Subaru P049B EGR Fault – Diesel Diagnosis & Common Cause

As soon as someone brings up Subaru diesel, many mechanics immediately think, “Oh, that’s an easy fix—it's a DPF issue or a split intake pipe.” But I take a different approach. DPF faults become much easier to diagnose once you understand how the system you’re working on actually operates. For a broader understanding of how diesel fuel systems interact and how to structure diagnostics correctly, start here: Know the diesel fuel system you are diagnosing.

In late 2014, Subaru released the Euro V version of its diesel range. A significant change was made to the emissions system by introducing a second exhaust gas recirculation (EGR) system. Models fitted with this later Euro V design include the Outback and Forester fitted with the 110kW 2.0-litre EE20Z F4 16-valve DOHC intercooled turbo common-rail diesel (CRD), auto (CVT) or manual BS, B15 BSD 12/2014-12/2020.

Exhaust Gas Recirculation System

When we talk about EGR, we are referring to the complete EGR system, not just the valve. This can also include connecting pipes, sensors, intake manifolds and EGR coolers.

The secondary EGR system on a diesel is called the low-pressure (LP) EGR and is located after the DPF. This design is not a modern innovation - the US heavy-vehicle industry was using this technology in the early 2000s. It helps reduce nitrogen oxides (NOx) at minimal expense, compared to the more expensive AdBlue system. The US government’s issue was the cost of storing AdBlue; the key stakeholders came up with the LP EGR system as a cost-effective alternative.

The LP schematics from DieselNet in image 2, are easy to understand. One of the key features of Subaru’s LP EGR is a fine gauze fitted between the DPF and the EGR itself (image 3). This stainless steel gauze filter helps to reduce excessive carbon buildup around the EGR valve and EGR cooler. This is a key issue for diesels, especially if they are not regularly serviced, with the EGR cooler and intake removed at set intervals for cleaning off the vehicle.

Diagnosis and/or early detection of the fault

Unfortunately, this service does not occur very often. Subaru DPFs are often abused by repeated forced regenerations — see repeated forced DPF regeneration problems — meaning the gauze easily becomes blocked. This triggers a P049B – EGR B flow insufficient fault code, and as a result, the vehicle goes into derate status.

Using any version of on-vehicle intake cleaning is not the correct service method. Anyone with diesel knowledge and experience would understand that this on-car chemical process has no chance of getting close to the EGR. In some cases, it has created the opposite effect, blocking the LP EGR.

EGR B, as it is known on the Subaru, has lengths of steel tubing to direct the flow (image 4).

One section of this tubing has a fine-gauze filter. This section has been described to look like an exhaust catalytic converter. When diagnosing the P049B – EGR B flow insufficient fault code, we recommend checking this gauze first for any restriction.

To locate, follow the EGR bypass pipe from the passenger side across to the driver’s side. Here you will find the small cat-converter design with a two-bolt flange on either end (image 5).

Remove and inspect for blockage (image 6).

Solution

- Clean the section with a reputable cleaner. If it is a high-kilometre car, and the DPF and intake manifold conditions are unknown, it would pay to clean these too.
- The manifold absolute pressure (MAP) sensor has also been known to build up with excessive carbon and must be removed and inspected. If severe, do not clean the MAP sensor – replace with genuine only.
- The MAP sensor is in the bottom section of the two-piece intake manifold. Both upper and lower intake manifolds must be removed and thoroughly cleaned.

Special note

Our Diesel Help members have successfully cleaned this filter without a return fault. Well, that was until we experienced a repeat offender. The vehicle returned after a week of driving approximately 500km. The gauze had blocked very quickly and required removal and cleaning again, three times in two weeks.

On further investigation, this vehicle had a new DPF fitted by the local Subaru dealer. However, for some reason, they had opted for an aftermarket DPF over the preferred OEM unit. Having experienced this on a further occasion, we have concluded that the aftermarket DPF was not sufficient for this engine. We suspect, allowing excessive soot to pass through the DPF substrate filter.

This could also mean that the DPF was not performing the required task – reducing emissions. In one of these cases, the technician had opted to remove the gauze. This, of course, will inevitably lead to other issues down the track, such as a blocked EGR cooler, a clogged EGR, and an intake restriction. Eventually, it will require a more expensive repair and damage to the vehicle. If any aftermarket DPF manufacturers or suppliers are reading this article, I am open to further discussions on this matter. Diesel Help Australia does not recommend tampering with emissions.

This Subaru case shows how individual faults sit within the bigger picture of how the engine and emissions components work together. Our guide on understanding the diesel fuel system you are diagnosing explains how to approach these faults methodically and avoid misdiagnosis.

For more real-world diesel fault case studies, explore our diesel diagnostic resources.