P0400

Detailed Overview

The P0400 code indicates an Exhaust Gas Recirculation (EGR) flow malfunction. This means the engine control module has detected insufficient exhaust gases being recirculated back into the intake system. In Audi vehicles, this is most commonly caused by carbon buildup clogging the EGR valve or passages, preventing proper airflow.

Common Causes & Symptoms

• • Carbon buildup clogging EGR valve or passages
• • Faulty or stuck EGR valve
• • Damaged or leaking vacuum lines
• • Faulty EGR solenoid or sensor

Typical symptoms include a check engine light, rough idle, hesitation during acceleration, reduced fuel economy, and possible engine knocking or pinging.

Step-by-Step Diagnosis

• 🔧 Inspect vacuum lines for leaks or damage
• 🔧 Remove and clean the EGR valve
• 🔧 Check EGR passages for carbon blockage
• 🔧 Test EGR valve or solenoid operation
• 🔧 Inspect for exhaust leaks affecting flow

How to Fix

• ✔ Clean or replace the EGR valve
• ✔ Clear carbon buildup from passages
• ✔ Replace damaged vacuum lines
• ✔ Replace faulty EGR solenoid or sensor

Cost Breakdown

DIY: $50 – $200
Independent Shop: $150 – $400
Dealership: $300 – $600

Frequently Asked Questions