This thread might help.

https://www.toyotanation.com/threads/computer-logic-before-and-after-code-p0401-is-thrown.1766026/

 

On your Camry's 5SFE, temperature is an indirect input at most.

From the service manual, here is what is necessary for the computer to throw a P0401 code and turn on the CEL:

"After engine is warmed up, intake manifold absolute pressure is larger than value calculated by ECM while EGR system is ON (2 trip detection logic)."

When the EGR System is ON, the following are true:

-- the EGR VSV does not have 12 volts applied to it;
-- the EGR VSV is electrically Off;
-- EGR VSV port E and port G are connected;
-- EGR VSV port E transmits vacuum to the top of the EGR Valve;
-- the EGR Valve is open;
-- exhaust gas is being recirculated.

A drawing to help:

I have had a fair amount of experience with P0401 on my 3SFE Rav4. I have also read many reports on eliminating this code on 3SFE and 5SFE engines. (The 5SFE engine happens to use the same cylinder head as my Rav4.) I know people do not want to fire the parts cannon. But the problem is that successful bench testing of either the EGR VSV or the EGR valve will not confirm these parts will operate correctly under all engine conditions. Also I think the age of many of these components translates to replacement being a sound investment, even if the problem is not fixed immediately. Lastly the hours DIY-ers and technicians alike often give to P0401 in my opinion argues for a systematic replacement of parts until the problem is solved. For these reasons I think the strategy to adopt is a step-by-step approach of replacement as follows.

-- Replace all EGR system vacuum hoses with high temperature resistant silicone hoses. Does this fix the code? If not, one has still just eliminated the chances of vacuum leaks in the future. The hoses are available on eBay. Some help appears here: https://www.rav4world.com/threads/vacuum-line-replacement-with-high-temp-silicone-lines.255961/.

-- Clean the EGR valve and all connected ports. This includes the ports in the throttle body and the large pipe connecting to the bottom of the EGR VSV. Do the new hoses and clean EGR valve fix the code?

-- Go ahead and replace the EGR VSV with the Aisin version. It is typically available from rockauto or ebay or both. Does the new EGR VSV fix the code?

-- Replace the EGR vacuum modulator. Aftermarket is fine in my experience. Does the new modulator fix the code?

-- Replace the MAP sensor, preferably starting with a salvage yard one. Does the new part fix the code?

-- Check the EGR VSV circuit as given in the service manual, SFI Section. This is rarely the cause of the P0401, but the circuit's testing is free to do for anyone who has a multimeter. Go ahead and eliminate a possible fault in the wiring.

– Replace the gas filter. In 2023 I cleaned my old one with carburetor cleaner. Carburetor cleaner may or may not destroy the filter media. The main filter media appears to be brass wool. A magnet will not stick to the media. Here is a photo showing the location of the gas filter on a 3SFE engine.

-- If none of the above fixes the P0401, then I would next consider replacing the EGR valve (possibly starting with a salvage yard one).

-- Finally I would consider replacing the computer.

By my count, P0401 is the second most frequently reported code at this Camry sub-forum. It does not always get resolved.

I attach the service manual section for troubleshooting P0401.

 

Thanks for the replies! (plus corrected reference to the vacuum modulator)
FYI what I've done so far:

1. I vacuum tested the EGR valve and found a slow vacuum leak, so got a new one.
   1. With 154k on this old beauty, there was barely any exhaust carbon build up on the EGR and the hoses were relatively clear.
   2. Everything came off easy, except the paper gasket between the EGR and the intake manifold
2. While troubleshooting, the R port of the Vacuum modulator instantly broke off while removing the old hose, so I also replaced the vacuum modulator.
3. I went with Dorman for both parts, so I'll see if going cheap it going to haunt me down the road.
4. Vacuum Lines:
   1. I replaced the vacuum hoses between the throttle body and the vacuum modulator with 1/8" hi temp silicone hose.
   2. I cleaned the short hose between EGR and vacuum modulator.
   3. I'm leaving both VSV vacuum lines as I'm not feeling like jacking up the car at this point to get to it.
5. I cleared codes and road tested one run EGR test today. I'll run a 2nd cycle later tonight while going to work. Keeping my fingers crossed!

 

While troubleshooting, the R port of the Vacuum manifold

The part with the R port is called the "EGR Vacuum Modulator." A manifold is completely different.

My Rav4 uses the same EGR vacuum modulator as your Camry. I replaced my Rav4's EGR vacuum modulator with an aftermarket one two+ years ago. Doing so eliminated the P0401 code. The code has not returned.

 

I hope its possible to get that VSV out from the bottom. It sounds like many relocate the VSV to the rear firewall where its more accessible for future needs. Do you know if the electrical connector will easily route to a higher location, or is an extension plug needed?

