P0408 OBD-II Trouble Code: Exhaust Gas Recirculation Sensor B Circuit High

Our certified mechanics come to you · 12-month, 12,000-mile warranty

SCHEDULE P0408 DIAGNOSTIC Get a fair and transparent estimate upfront
Red-stars EXCELLENT RATING ON

Cost of diagnosing the P0408 code

P0408 is a diagnostic trouble code (DTC) for "Exhaust Gas Recirculation Sensor B Circuit High". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

Cars Estimate Credit towards follow-up repair Earliest Availability
Ford$20.00
Toyota$20.00
BMW$20.00
SCHEDULE P0408 DIAGNOSTIC Get a fair and transparent estimate upfront

P0408 code definition

The P0408 code is stored when the PCM detects an issue with the EGR (exhaust gas recirculation) system. After one or more (depending on the vehicle manufacturer’s specifications) instances of this, the code will be stored, and the Check Engine Light will illuminate.

What the P0408 code means

When the EGR system is working properly, it will recirculate inert exhaust emissions back into the engine cylinder. These gasses burn at a lower temperature, thereby lowering the temperature of combustion for the engine fuel, which reduces the emission of nitrogen oxides. When the EGR system is activated, the PCM should detect a certain degree of manifold air pressure change.

If it detects a change that is outside the acceptable range, it will record the P0408 code. Some manufacturers’ vehicles will record the code after one instance, while others will record it after multiple instances.

What causes the P0408 code?

A few things could be occurring to cause a P0408 code to be stored. These include:

  • A damaged or defective EGR temperature sensor
  • A fault in the EGR temperature sensor’s circuit
  • A shorted or open connection in the EGR volume control solenoid valve circuit
  • Excessive carbon buildup on the EGR’s valve
  • A damaged or defective EGR volume control solenoid valve
  • An open ground circuit for the EGR sensor
  • An internal failure in the solenoid or EGR sensor

What are the symptoms of the P0408 code?

When the P0408 code is stored, the Check Engine Light will always illuminate. Other than this symptom, there may be no discernible change in the performance of the vehicle. However, some other symptoms that the driver may notice include:

  • Engine surging during operation
  • Intermittent engine stalls

There will likely be an increase in nitrogen oxide emissions, due to the increased combustion temperature, but this will usually be undetectable to the driver or owner of the vehicle until an emissions test is performed.

How does a mechanic diagnose the P0408 code?

The first step that any mechanic will take to diagnose a P0408 code is to use an OBD-II scanner to identify it and any other codes stored by the PCM. Then, if the vehicle stalls after starting and/or will not run consistently, they will generally unplug the EGR valve before clearing the code and then restarting the vehicle.

If the code appears again, they’ll then remove the EGR valve and clean out any built-up debris before reinstalling it and starting the vehicle again. If the problem still isn’t solved, they will block the EGR’s port and try to crank the engine. If this seems to solve the problem, then the cause of the code is most likely a faulty EGR valve, which is stuck in the open position. Replacing it should resolve the code and allow the engine to start consistently.

If the EGR valve is vacuum controlled, and unplugging it works to allow the vehicle to allow the vehicle to start and run, the problem is likely with the EGR solenoid. If this is not the case, the mechanic will continue taking steps to diagnose the problem, including visual inspections of all wiring, circuits, and connections. After each test and repair is done, the mechanic will clear the code and retest it to ensure that it does not appear again.

Common mistakes when diagnosing the P0408 code

The most commonly reported misdiagnosis of a P0408 code is immediately assuming that the EGR valve is the problem. Replacing this without inspecting any other potential problems will result in spending unnecessary time and money without actually addressing the problem causing the P0408 code to be recorded.

What repairs can fix the P0408 code?

Some common fixes for a P0408 code include:

Need help with a P0408 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6220.

trouble codes
P0408
Check Engine Light

Fast and easy Check Engine Light is on Inspection service at your home or office.

SCHEDULE P0408 DIAGNOSTIC
Get a fair and transparent estimate upfront

Recent Check Engine Light is on Inspection reviews

Excellent Rating


YourMechanic Check Engine Light is on Inspection Service

Average Rating

4.8/5

Number of Reviews

4,810

Rating Summary
4,424
187
47
28
124
4,424
187
47
28
124

Joel

19 years of experience
741 reviews
Joel
19 years of experience
Acura RL - Check Engine Light is on - Jacksonville, Florida
very knowledgeable and precise. able to fix electrical issues in my car. Right on time for the appointment.

Christopher

15 years of experience
41 reviews
Christopher
15 years of experience
Honda Accord - Check Engine Light is on - Fairburn, Georgia
Christopher was excellent! He explained everything in detail to make sure I understood. He was professional and provided me with the next course of actions to resolve my care issues. I'm glad he was sent to service my car.

Alexander

18 years of experience
58 reviews
Alexander
18 years of experience
Saab 9-7x - Check Engine Light is on - Kissimmee, Florida
He was very friendly and very helpful . I felt like it was being around a close friend , also gave me so very good advice on my car & future advice about it . Highly recommend mechanic, would use his services again .

Matthew

31 years of experience
639 reviews
Matthew
31 years of experience
Jeep Patriot - Check Engine Light is on - Norfolk, Virginia
Matthew was on time which was a relief, he is very knowledgeable and was easy to understand what was wrong and what was being done. I am recommending to friends and family due to the great experience Matthew gave me. Extremely professional 10/10

More related articles

P0505 OBD-II Trouble Code: Idle Control System Malfunction
P0505 code definition Idle Control System Malfunction What the P0505 code means P0505 is an OBD-II generic code that the engine control...
P0720 OBD-II Trouble Code: Output Speed Sensor Circuit Malfunction
P0720 means there is an issue with the output speed sensor circuit. Issues may include engine stalling, misfiring and hesitation when accelerating.
P0103 OBD-II Trouble Code: Mass Airflow (MAF) Circuit High Air Flow and High Voltage Output
P0103 Trouble code definition Mass Airflow (MAF) Circuit High Air Flow and High Voltage Output The Code P0103 code is often...

Related questions

Shuddering and check engine lights blink so

This may be a sign of a dirty or failing mass air flow sensor. The mass air flow sensor is a unit in the intake system that monitors air intake as it is mixed with fuel prior to being injected...

Multiple trouble codes, no spark, no start.

You are getting codes to each and every major sensor on the engine that is needed to run your motor. You either have a major malfunction with the ECU or a problem with the harness not being wired correctly. You...

My vehicle is shaking when I drive past 50 mph

Hi there. From the description you have given, the drivability problem your vehicle has may be a fuel delivery problem. This would account for your vehicle not wanting to start that one day. Next day, you were entering the highway...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1-800-701-6230 · hi@yourmechanic.com