Code: P0401 (1 Viewer)

[Desertmaster]

Hello Everyone,

I just got an engine light right after I filled up the Rig with Gas, drove to the Auto Zone and they gave me code #: P0401.....Can some one help explain what is this for.


Thanks for your help in advance.

Al

 

[MTNRAT]

Search PO401 or EGR and you will get lots of info.
Sean
Oh and welcome to ih8mud.

 

[MLX450]

P0401............the MUD initiation code......... lol

 

[Rookie2]

Make sure you gas cap is tight and sealing properly before you go do a bunch of other stuff.

 

[woody]

this thread....

https://forum.ih8mud.com/showthread.php?t=11933

...which is so conveniently stuck at the top of this forum section has a writeup on that code, and the repair procedure...

If you dont already have the FSM, get one....

My VSV is bad, have on on the shelf from crusierdan, just haven't gotten around to installing...fortunately, the light goes on for a day, then off for a few....I just keep thinking it'll stay off

 

[Desertmaster]

Thanks for the tips.....here are my findings:

- I removed the gas cap and re-tightened it again......the engine light is still on.

- I re-set the code but after a 15 miles trip, the engine light is back on.

- Per the technical write-up by Landtoy80 and the FSM I removed the EGR Vacuum Modulator cap and checked the filter and for any possible carbon contamination.....the filter is clean and I have no carbon build-up in the Modulator itself.


Things to note:

-Truck has 107,000 miles on the ODO

- The EGR Vacuum Modulator is Green Capped, not the Blue Capped (updated version).



Your advise is sincerely needed as I hate to throw parts at the problem:

-Do I change the EGR Modulator to the new version knowing it is not contaminated ?

-Do I change the EGR Valve as well ?

-I am not too sure if the VSV needs to be replacved as I do not have any evidence of contamination in the EGR Modulator and therefore it never made it to the VSV but possibly the EGR Valve is sticking (my thought).


Any thoughts you can provide will be appreciated.

Thanks

Al

 

[Gumby]

P0401? never heard of it.




i got mine to go off after two years. My front o2 has gone slow. One of the enable criteria for the EGR monitor is the O2 has to be working properly. As long as i have the o2 code i have no EGR code.


BTW - I passed emissions just fine with a slow upstream O2, a leaky downstream and no EGR circuit. Gotta love AWD.

 

[MLX450]

The EGR modulator diaphram may be damaged inside. It is the cheapest and easy to replace. If you do get one yup get the blue one.....good luck

 

[semlin]

first off did you check the vacuum hoses for blockage/contamination and to make sure they are connected up properly?

I would also clean the throttle body and make sure the egr ports on top are clear, pull the egr valve and clean the egr valve to intake plenum passage. this is good PM anyway so no harm and it might fix it.

then you can either replace the VSV then the EGR valve then the egr temp sensor until the code goes off or there are several diagnostic tests in the FSM that will narrow it down. If I was replacing the VSV I would be tempted to get a new modulator even if yours seems fine. If the problem is carbon getting through the modulator it will wreck your new vsv.

 

[Whipp]

I spent the last two weekends fixing this and some O2 sensor errors. I failed emissions with P0401, P0420, P0133, and P0130, even though the tail pipe emissions were good - Colorado could put it on an AWD dyno and it was running clean. Arizona just connected a computer to it and it failed. I'm in the middle of relocating, otherwise I'd throw in some pics of the parts I'm talking about. All my descriptions of orientation, directions, etc. will be from the Land Cruiser's point of view. I'm whipping this up from memory and brief reminders from the FSM pages I downloaded, and kinda rushing. If I'm unclear let me know.

P0401 is the error code for insufficient EGR flow. The Factory Service Manual is a big help here, too. It has an awesome diagnosis section.

