P0401moderate Severity

Exhaust Gas Recirculation (EGR) Flow Insufficient

The ECM is detecting that not enough exhaust gas is being recirculated back into the intake manifold by the EGR system.

Common Symptoms

Check Engine Light illuminated
Increased combustion temperatures (knocking or pinging sound)
Failed emissions test for NOx

Generic Causes

Clogged or carbon-blocked EGR valve
Blocked EGR cooler or internal passages
Failed EGR vacuum solenoid or position sensor

Known Cars Affected by P0401

We have cross-referenced P0401 against our database of known faults. Below are specific cars where this code frequently appears, along with the detailed diagnostic and repair instructions for that specific vehicle.

EGR Valve Failure

Moderate

2005-201160,000–120,000 miles£80–£300

The EGR valve is a common failure on the 1.6 TDCi engine, causing power loss, rough idle, and excessive smoke.

DPF Blockage and Regeneration Failure

Moderate

2013-201730,000–80,000 miles£150–£1200

The 1.5 TDCi diesel Fiesta is commonly used for short urban trips — exactly the wrong usage for a DPF-equipped car. Blocked DPF is the number one complaint.

EGR Valve Failure and Carbon Build-Up

Moderate

2006-201450,000–100,000 miles£80–£280

The Fiat/GM 1.3 CDTi is a capable diesel but its EGR valve blocks with carbon deposits regularly. This is one of the most common Corsa D complaints.

EGR Cooler Failure and Soot Ingestion

Severe

2009-201360,000–120,000 miles£400–£800

The 1.6 TDI common-rail engine has a well-documented EGR cooler failure. Coolant can leak into the intake causing rough running or, in severe cases, catastrophic engine failure from hydro-lock.

EGR Cooler and Valve Failure

Moderate

2007-201270,000–130,000 miles£600–£1200

EGR cooler failure on the N47 is common from 70,000 miles. Coolant leaks into the intake manifold and the EGR valve sticks, causing power loss and rough running.

EGR Valve Failure and Anti-Shudder Valve

Moderate

2003-201370,000–140,000 miles£200–£350

The EGR valve and the adjacent anti-shudder valve (throttle flap) are prone to failing due to heavy carbon buildup and stripped internal plastic gears.

EGR Valve Failure and Intake Carbonisation

Moderate

2013-201740,000–90,000 miles£80–£350

EGR failure on the 1.5 TDCi Fiesta is a common companion to DPF issues. Carbon deposits block the valve and intake manifold, causing power loss and rough running.

EGR Valve and Cooler Failure

Severe

2011-201860,000–120,000 miles£400–£700

The EGR valve and cooler on the Mk3 Focus 1.6 TDCi are known to fail together. EGR cooler failure can cause coolant to enter the intake — a more serious failure than a simple blocked valve.

EGR Cooler Failure and Intake Contamination

Severe

2012-201570,000–130,000 miles£500–£800

EGR cooler failure on the Astra J 1.6 CDTi causes coolant loss and in severe cases, coolant enters the intake manifold. This is a more serious failure than a simple EGR valve block.

EGR Valve Failure

Moderate

2004-201050,000–100,000 miles£80–£350

The Isuzu-sourced Z17DTH and Z17DTJ engines in the Astra H have a well-documented EGR valve failure. Carbon deposits cause the valve to stick, resulting in power loss and increased emissions.

EGR Cooler Failure and Coolant Loss

Severe

2008-201570,000–140,000 miles£500–£900

EGR cooler failure is common on the B8 A4 TDI. Internal corrosion allows coolant to leak into the intake — initially causing rough running and white smoke.

EGR Cooler Failure

Severe

2008-201770,000–140,000 miles£600–£1000

EGR cooler failure on the Insignia 2.0 CDTi causes coolant to enter the intake manifold — a serious failure that requires immediate attention.

EGR Valve Carbon Blockage

Moderate

2009-201650,000–100,000 miles£100–£400

EGR failure on the Scenic Mk3 follows the K9K pattern — carbon accumulation causes power loss and rough running, compounded by the MPV's urban use profile.

EGR Valve and Cooler Failure

Moderate

2013-202140,000–90,000 miles£300–£700

EGR failure on the Transit Custom 2.0 TDCi is extremely common in fleet use. The valve carbons up rapidly in stop-start environments.