This guy found a location that did not require an extension:

Go right to 4:40 or so to see the new location.

Consider searching youtube for more videos of EGR VSV relocation on the Camry.

Some people find a threaded bolt hole on the firewall? At least, that's popular with my 1998 Toyota Rav4. If the harness is not long enough, splicing in more wire is not too much of a chore. I bought the required connector online so I did not have any wire splices. This was overkill, but I learned a lot about id'ing connectors in the process.

I used a dremel and hacksaw to modify the bracket for my EGR VSV's new location.

If per chance you find getting the old EGR VSV to be too difficult, many people leave the old VSV in place, abandoned, and just unfasten the hoses and plug.

 

I watched that one this morning after I posted. I think just getting to the connector is THE challenge. Not worried about the VSV and Im going to replace the hoses too anyway. He had to stick that 22" extension etc to get the VSV out. I guess I'll see what to do once Im under there

 

OK from my OP, I was asking about what sensors are used to determine if the EGR system is malfunctioning. Articles provides show that P0401 DTC will trip if the air temp doesn't rise more than ~ 72F when EGR is ON (with two test cycles). Specifically, the text says: " ECM does not see the EGR temp rising more than 72 degrees F above the ambient air temperature".
But its using a MAP sensor instead of a temperature gauge, so how does the MAP pressure reading correlate to the temperature, or maybe the delta temperature? What values are the ECM sensing?

I was able to record some values while driving with my BlueDriver scan tool, such as: Intake MAP Sensor (7.7 to 28 InHg range), Intake Air Temp (136 to 126 F range), and Engine Coolant Temperature (189 to 187 F range). Not sure how to correlate these to the EGR system.

I'm looking a few troubleshooting steps ahead, and trying to see if I can use by BlueDriver tool to monitor values that would cause the problem.

1. Next on my list is to replace the VSV and both vacuum lines (though they appear to be working).
2. If that doesn't fix it, I'd like to find where the temp values are generated... MAP Sensor was mentioned as a possibility.
3. Also thinking the engine exhaust tube feeding into the EGR could be clogged
4. And lastly, the replaced EGR or Vacuum Modulator could be bad, though they appear to be operating.
5. What's left besides the ECM?

 

OK from my OP, I was asking about what sensors are used to determine if the EGR system is malfunctioning. Articles provides show that P0401 DTC will trip if the air temp doesn't rise more than ~ 72F when EGR is ON (with two test cycles). Specifically, the text says: " ECM does not see the EGR temp rising more than 72 degrees F above the ambient air temperature".
But its using a MAP sensor instead of a temperature gauge, so how does the MAP pressure reading correlate to the temperature, or maybe the delta temperature? What values are the ECM sensing?

You do not believe what the service manual says, as attached in post #3, and as repeated below?

"After engine is warmed up, intake manifold absolute pressure is larger than value calculated by ECM while EGR system is ON (2 trip detection logic)."

 

SO it's really the MAP sensor readings that is significant, and not a direct temperature reading

Yes.

I'm still trying to figure out how to detect this: while I can view the Intake Manifold Absolute Pressure readings in real time using by ODBII Live Data tool, I dont know how to compare it to the values calculated by the ECM.

Believe me, the fact that we DIY-ers do not know how the ECM calculates this reference "value" vexes me. I have been pondering PO401 and why it is the second most reported code at this forum for a few years now. Worse, too many threads report a P0401 going unresolved.

But I also figure the calculation of these values may simply be so sophisticated and so irrelevant to troubleshooting that I should not tarry over this aspect of the design. After all, the service manual is written for technicians and has them using specialized tools. Yet none of the service manual's instructions say anything about comparing MAP sensor readings to some pre-calculated value.

Strega315's posts #8, #10 and #14 in the thread linked above (www.toyotanation.com/threads/computer-logic-before-and-after-code-p0401-is-thrown) are certainly interesting. I attach the two pages from the service manual that flesh out what he posted a bit more (at least for me). But again, this info pertains to a 1MZ-FE engine and those 5SFE engines having an EGR temperature sensor.

Should I be able to view those calculated values somehow in ODBII for this vehicle?

I can only guess, and my guess is no.

You can certainly search this forum and elsewhere and see if anyone knows otherwise.

For me it is enough to know that the MAP sensor is sending an indication that pressure is too high (under certain conditions), and this is what will result in a P0401 code being thrown. I think: So what could cause the pressure in the intake plenum to be higher than it should be when the EGR system is on (and so EG is being recirculated)? Certainly if one disconnected the hose to the MAP sensor, this would result in too high a pressure. What else could cause too high a pressure?

