01 Drive Without a Thermostat? Here’s What You Need to Know • 02 You Could End Up Paying More For Insurance Just By Choosing… • 03 Why Your Car Cranks But Doesn't Start • 04 C0045 and C0046: Left Rear Wheel Speed Circuit Malfunction • 05 You Could Destroy Your Entire Engine If You Keep Repeating … • 06 What Every Driver Gets Wrong About This Tiny Part That Save… • 07 Is the Toyota Supra Saying Goodbye? Here's What We Know • 08 How to Start an Automotive Business With Almost No Capital • 09 Mastering the Art of Reading Engine DTC Codes without a Dia… • 10 New Engines Use Tiny Explosions Of Air Instead Of Spark Plu… •

DTC codes / Mercedes-Benz codes / P1404

P1404 · Mercedes-BenzEGR Control

P1404 on Mercedes-Benz vehicles indicates that the engine control module has detected a fault within the exhaust gas recirculation (EGR) control circuit, typically an electrical open, short, or performance deviation from the commanded EGR valve position. The computer monitors the EGR valve's electrical circuit and its effect on intake airflow, and it sets this code when the measured response does not match the expected value during a self-test.

SystemPowertrain
Code typeMercedes-Benz specific
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Rough idle or stumbling when the engine is cold or under light load
  • Slight hesitation or surge during acceleration
  • Reduced fuel economy over time
  • In rare cases, a noticeable decrease in engine power or a lean condition

Common causes, most likely first

  1. Carbon buildup or sludge on the EGR valve pintle causing it to stick or bind
  2. Failed EGR valve position sensor or internal electronics
  3. Damaged or corroded wiring harness connector at the EGR valve
  4. Open or shorted circuit in the EGR control wiring
  5. Faulty EGR vacuum modulator or solenoid (on vacuum-operated systems)
  6. Clogged EGR passageways in the intake manifold or valve body
  7. Engine control module software issue or, less commonly, a failed driver circuit

How to diagnose and fix it

  1. Connect a Mercedes-Benz compatible scan tool and record all stored and pending codes, then clear codes and test-drive to see if P1404 returns
  2. Visually inspect the EGR valve connector and wiring for corrosion, chafing, or loose pins, and repair as needed
  3. Measure resistance and voltage on the EGR valve control circuit according to the factory wiring diagram to rule out electrical faults
  4. Remove the EGR valve and inspect for carbon deposits; clean the valve and passages thoroughly if fouled
  5. Command the EGR valve open and closed with the scan tool while monitoring position feedback to verify smooth operation
  6. If the valve fails to respond correctly, replace the EGR valve assembly and clear codes
  7. After repair, perform an EGR adaptation or relearn procedure if required, then verify the code does not return during a road test

What Mercedes Techs Check Before Replacing the EGR Valve

  • A sticking EGR valve due to carbon buildup is the most common cause on Mercedes-Benz engines, and cleaning often resolves the code without parts replacement.
  • The wiring harness near the EGR valve is exposed to high heat and vibration, so inspect for brittle insulation and broken strands before condemning the valve.
  • On some models, a faulty mass airflow sensor or intake air leak can mimic EGR flow faults and set P1404 indirectly, so check fuel trims and airflow readings.
  • After replacing the EGR valve, some Mercedes-Benz models require a software update or adaptation reset to prevent the code from reappearing.
  • Do not overlook the EGR passage in the intake manifold; a clogged passage can cause the valve to appear faulty even when it is functioning correctly.

Can I keep driving with P1404?

You can generally continue driving with P1404, but avoid prolonged idling and heavy acceleration until the fault is repaired, as a stuck EGR valve can cause rough running and increased emissions. If the engine begins to misfire or lose power noticeably, have it inspected promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Mercedes-Benz vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage