The P0401 is a common error code for Volkswagen TDIs, especially the 2009-2014 2.0L Common-Rail Motors.

P0401 Definition and Symptoms

Cars with the P0401 error code will show a Check Engine Light. TDIs with this error code most often do not exhibit any other symptoms and the vehicle otherwise runs fine. The P0401 error code is described as “EGR System Insufficient Flow” in most diagnostic tools.

P0401 Technical Information

For the purposes of P0401 repair the 2009-2014 Volkswagen Jetta, Golf, Beetle, Sportwagen as well as the Audi A3 TDI all use the same engine and emission style.

It is important to understand that these vehicles use two separate EGR systems – a high-pressure and a low-pressure EGR. The high-pressure EGR recirculates hot exhaust gases as they exit the engine back into the intercooler system, which is to say after the turbocharger. The low-pressure EGR takes exhaust after it has been treated by the CAT and DPF and recycles it back into the intake, meaning before the turbocharger.

So, for the P0401 emissions code, the insufficient flow error refers to the low-pressure EGR system, not the high-pressure one. In plain English, the P0401 code means that not enough exhaust gases are flowing through the low-pressure EGR system.

P0401 Causes

While a faulty Exhaust Flap can cause this issue, the vast majority (and all the cases we have seen) for the P0401 error code is a clogged low pressure EGR filter.

Before you replace the low pressure EGR filter, however, you must first check to be sure that the clogging was not caused by a failed DPF – or diesel particulate filter.

For vehicles that have experienced clogging in the low-pressure EGR filter, most (if not all of them) also had a failed DPF. To get more specific, this issue stems from the DPF cracking and that causes the EGR filter to clog.

P0401 Repair

Fixing the P0401 emissions code will at minimum require a new low pressure EGR filter. However, if the DPF is found to be faulty as well then it must be replaced as well.

What if I ignore P0401?

Whenever a TDI model has an error code related to emissions, the vehicle will no longer perform active regenerations. While this will do long-term damage to the DPF, since the most common cause of the P0401 error code is a failed DPF, it can be argued that ignoring the problem won’t do immediate damage. However, other emissions and engine control systems will not be operating efficiently, which could lead to other problems in the future.

Summary

Because of the risk of damage to the DPF, this issue should be addressed sooner rather than later. You may be fortunate to only need a low-pressure EGR filter instead of a DPF. However, bear in mind that a clogged EGR filter could be caused by a failed DPF, so you may need to replace both. In the event that you can replace only the EGR filter, your mechanic may have to run a manual regen to clean up the accumulated soot and make the DPF light go out. In addition, a forced NOX regen may be necessary. While these take time, they don’t require hardware replacement, which is a good thing.