the P0401 error code.

 

MAF sensor is very easy to access, two screws and electrical connector. It's central in this pic:



There's a specific cleaner for it, and carb cleaner shouldn't be substituted; the latter may leave oil residue. One guru here used isopropanol I believe. Again, just the 99% stuff, not rubbing alcohol which has oil. Oh heck, get the right stuff, lol. From dealership?

I've no inclination to do ours yet, the car is very low miles, as you can judge from the clean pic.

 

How was the intake manifold cleaned? Was it removed so that the passages could be cleaned out with solvent? If not maybe that is necessary.

The repair manual suggests the EGR valve, EGR passage, "EGR with cooler pipe" <whatever that means, another Japanese to English translation which is not obvious to me>, or the manifold absolute pressure sensor are possible causes of the P0401 code.

The code is logged when the engine ECU detects little difference in manifold pressure as the EGR valve opens and closes while the car is moving forward and the accelerator pedal is released which cuts fuel supply to the engine.

 

There's the EGR cooler sub-assembly, with a pipe coming off it. I'd guess this is the cooler pipe.

 

I will try that. Thanks for the attachment instructions.

 

I am pretty sure the intake manifold cover was removed and cleaned with solvent of some kind. The mechanic just mentioned it was sticky with the oil and carbon mixture and he cleaned that.

Yes, I have cleaned the EGR passage. and replaced the egr valve. But the EGR cooler pipe is way beyond my skill level to replace. From my understanding, the cooler is connected to the egr valve and is behind it, with a coolant hose or hoses going to it. I have not replaced that.

Your "code is logged" description makes sense, but I don't know what to make of it with my car. For instance, the check engine light went off and stayed off for 2 weeks. I just decided to drive the Prius, just like I would any other non-hybrid. I didn't try to start from a stop light or stop sign on battery as long as possible; I didn't try to hyper-mile. Just drove it irregardless of the mpg. So, I was using the gas engine more.

Then a couple of days ago I was driving down a two lane back road, hyper-miling, going about 35 miles an hour for a while. It was then the light came back on and has stayed on. I have since driven it hard on the interstate - up and down hills - about 200 miles. The check engine light has stayed on.

Is the manifold pressure sensor the same as the vacuum switch sensor ? Is the mass air flow sensor a different sensor as well, or is it the same as the manifold pressure sensor or vacuum switch sensor? I have also been told it could be the vacuum switch sensor. But am confused in trying to research it.

 

Okay, I understand what you are saying. The cooler pipe is different than the egr cooler connected to the back of the egr valve.

 

The intake manifold is a black plastic assembly. This would not be easy to remove.

The 35 mph speed and pulse and glide driving is exactly what would cause the EGR light to go on if the system has a problem. A speed of 25 - 37 mph for more than five minutes, and then releasing the accelerator pedal for more than 5 seconds, is the driving pattern suggested by the repair manual when troubleshooting the system.

The mass air flow sensor detects the flow and temperature of air entering the engine intake. This is located above the throttle body assembly.

The manifold absolute pressure sensor is mounted to the intake manifold and detects the pressure within the manifold. This is downstream from the throttle plate, so the sensor is exposed to engine vacuum produced when the engine is spinning while the throttle plate is closed.

I am not aware of a vacuum switch sensor as part of the Prius engine management system.

Maybe you should remove and clean the manifold absolute pressure sensor, which requires removing the air cleaner housing for access.

 

Yes, very clean in that picture. Impressive. I purchased some mass airflow sensor cleaner. Took the mass airflow sensor off. Sprayed the cleaner. Will now see if that does anything different. Thanks again.

 

Thank you for the advice and help. I am running out of time for today and will try to clean that soon. Again, thank you for sharing your knowledge.