Code: P0401 (1 Viewer)

[OGBeno]

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

Yes, this is definitely a more prudent mindset to have.

 

[Jrob80]

A P0401 itself will NOT lead to engine failure.

No, I agree I did not believe that P0401 would lead to headgasket/engine failure. I knew from another thread that NLXTACY has a P0171 and a while back and I thought to myself that I better not let these little things go for to much longer or build up.

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

I remember reading that now that you say it.

 

[weaseltraps]

I started with the easiest and removed the temp sensor which was covered in black soot/carbon. I wire brushed it clean, cleared the code and it hasn't come back in a year.

 

[muddinmetal]

Project FJ80,

Someone has deleted so of your smog components, this is a somewhat common modification which you can find plently of info about here on this site...

 

[dtrain]

​

I started with the easiest and removed the temp sensor which was covered in black soot/carbon. I wire brushed it clean, cleared the code and it hasn't come back in a year.

Where is this sensor?

 

[OGBeno]

DS of the intake manifold. It has an electrical connector and screws right into the upper portion of the plenum towards the back.

 

[dtrain]

I started with the easiest and removed the temp sensor which was covered in black soot/carbon. I wire brushed it clean, cleared the code and it hasn't come back in a year.

DS of the intake manifold. It has an electrical connector and screws right into the upper portion of the plenum towards the back.

Thanks Beno!

 

[Olaf]

This is a phenomenal thread. Been chasing down codes for o2 sensors. Just replaced with denso's. Had to splice the wires so called denso to make sure I got it right. Left with p0125. Lots of information about it. Was about to change my ECT sensor, couldn't locate it. Found this thread for help.

My mileage was way down, but improving since denso's. Not sure what to do next. Recently replaced distributor cap and rotar. Plugs and wires, air filter, coolant and thermostat.

 

[Olaf]

This is a phenomenal thread. Been chasing down codes for o2 sensors. Just replaced with denso's. Had to splice the wires so called denso to make sure I got it right. Left with p0125. Lots of information about it. Was about to change my ECT sensor, couldn't locate it. Found this thread for help.

My mileage was way down, but improving since denso's. Not sure what to do next. Recently replaced distributor cap and rotar. Plugs and wires, air filter, coolant and thermostat.

Cruiser Dave your link was all I needed, https://forum.ih8mud.com/80-series-tech/538233-water-temp-sensor.html. The sensors in the parts books have different connectors that list as exact fits, most don't but the one in your thread does. Really appreciate your pics.

Fixed and getting 13 plus mpg at 75 mph. Thanks!

'97lx, 290,000 arb, mate blk, emu lift, engine rebuilt, etc...

 

[SierraMtns]

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

Craig thanks for posting this it really help.

 

[decoco]

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

It's taken me 3 years, 3 months and about 40,000 miles to get initiated. fml.