EGR Cooler Failure and Coolant Loss

Severe

2012-201960,000–130,000 miles£500–£800

EGR cooler failure on the Kuga Mk2 causes coolant to enter the intake. Left unchecked this results in rough running, white smoke and potential engine damage.

EGR Valve Failure

Moderate

2007-201460,000–120,000 miles£200–£450

EGR failure on the Mondeo Mk4 TDCi causes power loss and rough running. Carbon build-up on the valve is the primary cause, accelerated by urban use.

EGR Valve Failure

Moderate

2009-201440,000–80,000 miles£150–£350

EGR failure on the 1.2 TDI Polo is a regular occurrence. The small-displacement engine's EGR system clogs readily in urban use.

EGR Cooler Failure

Severe

2010-201470,000–140,000 miles£500–£900

EGR cooler failure on the Passat B7 2.0 TDI causes coolant loss and white smoke. The internal corrosion of the cooler allows coolant into the intake manifold.

EGR Valve and Cooler Failure

Severe

2007-201660,000–130,000 miles£500–£850

EGR failure on the Tiguan Mk1 2.0 TDI causes power loss and in more serious cases, coolant loss from the EGR cooler. Both components often need addressing together.

EGR Failure and Coolant Loss

Moderate

2007-201070,000–140,000 miles£700–£1200

EGR cooler failure on the E60 520d N47 causes internal coolant leaks into the intake manifold. Combined with EGR valve carbonising, both components typically need replacing together.

EGR and Timing Belt Failure

Critical

2008-201780,000–140,000 miles£500–£1500

The Q5 TDI shares the EGR cooler failure of the wider VAG TDI family, plus the timing belt must be replaced at 140,000 miles — a job many owners defer dangerously.

EGR Valve Failure

Moderate

2006-201350,000–100,000 miles£100–£400

EGR failure on the Note dCi produces power loss and rough running. The small engine is sensitive to any reduction in airflow caused by EGR carbon build-up.

EGR Valve Carbon Build-Up

Moderate

2012-201940,000–90,000 miles£100–£450

EGR failure on the 208 BlueHDi follows the established PSA pattern — carbon deposits restrict the valve, causing power loss and rough running.

EGR Valve Failure

Moderate

2007-201350,000–100,000 miles£100–£450

EGR failure on the 308 HDi causes the familiar symptoms of power loss and rough running. The PSA 1.6 HDi EGR system is well-known for carbon build-up.

EGR Valve Carbon Blockage

Moderate

2008-201650,000–100,000 miles£100–£400

EGR valve carbon build-up on the Megane Mk3 dCi restricts airflow and causes power loss. It is one of the most frequently reported faults by Megane diesel owners.

EGR Valve Carbon Blockage

Moderate

2005-201450,000–100,000 miles£100–£400

EGR failure on the Zafira 1.9 CDTi is common in MPV use. The valve carbons up rapidly with the stop-start urban driving profile typical of family vehicles.

EGR Valve Failure

Moderate

2007-201440,000–90,000 miles£80–£350

EGR valve carbon build-up on the 1.4 TDI Fabia restricts airflow and causes power loss. The small engine is particularly sensitive to EGR restriction.

EGR Valve and Swirl Flap Failure

Critical

2010-201660,000–130,000 miles£30–£4000

The N47-powered Countryman shares the EGR and swirl flap failure modes of the 3 Series. Both are serious issues that should be addressed preventatively.

EGR Valve Failure

Moderate

2004-200780,000–150,000 miles£150–£450

EGR failure on the Corolla Mk9 diesel causes power loss and rough running. While Toyota quality means the EGR lasts longer than European equivalents, it is still a wear item.

EGR Valve Failure

Moderate

2005-201850,000–100,000 miles£80–£350

EGR failure is the most common fault reported on the Punto Mk3 1.3 Multijet. The small diesel's EGR valve blocks with carbon rapidly in urban use.

EGR Valve and Cooler Failure

Severe

2014-201970,000–140,000 miles£600–£1100

The Ingenium 2.0 TD4 engine in the Discovery Sport develops EGR valve and cooler failures. The cooler can cause coolant to enter the intake on higher-mileage examples.