My understanding of the EGR system is this: The modulator (green/blue top) is essentially a vacuum actuated valve, that applies vacuum to the EGR valve (the metal flying saucer and plumbing next to the EGR modulator). When there's little vacuum (high engine load), there's little or no EGR. The VSV is a soleniod actuated valve that, closes a connection between the EGR valve and the throttle body, ahead of the throttle plate (no vacuum). When the VSV is open (no voltage applied by the ECU), there is a direct connection to the vacuumless TB, so the EGR stays off - typically, this occurs at idle, or when the engine is cold. When the VSV closes, the vacuum from the modulator can build up, opening the EGR valve.

First, check all your vacuum hoses. A small leak can cause this stuff to not work.

Then check the modulator. The writeup has great instructions, I won't duplicate them here. My filter was clean enough to eat off of, I think the previous owner replaced it, still had the error code, and decided that it'd be easier to pull the Maintenance Indicator Lamp in the dash than deal with the VSV. 1/2 of my vacuum lines were nice and pliable, and the other 1/2 weren't.

EGR valve:
To test that your EGR valve is good, do the following: Plug the vacuum port on the right side of the EGR valve (stick you finger on it, whatever). Find the vacuum line that connects between the left side of the EGR valve and what looks like the intake chamber. It's really connected to a brass tube that runs through the chamber to the VSV that sits inside the curve of the chamber. Pull off the whole line and make sure it isn't clogged, then replace the end on the EGR valve, and with the engine idling, apply vacuum to it. The FSM shows some sort of vacuum pump. You could connect it to another vacuum line or port on the engine. I used my mouth, and just uh, well you get the idea... didn't taste too bad. The engine should falter/shudder/stall, indicating that the EGR valve is good. If not, there's probably something wrong with your EGR valve, try removing and cleaning/replacing it.

Temperature probe:
This sits on the intake chamber to the left of the EGR valve, with about a 4" pig tail that connects to the wiring harness. Unplug it and then unscrew it from the intake chamber. Measure the resistance across the contacts in the plug. At roughly room temperature, the resistance should be in the range of 188K to 439K ohms. Boil a small pot of water, and while watching the resistance, put the tip of the probe in the water. It's probably ok to put the whole thing in there, but I didn't, just to be safe. The resistance at 100C/212F should be in the range of 11K to 16K ohms, the response is pretty quick. If you want, heat some oil to 150C/302F (measure with a candy or meat thermometer), and repeat. The resistance at this temperature should be in the range of 2K - 4K ohm. If not, your probe needs to be replaced. My probe was good.

VSV: The VSV a pain to get to, but not too bad to diagnose. Remember that vacuum line I told you to suck on earlier. This time, pull off the end that connects to the EGR valve, and suck on that, leaving the other end on the brass tube. You shouldn't have much resistance. It tastes just about as good as the last time. If you can't suck any air through, that means your VSV is plugged, and you EGR is on all the time (with the flow rate controlled by the EGR modulator). Of course, your problem is "insufficient flow", so this is unlikely to be the case.

VSV electrical:
Find the plug where the VSV connects to the wiring harness. It's up towards the front of the intake chamber, near the plug for the idle controller, it has 4 wires arranged 2x2. I really wish I had my digital camera. On my '96 Land Cruiser, the two wires connecting to the VSV are yellow, and are the top pair. Trace the wires and double check on your vehicle. I'd hate to tell you to do something that could fry expensive stuff (see the 2nd paragraph down) because the wiring is different. Check the resistance between these two contacts. It should be 30 to 34 ohms. If not, either your VSV is bad (my case), or the wires between the harness and the VSV are bad. Also, measure between each of the terminals and ground. There should be no connection.

To test the wires, you'll need to unplug the electrical plug on the VSV. I removed the throttle body to do this, it wasn't as bad as I thought it'd be - two electrical plugs, 4 bolts, and two coolant lines, if I remember correctly - watch out for the gasket too. Then you'll have enough room to stick a few fingers down and unplug it. Short the contacts on one end of the VSV harness - I used alligator clips - and measure the resistance between the terminals on the other end of the harness. If there's infinite resistance, you have a faulty wiring harness between the VSV and the main vehicle harness.