Subsequently I find the design of the system to be too complex for me to say anything laser-focused on such-and-such aspect of such-and-such EGR system component.

Personally I have homed in on how the soot in the exhaust gas may tend to foul all the components: Throttle body (especially intricate ports inside it), EGR Valve (with similar intricate ports), possibly the EGR VSV, the EGR vacuum modulator and maybe the MAP sensor. The EGR vacuum modulator and MAP sensor systems both have non-trivial filters serving them. I have found these filters plenty sooty in the past. Someday I am going to take a dremel saw to an old EGR VSV and see if its insides are fouled.

The soot the EG deposits is so sticky. At this point I clean my 1998 Toyota's throttle body about once every two years or else I start seeing effects. After two years, the layer of soot on the inboard side of the throttle body butterfly valve, for one, is dramatic and sometimes, enough to cause sticking when I press the gas pedal.

I appreciate your intense look at all this. Maybe this thread will turn up new insights? Mostly I hope you solve this for your Camry.

 

https://www.toyotanation.com/threads/what-sensors-are-used-to-trigger-a-p0104-egr-insufficient-exhaust-gas-flow.1781766/?post_id=14977812&nested_view=1&sortby=oldest#post-14977812:~:text=But%20again%2C%20this%20info%20pertains%20to%20a%201MZ%2DFE%20engine%20

and%20those%205SFE%20engines%20having%20an%20EGR%20temperature%20sensor.

-- The Aisin VST-006 comes with the bracket. To help confirm details of any part, remember one can always google on the part number; click on the images tab; and look at the images that come up.

-- I thought the bolt holding the EGR VSV to the block had a 10mm head. If you were using a 12mm socket, maybe try one more time with a 10mm socket?

1st thing: New VSV fixed my P0401 problem. yaaah!

About your previous comments:
1. I did use the Aisin VST-006, and it does NOT come with a bracket, and all the pictures on Amazon show it without a bracket, too. There are other off brands that DO come with a bracket, but reading here it seemed best to stick with the AISIN OEM. I'll hit pick n pull eventually, but for now zip ties to the MAP sensor bolt is good enough for today.

2. The VSV bolt under the intake manifold is most definitely 12mm. The two YouTube videos I watches also point this out.

Last question: Did you find an aftermarket cable extender for the VSV valve, or did you just make one? Have any M/F connector part numbers from that task?

Cheers

 

Got it, re the bracket and bolt head size.

I bought connectors and made a "clean" extension as follows:

From the 1999 service manual, the p/n for the female side of the connector is Toyota OEM 90980-11156 (replacing 90980-12634). I believe that at least part of the Toyota p/n is printed on the connector. A magnifying glass may be necessary to see the p/n on the connector, if it is there.

Go to Connector Kits, Terminals, Wiring, Tools | Corsa Technic and put 90980-11156 into the search window. A connector one can buy will come up. Also the listing will show the male side, mating connector, which one can also buy.

I bought the male side from corsa-technic. The latter's service was good. I bought a female side pigtail from a salvage yard. I had some spare pins et cetera and then assembled the extension. If one is new to assembling connectors from scratch, the pins can be tricky.

Edit:
The Corsa-technic male connector purchase included the pins and tiny rubber seals.

 

Got it, re the bracket and bolt head size.

I bought connectors and made a "clean" extension as follows:

From the 1999 service manual, the p/n for the female side of the connector is Toyota OEM 90980-11156 (replacing 90980-12634). I believe that at least part of the Toyota p/n is printed on the connector. A magnifying glass may be necessary to see the p/n on the connector, if it is there.

Go to Connector Kits, Terminals, Wiring, Tools | Corsa Technic and put 90980-11156 into the search window. A connector one can buy will come up. Also the listing will show the male side, mating connector, which one can also buy.

I bought the male side from corsa-technic. The latter's service was good. I bought a female side pigtail from a salvage yard. I had some spare pins et cetera and then assembled the extension. If one is new to assembling connectors from scratch, the pins can be tricky.

Thanks for the info! I'm an old IT guy, so pins and connectors dont bother me. I probably have a pin extractor tool in my tool kit.

So glad I fixed that P0401.... when I road tested the new VSV, it was still failing and I was BUMMED! Opened up the hood and found the connector fell off! Lol. It's kind of stretched after I rerouted it and the connector latch isn't holding very strongly, so I now have a zip tie to keep it in place. Off to Pick n pull tomorrow for some spare brackets and maybe another connector

Cheers!

 

Sooooo... while my P0401 problem was resolved back in May, I've been trying to get my Emissions Readiness Test to complete. All completes except the Catalyst readiness test. I'll post a new thread for this problem!