So, every DTC has a range of parameters that will cause it to trigger. My question: is there a list of parameters/sensor readings that can let you know what might trigger a DTC?

I’ve been getting ‘P0401-00 (AC) Exhaust gas recirculation flow insufficient detected’ on my 4.4TDV8 intermittently but am almost certain that it’s not my EGR. Why? I had a brand new EGR fitted under warranty less than a year ago. I can’t rule it out, but let’s be honest, the chances are v low.

I’ve heard that the MAP sensor on top of the plenum can trigger it if it gets blocked. I’ve been keeping an eye on that sensor and keeping it clean so, unless the sensor is on its way out then I doubt that’s it. I’ve recently heard that the temp sensor in the EGR can also need replacing as the sensor sometimes doesn’t register heat very well if faulty/on its way out, and that will also register the fault. Both the MAP and EGR temp sensors may be red herrings but, the point is that there must be a number of different things that will trigger any particular fault; it can’t just be one parameter.

So, back to original question, is there a source that will tell me all of the different possibilities that can trigger a fault code?

Many thanks in advance as ever! Andy

2010 4.4TDV8 Vogue SE in Santorini Black with Ivory interior
2017 Audi SQ5 3.0 V6T Quattro in Volcano Red
2001 Audi Allroad 2.5 TDI manual with low-range in Highland Green. Currently SORN whilst undergoing some serious restoration!

Have you looked at the DTC chapter in the Workshop Manual? Free on the Wiki. Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
2025MY Defender D350 90 in Silicon Silver on coils
1974 Series 3 Lightweight = "Millie"
Many, many other Landies over the years
My preferred specialist: www.glenrands.co.uk
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put borescope camera inside egr tube and look gor soot. If you have bad / old injectors then it will give lot of it.

Thanks for the replies both. There’s little useful detail in the DTC Documents sadly. As I said in my OP, I’m not worried about the EGR as it’s not even a year old.

What I’m trying to ascertain is what list of triggers can cause this (or any other) DTC. As it’s an ECU-based decision, there must be something out there which would list all the possible causes for each possible DTC. Surely?

I heard that TOPIX has more info that the WM, so is there someone with access that might have a look to see what it says ref P0401-00?
TYIA Andy

2010 4.4TDV8 Vogue SE in Santorini Black with Ivory interior
2017 Audi SQ5 3.0 V6T Quattro in Volcano Red
2001 Audi Allroad 2.5 TDI manual with low-range in Highland Green. Currently SORN whilst undergoing some serious restoration!

Last edited by Ajmngn on 17th May 2023 10:19am. Edited 1 time in total

Have you checked the short pipe that connects to the intake plenum for soot blockage? Also the ECU will try to increase EGR flow by closing the throttle plate to create negative pressure in the intake to pull more EGR gas through so if this is sticky / intake 'gunked' it may be the issue. (The commanded and actual throttle position and EGR position are viewable in the GAP IID tool BTW). Lastly an air leak in the intake will both cause more soot and prevent the throttle plate from creating a low pressure intake condition. 2012 TDV8 Vogue (L322)

There are a few places now that can "delete" the EGR and DPF  Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
2025MY Defender D350 90 in Silicon Silver on coils
1974 Series 3 Lightweight = "Millie"
Many, many other Landies over the years
My preferred specialist: www.glenrands.co.uk
--------------------------------------------------

Was the vacuum actuator changed as part of EGR valve?

I had the same code, my vacuum actuator had failed.

As part of diagnosis i checked operation of electric solenoid at the front of engine bay.

I have had the vacuum actuator changed when the new EGR was fitted. I haven't looked at the solenoid yet, but will see how easy it is to access when I get a chance to have a look. I assume you mean part no.10 on the picture?

Click image to enlarge

So, an interesting development this evening…

I was washing the car this eve and had a bit of extra time whilst still light so thought I’d try and see if I could access the electrical connector on the EGR Valve Motor (referred to as the Electric Actuator in the DTC Tables). I had to remove the pollen filter housing which is super quick and, lo and behold I could just about get to it with the bonnet in service position. BEWARE - don’t attempt this whilst engine hot as you will burn yourself!

I managed to unclip the plug with a pick tool and flathead screwdriver and, looking inside it was quite dirty. So, after plenty of contact cleaner and then pressing the plug back and then pulling apart about 20 times to clean the surfaces, I replaced it back in the EGR Valve Motor. There’s another plug about 15cm further up the branch that the first goes to and where it joins another bunch of wires from elsewhere, so I did that plug too. Anyway, the whole thing took about 10 mins to do. Easy.

Anyway, I thought I should do for a test drive and see if I could see any difference using the GAP tool to monitor the live values. So, I’ve either fixed it or f*cked it! I’m pretty certain it’s the former as all I did was put contact cleaner on the plugs and open and close them.

The GAP tool shows the values below and are what I’ve been keeping an eye on for the last week or so. The top two values were previously fluctuating between 0% when under hard load to about 50% almost constantly when driving. However, they did this until the engine had warmed up and then just went to 0% and stayed there this eve. That’s never happened before so, I have to assume that cleaning the contacts in the circuit has made a difference to the readings, which would make perfect sense if there was resistance through corrosion or dirt on the connections.

The bypass duty valve cycle is the vacuum actuator and is either 100% or 0% depending on whether you’re on or off the throttle which is as expected, and the other two values seem to remain fairly static as they did before.

Long story short, it’s clearly operating differently as a result of my investigation this eve so as I said, this is either fixed or f*cked. I’ll only know once I’ve driven lots more miles as, if the EML light stays off and the DTC doesn’t return, then I guess I must’ve fixed it! Fingers crossed for me!! 🤞

Click image to enlarge