If your wiring is all good reconnect the plug at the VSV. Don't reconnect the 2x2 plug. Double check all your wires. This is where you could toast stuff. Apply 12V across the two terminals that run to the VSV. I used aligator clips for this - make sure they aren't touching each other in the plug - and just connected them to the battery. My VSV was bad, but I imagine you'd hear a click. That vacuum tube you sucked on twice already - suck on it again, you shouldn't get any flow through the VSV when voltage is applied. If you can suck air through it when voltage is applied, replace the VSV.

I'm going to leave my old VSV where it is and install my replacement somewhere more accessible, just rerouting the vacuum lines and the VSV harness, because... I'm lazy.

I think that's about it for all the diagnostics. Again, ask any questions. PM me if you want my phone number to ask questions in realtime.

- Craig

 

[masterhu]

3SFE RAV4 P0401, VSV plug

Whipp,

Thank you very much for the insightful post on fixing P0401. I have a 1999 RAV4 with same code, VSV is even harder to reach due to compact RAV4. I was thinking about leave the old VSV there, just mount another VSV switch on top and move the 2 tubes and 1 plug over, then I found your post!!!

Did you actually do it? I am trying to identify the two wires goes to VSV solenoid, and just solder the wires and seal/tape them. I am as "lazy" as you for this

they say "lazy" people invented the wheels

thanks,

steve

 

[asmxxiv]

I am currently replacing my EGR valve and had some trouble getting the existing EGR out:

I took the three small (12mm, I think) nuts, which secure the valve horizontally, off. I then tried to slide it along the threaded rods it sits on, but the threaded connection on the lower side of the EGR prevented this.

Do you have to slide the EGR completely off and rotate the EGR itself to unthread this, or does the couple rotate independently, similar to a garden hose? I tried to loosen this with vice grips but it's tight back there. Also applied some liquid wrench.

Any suggestions would be appreciated.

Thanks,
Andy

 

[semlin]

the egr pipe has a nut on it like a plumbing compression fitting. it is can be unscrewed to remove it from the egr valve body at which point the nut slides away on the pipe.

however, you also have to remove those studs not just the nuts to get the egr valve off.

 

[Houston FZJ80]

When I tore my engine apart to do some PM, I found the hard vacuum line that passes through the intake manifold in the rear almost completely blocked, which would cause that code... easy fix, but make sure you extract the junk with a small drill bit coated with Vaseline or something sticky to keep the junk on the drill bit... instead of just pushing it through, which would just send the junk that's in there down to the hose that is really hard to get to...

 

[Project FJ80]

I know this is an old thread, but
I checked my vacuum hose from the manafold to the T and I had a BB lodged in there.
I never thought about checking that hose, or why someone would put a BB in there.

 

[NLXTACY]

My PO401 turned into a PO306 which turned into a headgasket which turned into a motor that is gone. And that was this morning

 

[Jrob80]

My PO401 turned into a PO306 which turned into a headgasket which turned into a motor that is gone. And that was this morning

THIS MORNING? That kind of scares me a little bit I have had a P0401 popping up every now and again and haven't taken the time to get into it much. What also scares me is that I have the P0171 like you did with Landtanks MAF.

I better get on top of this before something really bad happens.

 

[NLXTACY]

I think you are ok JROB80. I'm actually a really bad example. My headgasket went out two years ago but I kept doing the Bar's fix and it kept working...until two years later, this morning

 

[OGBeno]

THIS MORNING? That kind of scares me a little bit I have had a P0401 popping up every now and again and haven't taken the time to get into it much. What also scares me is that I have the P0171 like you did with Landtanks MAF.

I better get on top of this before something really bad happens.

A P0401 itself will NOT lead to engine failure.

 

[NLXTACY]

A P0401 itself will NOT lead to engine failure.

Yeah don't think I am saying the sky is falling.

But if you are seeing several things going on at one time you should take care of the issue before the dominoes start falling.