Trouble Code P0401?

Tiny
PERIDOG
  • MEMBER
  • 2001 TOYOTA CAMRY
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 92,600 MILES
My engine light is on and I can't get it smogged. All the tests passed with the exception of the light. I've cleaned out the EGR valve and reinstalled it. It seemed to work properly. I've checked all the hoses and they were ok. What could I check out next?
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Friday, April 3rd, 2009 AT 9:18 PM

35 Replies

Tiny
WILLIAM OLIVER
  • MEMBER
Po 300 means multiple misfire any # 1 2 3 or 4 ck plugs, wires and ck compression. And po401 I guess is egr system malfunction. Replace vacum switch valve for egr wich is on the back of the engine close to power stering pump around that area. Did help?
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Tuesday, April 7th, 2009 AT 7:50 PM
Tiny
DIONJMUNOZ
  • MEMBER
  • 2000 TOYOTA CAMRY
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 98,000 MILES
Retrieved code P0401 insufficiant egr flow detected. Would like to know correct repair procedure.
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
RASMATAZ
  • EXPERT
A code P0401 most likely means one or more of the following has happened
There is a blockage in the EGR (tube), most likely carbon buildup
The EGR valve is faulty
The EGR valve may not be opening due to a lack of vaccuum
Possible Solutions
In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.
Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's, There should be a noticeable difference in RPM's with the EGR open
Clean out the EGR valve and/or tubing to remove deposits
Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
GRINDLESGARAGE
  • MEMBER
  • 1999 TOYOTA CAMRY
1999 Toyota Camry 4 cyl Front Wheel Drive Automatic

code p0401 will not go away. Changed all the components of the egr system and made shure all the passages were clear. Code p0401 still comes back. What is the problem and what monitors the egr flow?
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
RACEFAN966
  • EXPERT
OK so this is an EGR insufficient flow detected. This can be one of the following. EGR Valve, EGR Vacuum Switching Valve (VSV), Map Sensor, bad ECM or a vacuum hose disconnected or cracked or a short in the wiring to the EGR VSV. I can help with some testing but I need to know how comfortable you are with doing some tests and I also need to know if you have a scan tool that will allow freeze frame while driving and do you have a digital multi meter. Let me know what parts you did replace from the list above too. Thanks.
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
JEREMY HENRICH
  • MEMBER
  • 1998 TOYOTA CAMRY
  • 2.2L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 242,000 MILES
My car has had the check engine light on. Excessive exhaust smell inside car. Throws code p0401. I have changed EGR valve. The air intake hose has a pretty bad leak too. I need to replace. Could this be causing all the trouble? I am going to replace regardless, but want to know if there may be another issue I am missing. I also had replace a part next to the EGR valve that has three vacuum lines hooked up into it. It does not bolt on or anything just snaps into a bracket that holds it in place.
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
HMAC300
  • EXPERT
Misfires can be anything like a sensor injector. The 401 code is EGR is not working, vacuum leak, map sensor or ECM. I am sending a picture of what I think you may have replaced, it does have an electrical connection as well. So it may be hooked up wrong or something similar.
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Saturday, November 14th, 2020 AT 2:28 PM (Merged)
Tiny
JCANTERGIANI
  • MEMBER
  • 1997 TOYOTA CAMRY
  • 2.2L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 184,000 MILES
I keep getting this code, even though I have now replaced ALL the components in the EGR system: EGR valve, EGR modulator, EGR Vacuum Switching valve (VSV), and all 5 of the EGR vacuum hoses. I used genuine Toyota parts. All ports and pipe are clear. Both EGR gaskets were replaced. I am getting 12 volts at one of the two terminals that connects to the VSV. Don't know what else I can do. I'm having no other codes or problems with the car. Could it be the ECU is bad anyway, even though there are no other issues? P0401 code
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HMAC300
  • EXPERT
These codes can be caused by a faulty EGR valve here is a video that shows how to test it with the location of the valve in the diagrams below

