P401 egr insufficient flow

**DMitch16** · 11-01-2016,06:49 PM

The glow plug light comes on for most non serious engine faults and will say via the dash MFD "Engine Fault - Go To Workshop" or something similar (the most serious ones are when you hear a massive bang and hear metal bits clanking around but if nothing mechanical drastically fails the car for more potentially damaging faults usually screams at you and the dash MFD tells you to STOP!).

The emissions warning one (engine shaped on a VW TDI) usually reacts to inconsistent readings from the DPF pressure difference sensor at the top of the engine (it has an electrical connector and 2 rubber hoses connected to it that are in turn connected to narrow metal pipes bolted to the DPF that read the pressure before and after the DPF filter can. This is so the ECU can determine the gas flow pressure through it. The ECU then uses these readings to make small adjustments to the running or to invoke a regeneration should the soot index level of the DPF be high at this time and other regeneration conditions are met. If it doesn't like the reading being returned it may illuminate the emissions fault light on the dash. Mine has in the past come on and after a few trips and gone off again but it should not be a regular recurrence which would indicate some sort of problem still.

There is of course much, much more monitoring and adjusting going on other than this in the ECU and across the engine systems so this is just to give you an idea.

**Nasif Khan** · 11-01-2016,07:29 PM

Thank you for your detailed reply.
Can that sensor you mention be at fault?

**martin1810** · 21-01-2016,02:23 PM

P401 egr insufficient flow usually means what it says and the egr valve isn't opening and closing properly. The normal thing to do is replace the egr with a new one.

**Nasif Khan** · 21-01-2016,02:52 PM

Replace the whole egr system or just the valve?

**Nasif Khan** · 01-02-2016,07:48 AM

Can a blockage in the silver pipe that leads up to the egr/throttle body cause this code to come up?

**Nasif Khan** · 06-03-2016,11:32 AM

Bump

**DMitch16** · 06-03-2016,02:03 PM

Just the EGR valve. Make sure all vacuum lines have been inspected / tested. It is possible that the EGR pipe itself is restricted but usually unlikely. A build up of soot coating the inside of the pipe is likely but not to the point that it becomes too narrow to allow sufficient flow. The smaller vacuum solenoid valve nearer the EGR cooler could be faulty however not allowing full travel when needed. If in doubt remove the pipe and use a flexible pipe brush to clean inside. That would be the cheapest option and one thing ruled out if the problem still persists. Then move onto the cooler flow solenoid valve, check and or replace. Finally if the problem is still there replace the EGR valve itself which will be the most expensive part at around £150 (if electric type) or £55 (if pneumatic vacuum type).

**Nasif Khan** · 02-05-2016,02:00 AM

Sorry to resurrect old post.
I've changed the egr valve with a brand new one....reset the MIL light and after 90 miles the MIL light came back on.

You have mentioned the solenoid valves...where csn I find these?

Not sure if this helps...The MIL light does go off itself every 80 to 100 miles.

**DMitch16** · 02-05-2016,11:00 AM

Read this from Ross-Tech but note near the bottom that some CR Tdis have an EGR filter fitted somewhere in the circuit which can become blocked.

16785/P0401/001025 - Ross-Tech Wiki

I have just worked on a friend's CBDC engine so I will take a look at his. Some non VW cars have had a similar issue with the Exhaust Gas Pressure sensor across the DPF being at fault (one guy even put in a needle valve between the high and low pressure sides to balance the pressure so the code went away - not sure that is the right way to solve the problem and whether long term damage would happen as a result). The sensor is easy to replace but the 2 pipes must be connected the right way around.

**Nasif Khan** · 02-05-2016,11:14 AM

Funny you mention the pressure sensor. I replaced 2 on the DPF. I believe from memory it was before and after the DPF or before and on the DPF. It's one or the other. However, this didn't cause the MIL light...The flashing glow plug came on.

I'll await a response once you've had a look. I'm able to do basic things but am no mechanic.