Quick answer: The P0408 code indicates that the Powertrain Control Module has detected an abnormally high voltage signal coming from the Exhaust Gas Recirculation Sensor B circuit. This code affects the emissions control system and requires moderate attention. It will not leave you stranded immediately, but you should address it soon to avoid failing an emissions test.
Seeing a Check Engine Light pop up on your dashboard is never fun, especially when you plug in an OBD-II scanner and find a specific code staring back at you. If your scanner reads P0408, your vehicle is telling you there is an electrical issue within the Exhaust Gas Recirculation system. In this guide, we will break down what causes this fault, how to diagnose it safely, and what it will take to get your car running cleanly again.
What Does P0408 Mean?
The Exhaust Gas Recirculation system helps reduce nitrogen oxide emissions by routing a measured amount of spent exhaust gases back into the engine intake. To ensure this system works accurately, the Powertrain Control Module monitors the position of the EGR valve using a feedback sensor. The P0408 code specifically triggers when Sensor B-often a secondary sensor or a specific circuit designated as “B”-returns a voltage reading that is higher than the maximum threshold programmed by the manufacturer. This usually points to a short to power, a disconnected sensor, or an internal fault within the sensor itself.
Symptoms
- Check Engine Light illuminated on the dashboard
- Slight drop in fuel economy during highway driving
- Rough idle or engine hesitation when accelerating from a stop
- Engine failing a state emissions inspection or smog check
- Engine pinging or knocking under heavy acceleration due to incorrect combustion temperatures
Possible Causes
Before throwing expensive parts at your vehicle, take a breath. The most common mistake is replacing the most expensive part first, such as buying a brand-new EGR valve assembly when the real culprit is simply a loose wire.
- Corroded, damaged, or loose electrical connector at the EGR sensor
- Frayed or broken wiring in the harness leading to the EGR sensor
- Faulty EGR valve position sensor B
- Carbon buildup preventing the EGR valve from moving smoothly, altering the sensor reading
- Failing Powertrain Control Module in rare cases
Check These Yourself Before Visiting a Mechanic
- Locate your EGR valve and inspect the electrical plug to ensure it is firmly clicked into place and free of corrosion.
- Trace the visible wiring harness leading away from the EGR sensor to check for chafing, melting, or cuts against hot engine components.
How to Fix It
- Disconnect the negative battery cable, unplug the EGR sensor, and clean any dirt or oil buildup from the pins using electrical contact cleaner.
- Use a digital multimeter to test the reference voltage and signal return wires for continuity and shorts to power.
- Remove the EGR valve and check for excessive carbon deposits, cleaning or freeing up the pintle valve mechanism if it is stuck.
- Replace the EGR sensor or the entire EGR valve assembly if electrical testing confirms the sensor internal circuit has failed.
- Clear the trouble code with an OBD-II scanner and take the vehicle for a test drive to ensure the light stays off.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Wiring harness repair and connector cleaning | 10 – 30 | 75 – 150 |
| EGR valve cleaning and carbon removal | 15 – 40 | 100 – 200 |
| Replacement EGR valve position sensor | 50 – 150 | 100 – 200 |
| Complete EGR valve assembly replacement | 150 – 400 | 150 – 300 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0408 code?
Yes, you can usually drive your car short distances with this code. Your engine will still run, but it may have a slightly rough idle and will certainly fail an emissions test. It is best to schedule a repair within a reasonable timeframe.
How do I clear the P0408 code?
You can clear the code using an OBD-II scan tool after making the necessary repairs. Simply plugging in the tool and selecting the clear codes function will turn off the Check Engine Light, though it will return if the underlying fault is not fixed.
Will cleaning the EGR valve fix a P0408 code?
Cleaning the valve can help if carbon buildup is causing the mechanical pintle to stick, which indirectly throws off sensor readings. However, because P0408 is specifically a high voltage electrical circuit fault, you will often need to repair wiring or replace the sensor itself.