https://youtu.be/DqUCbbKdWf4

Check out the diagrams (Below). Please let us know what happens.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
JCANTERGIANI
  • MEMBER
Yes, I know all of those tests. THE PARTS ARE ALL NEW GENUINE TOYOTA PARTS. The hose from the intake to the MAP sensor is not leaking, or plugged, and the MAP sensor holds vacuum when applied. I erased the codes and went on the Toyota EGR drive cycle and followed it exactly. It always trips the pending code during the drive cycle and the it becomes a current code on the 2nd drive cycle. The 5 EGR vacuum hoses are routed correctly.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HMAC300
  • EXPERT
Did you check the vsv likit said in second pic and test vacuum through it? Is the egr plugged or partially plugged including passageways. It also can be a bad ecm.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
JCANTERGIANI
  • MEMBER
Yes, I did. All the parts are new Toyota parts anyway. Nothing is plugged up. Is there a way to check using a multimeter to see if the ECU is switching the VSV on/off? I do get 12 volts at one of the terminals on the plug that connects to the VSV. Is the other terminal supposed to get 12 volts at times to "ground side switch" it? I'm not sure how that can be tested. Is there a way to test the ECU first?
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HMAC300
  • EXPERT
The pink wire on vsv sensor goes to port 7 on pcm so check that for continuity. The other wires tie in to different sensors as you can see in pic. The second pic with violet wire leads to pin 8 on pcm do the same with that. If everything is getting vacuum then I'd have to go with ecm isn't sending power to operate the system which a pro will have to install due to security systems in them. Icannot send the wiring diagram as it's to big so if you want to delve further then you can either buy a manual, rent one from here under manuals for your car and I think they allow you for a week as I'm not sure or see if a library has one for it.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
JCANTERGIANI
  • MEMBER
It was the ECU. I went to the junkyard and found one for $39. I installed it and the car started and drove fine. After completing the EGR drive cycle, the readiness monitor completed and there is now no more P0401 code. I didn't have any problem with security or anything else with replacing the ECU by myself. Maybe it's newer cars in which that is the case. Anyway, the problem is fixed with a replacement ECU.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HSANO6294
  • MEMBER
  • 1997 TOYOTA CAMRY
  • 2.2L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 225,000 MILES
My engine is throwing a P0401 Insufficient EGR Flow error code. The engine was running smoothly, but with the error code I would not pass California smog inspection. The engine was running very smoothly so it was obviously not a major issue with how the engine was running.

After troubleshooting the EGR Valve, vacuum switching valve, and vacuum modulator, I found that the vacuum modulator was defective.

I replaced the vacuum modulator and drove two hundred miles of mostly city driving to reset the computer. Even though the engine was running pretty smoothly previously, I noticed that the engine was running even smoother after replacing the vacuum modulator.

When I brought it back to the smog test center, it immediately threw the P0401 error code again. The technician suggested cleaning the EGR down tube and driving another two hundred miles mostly city miles to let the computers reset.

What do you suggest I test next?

Also, I have noticed some mechanics refer to a bottom mounted vacuum switching valve but mine appears to be top mounted. Are there two vacuum switching valves on this vehicle, or are some top mounted while other Camry's have them on the bottom?

It has been many years since I have done my own auto repair. Back in the 1980's and early 1990's, I enjoyed do my own work on mostly 1960's to mid 1970's cars. This is my first time working on a car with (relatively) modern emissions controls. I have an engineering BS and MS and working knowledge of automotive systems, and consider myself a mid-level home mechanic.

Thank you!
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HSANO6294
  • MEMBER
I noticed that the two vacuum hoses connecting the throttle body and the vacuum modulator are showing aging signs (surface cracks) but appear to hold vacuum. They seem to be easy parts to replace to rule out vacuum leaks there. Do you concur? If I were to replace them, do I need to use genuine Toyota parts, or can I use equivalent lengths of generic tubing from my local parts store?
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HMAC300
  • EXPERT
Get the ones at auto parts stores. then try cleaning the EGR tube like mechanic said.
https://www.2carpros.com/articles/how-exhaust-systems-work
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HSANO6294
  • MEMBER
Thank you HMAC300. Do I need to remove the EGR Valve from the intake manifold to clean the EGR tube, or can the EGR tube be removed/cleaned with the valve in place?
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HMAC300
  • EXPERT
Take the valve off and clean it that way and check pintle valve on EGR it may be clogged or not working. Try putting vacuum to it see if it does or not.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)
Tiny
HSANO6294
  • MEMBER
My previous tests applying vacuum to the EGR valve caused the engine to stumble and/or stall. I think the EGR valve is working okay. I will take a closer look when I have removed it. The nuts to the intake manifold studs came off easily. I am currently soaking the bolts that hold it to the bottom tube in penetrating oil as they are quite frozen.

I have changed the two Vacuum Modulator to Throttle Body tubes.
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Saturday, November 14th, 2020 AT 2:29 PM (Merged)

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