◆OBD Codes

P0400

Exhaust Gas Recirculation Flow Malfunction

Moderateegr

Overview

The P0400 code indicates the PCM has detected a exhaust gas recirculation flow malfunction fault in the exhaust gas recirculation (EGR) system. The EGR system reduces nitrogen oxide (NOx) emissions by recirculating a controlled amount of exhaust gas back into the intake manifold. An EGR malfunction can cause rough running, engine knocking, and increased emissions.

Symptoms

  • Check Engine Light (MIL) illuminated
  • Rough idle
  • Engine hesitation or stumbling
  • Engine knocking or pinging

Common Causes

  • Carbon buildup in EGR valve or passages
  • Failed EGR valve (stuck open or closed)
  • Faulty EGR position sensor
  • Clogged EGR passages or tubes

Diagnosis Steps

  1. 1Connect an OBD-II scanner and read all stored trouble codes and freeze frame data
  2. 2Inspect the EGR valve for carbon buildup; clean or replace as needed
  3. 3Test the EGR valve operation by applying vacuum or using the scan tool
  4. 4Check EGR passages for carbon blockage
  5. 5Test the EGR position sensor and wiring with a multimeter

Estimated Repair Cost

$150 — $600

Cost varies by vehicle, location, and repair shop. Includes parts and labor.

Commonly Affected Vehicles

Nissan FrontierFord 6.0L/6.7L DieselRam 1500BMW Diesel models

Related Codes

P0402P0406P0401

More Egr Codes

P0401Exhaust Gas Recirculation Flow Insufficient Detected
Moderate
P0402Exhaust Gas Recirculation Flow Excessive Detected
Moderate
P0403Exhaust Gas Recirculation Circuit Malfunction
Moderate
P0404Exhaust Gas Recirculation Circuit Range/Performance
Moderate
P0405Exhaust Gas Recirculation Sensor "A" Circuit Low
Moderate
P0406Exhaust Gas Recirculation Sensor "A" Circuit High
Moderate

Frequently Asked Questions

Yes, EGR valves clogged with carbon can often be cleaned using throttle body or EGR cleaner. This is a cost-effective first step before considering replacement.