Can I pass CA smog with P0401?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

B

Blue Bomber

SILVER Star
10 Year 15 Year 20 Year
Joined
Mar 24, 2004
Threads
15
Messages
118
Got error code right after purchase 2 years ago, and I deleted codes just prior to smog test. I passed. Has anything changed in the last two years that would prevent this from working again? Search has indicated that this should not work. Any thoughts? I don't have $40 to toss on a trial test to find out.

I know I need to fix it, but I get 14 MPG and the truck runs great. I just don't have time or energy to exorcise the demon, but I will stick my head in there in about a month or so. Lots of reading on Mud has helped out a ton, and I am formulating a game plan.
 
I'm not sure how smog is done down there, but here in WA they don't actually run the vehicle on a dyno with a sniffer in the pipe. They just plugged a reader into my OBDII port and read my codes. In my case I failed due to the P0401. Clearing does not help because it deletes everything in the ECM's memory, and that shows on their reader too.

That was my experience :)
 
I am surprised that you passed 2 years ago if it was a P0401 back then. No legitimate tester would pass a vehicle with a MIL as all MIL codes are emissions related.

The P0401 is a 2-trip logic meaning that the ECM sets a "not ready" status if you clear it and no legitimate emissions tester would pass a vehicle in a "not ready" status.

It is possible that you have an intermittent P0401 meaning that you can clear it, go through 2 drive cycles which sets the "ready" status, then pass the test. They wouldn't be able to detect this condition. If you have an OBD-II scanner you can clear the code and check the ready status yourself.

-B-
 
FYI i just passed smog about a month ago with a code 28 (02 sensors). Don't ask me how the hell i did, but my numbers were fine. I knew i was throwing a code, but didn't have time to look it up before i brought it in. Well right before i brought it in i disconnected the batteries and passed first time with flying colors. Also, i passed with the aux tank (*they test your fuel line pressures now).

If you cant fix it, try the battery trick.. somehow, someway it worked for me. I was expecting to fail.

Not meant as a hi-jack.. but can someone explain to me how the heck i passed with bad 02 sensors? Just seems wild that our rig passed.
 
P0401

I was just about to ask about disconnecting the batteries. I too have a P0401.


Beowulf said:
It is possible that you have an intermittent P0401 meaning that you can clear it, go through 2 drive cycles which sets the "ready" status, then pass the test. They wouldn't be able to detect this condition. If you have an OBD-II scanner you can clear the code and check the ready status yourself.

-B-
Click to expand...

can you explain a little more on the 2 drive cycles?
 
SOCALFJ said:
Also, i passed with the aux tank (*they test your fuel line pressures now).
Click to expand...
I believe the fuel tank pressure test is only done in pre-OBDII vehicles as anything later will throw a code if the system is leaking.
 
If you disconnect the battery and make the check engine light go off... you need to drive the truck for a good 40-100 miles to make sure the code doesn't reappear and also to let the computer relearn the settings. It will put the computer in the "ready" state which will finally allow it to be tested.

This is how it works in CA anyway.
 
cruiserruf said:
can you explain a little more on the 2 drive cycles?
Click to expand...

Certain malfunctions are detected by the ECM using 2 drive cycles. I can never recall the details, but you have to reach a specific speed (~50mph) for a specific length of time (~2 min) with a specific coolant temp (vehicle must be 'warmed up') and some other things that I cannot recall (maybe intake air temp). The ignition must be turned off between the 2 cycles.

Anyway, the P0401 is a 2 drive-cycle failure and the ECM will not give a "ready" status until the 2 cycles have completed. Also, the P0401 will not give a failure unless it has failed on 2 successive drive cycles. If the P0401 happens it will give a stored code but the MIL will not come on. Once 2 drive cycles have completed without another P0401, the stored code will be cleared.

I am not sure on O2 sensor codes. My Taco threw an O2 code about a year ago. I cleared it and it went about 10 months then threw another. I cleared that one about 4 months ago and nothing since. My 80 threw an 02 code about 6 months ago and I cleared it and it has not returned. However, I am expecting to need 02 sensors as the truck is 11 years old and has 173k miles on the original 02 sensors so it is time.

SoCalFJ didn't fail because he cleared the code and his truck is not OBD-II so there are no stored codes and no way for the diagnostic machine to read the O2 sensor values in real time. He still probably has weak or failing O2 sensors like I have on my 80 and my Taco.

-B-
 
Since the 80 is AWD, I've never had my truck put on a dyno!

I have cleared the CEL code (02) a couple of times prior to getting a smog check in California and I've passed both times.

My last smog check had to be at a Gold station and the technician told me that he has to do a visual inspection of the emissions system because the machines will pass a truck even if it's missing the cats.

It's good to know that you can only clear some of the codes. My code comes back about 1 month after clearing it.
 
I am trying to fix the P0401...finally. Tested the components today, to the best of my equipment. The EGR modulator was definitely toast, so I put in a new one. Totally full of carbon and did not operate per specs. Blue one appears more adequate for the job at hand.

The EGR appears to be running well. The tube going through the intake was highly clogged with carbon deposits. If/When MIL re-appears, i will pull the intake to remove carbon, and verify operation of VSV. I am aware of the method to do it from below, but want to remove carbon as well. EGR will be replaced after that, if necessary.

I am going to equally apply my faith to all known religions. Vehicular sacrifice is not out of the question.
 
You are 100% on target with this plan. This is precisely the approach that many people refuse to take. The only thing that I would add is to be sure that all the hoses are attached, leak free, and not clogged with carbon as a first step.

-B-
 
About 3 or 4 months ago...
I had a P401. I cleared the code.
I passed.

The MIL came on 2 starts later.

:meh:



Is it possible? yes.
Would I suggest tackling the root of the problem? yes.

Have I solved my P401 issue? no.



:doh:
 
Has anyone gotten a smog referee to give the OK on a smog test with a P0401?

The EGR modulator replacement did not solve the problem. Intake cleaning, especially the clogged metal passage is next.
 
I'm due for smog again...

IIRC, two years ago I took it to autozone, had them check it and tell me it was a P401. I asked them to reset it and drove it from there to the test station. It passed. I actually tried to fix it once... replaced the EGR and modulator, tested the VSV (which worked fine), and checked all the tubes. P401 actually went away for two weeks then came back.

So now I may have to tear apart this weekend for a full cleaning as mentioned earlier or may try the Sea Foam procedure... not quite sure yet.

All else fails, I may go back to autozone with a naive look on my face.

WET
 
Last edited:
I have cleared out the intake passage, and removed the VSV for testing (removed intake). Get a long 12 mm ratcheting wrench. Mega carbon buildupeverywhere. The VSV is not operational, so it will be replaced ASAP. Also found a few leaky vacuum hoses, so those will be replaced as well.
 
New VSV installed, also replaced vacuum tube with a massive tear in it under the intake. The plastic air tube was also replaced with a new one (that smells like fish?) and is pliable. Better than duct tape.

Fingers crossed, praying to many gods, and knocking on wood. Animal sacrifice is not out of the question.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom