Check engine light came on this morning-P0101

Since everything is running correctly, clean them, just to make sure.
Then just drive the car. It may take a several cycles of the car starting/shutting off before it resets.
When I cleaned the egr circuit a few months ago, I had forgotten to install the connector on the MAF sensor.
At least it's the easiest one!
I cleared the code a few times with techstream, but it kept coming back. I had read something on Prius Chat
that the computer needs to see a consistent reading for X amount of cycles before it cleared the code.
The light went off on it's own after a few cycles of stoping and starting the car. I had errands to run at several
stores that day. I didn't even notice it was off until the next day!
So maybe clean both sensors with the proper cleaner, clear the code. Then just drive it since it runs fine.
Hopefully, in a day or two it will go off on it's own.

And I would run techstream and test both sensors and verify either of them are bad before spending the money
to replace them. Because you can't usually return electronic parts.

 

P0101 is basically a sanity check on the mass-airflow sensor:



You can see what the ECM is doing there, it's kind of clever.

First, it doesn't want to be making such judgments under any kind of extreme conditions, so it only does this check when up to operating temperature, and driving at moderate throttle.

Then, it has two different ways it could calculate how much load is on the engine. The first way is from the mass-airflow reading; how much air is being charged into the engine is pretty much the by-the-book indication of how hard the engine is working.

But the ECM is also built into a hybrid car that's bristling with other ways of knowing how much power is being asked of the engine, such as electrical readings from the transmission.

So, if you have two different ways you could calculate the load on the engine, and you want to do some kind of basic sanity check, what could you do? Well, you could calculate the load both ways, and divide one answer by the other, and hope the ratio is pretty close to 1.

Toyota, they picked some limits on both sides, and said if the ratio came out less than 0.829 (so the MAF reading says the engine is working less hard than the driving conditions indicate), or more than 1.153 (so MAF says the engine is working harder than driving conditions indicate), they're going to say "something doesn't add up here", and set the P0101 code.

Chances are a discrepancy like that, in how hard the engine is "really" working compared to the airflow they think is going in, will also require largish changes in fuel injection to one side or the other of what would be calculated from the airflow, so they check for that too.

You can see it's one of those trouble codes that covers two pretty-much-opposite scenarios—I mean, look at it one way, they're just saying "we defined a range we'll consider believable, and this code is for when things are outside that range", but look another way, there's a pretty big difference between the airflow reading telling you the engine is working less hard than you think, or that it's working harder than you think, and between the mixture coming out richer than you calculate, or leaner than you calculate. There would be pretty much different universes of explanations to check for, depending on which of those scenarios you're in.

That's why the "inspection procedure" in the manual starts right off by reading the freeze frame from the ECM so you can see what the readings were when the code was set, and find out which side of normal things were off to.

So probably following that procedure will be a good start. Sure, the EGR valve is one of the listed trouble areas, but don't overlook the rest of them either. Again, after figuring whether it seemed like more air or less air was flowing than expected, a bit of thinkery should come up with which areas might be worth a closer look.

As far as taking the EGR motor apart and reassembling it, it's so mechanically simple in there I don't think there's very much to go wrong. If it's like the one I opened and the stator is rusty and touching the rotor, that could interfere with operation, but once the rust is cleaned up and the parts fit, it seems like if you manage to get the parts in place and stator snugged down so the flange is seated, you've probably done it right.

Conveniently (for purposes of assembly confidence), I found it kind of hard to reassemble without the valve ending up slightly open. You can spin the rotor all the way down on the shaft, but the spring wants to spin it back up some, and sooner or later, while you're putting the stator back on, you're gonna have to let go. So the rotor spins back up a little ways, and then as you finish snugging the screws down, sure enough, you've pushed the valve a little bit open.

The reason that's convenient is it gives you a nice easy way to confirm everything moves smoothly. Just plant your thumb on the slightly open valve pintle and push it back closed. It should not be very hard to do, and you should hear the rotor inside quietly whirring (no scraping or roughness) while you do. If that's what happens, I'd say you're good, as far as the EGR valve goes.

It would be less convenient to test if you actually managed to reassemble with the valve completely closed, because then the only way you would have to test would be by trying to pull it open, and it's pretty hard to get a decent grip on for that (and I would be leery of trying to grab it with tools and maybe mess up the seating).

 

You do this just with the valve in your hands, not connected to 12 volt? It's purely a mechanical reaction? Or while connected to 12 volt? I haven't had the valve apart myself, not at all clear on the internals.

 

Yeah, just sitting in your hands.

Making a six-terminal stepper motor (which is what that is) electrically turn is kind of a fussy business. You could make up a board with four two SPDT toggles or buttons and train yourself to 'play' it (one of 110 steps at a time), or a digital output board and write a little program to do it. Definitely not a "just tickle the terminals with a battery and watch it spin" proposition.

In most cases, when you have any steady electrical connection to it (that you're not 'playing' in the right sequence for rotating one way or the other), a stepper motor will stay exactly where it is with great stubbornness. That's one of the things people choose them for. So the only time you're gonna find it easy to move by hand is when there's no power connected.

 

Hopefully it will clear the code soon.

 

I'm very happy for you! And glad it was something simple.
Both MAF and MAP sensors are very sensitive. Glad cleaning them took care of things.

With the rear seat, they are TOUGH! Stupid idea!
I use a pry bar carefully to push up on the seat on one side of the clip and my fingers on the other,
then give it a good yank. It's till a pain!

Thanks for the update!