Technical Description of OBD-II Trouble Code
The circuit for the Exhaust Gas Recirculation Sensor B is registering a high reading.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
The purpose of the EGR (Exhaust Gas Recirculation) system is to lower combustion temperatures under certain conditions, which helps reduce emissions of oxides of Nitrogen (NOx). It achieves this by recirculating inert exhaust gases back into the cylinder to be burned with the air/fuel mixture. Inert exhaust gas burns slower and reduces combustion temperatures. The EGR valve is controlled by the computer and opens to allow the flow of exhaust gas into the cylinders.
The EGR valve may be operated by vacuum. In such cases, a PCM (Powertrain Control Module) controlled EGR vacuum solenoid is used to deliver vacuum to the EGR valve. When activated by the PCM, the EGR solenoid opens, allowing engine vacuum to reach the EGR valve. This engine vacuum opens the valve, allowing exhaust gases to pass through and enter the combustion chamber. Other EGR valves are electronic and directly controlled by the PCM. The PCM activates solenoids that are integrated into the EGR valve. These solenoids open for varying durations, allowing the necessary amount of exhaust to pass through. The EGR system is continuously monitored for faults. Most EGR valves have a feedback sensor that informs the computer of the actual EGR position. This sensor typically provides a voltage reading between .4 and 5 volts.
If the EGR position sensor maintains an abnormally high reading for an extended period of time, this code may be triggered.
Note: This code is essentially the same as P0406. The difference is that the P0406 DTC refers to the “A” circuit, while P0408 refers to the EGR “B” circuit. Consult a vehicle-specific repair manual to locate the “A” and “B” circuits in your vehicle.
This code is a generic powertrain code that applies to all makes and models of vehicles (1996-newer). However, the specific repair steps may vary slightly depending on the model.
Symptoms
Symptoms of a P0408 DTC may include:
Causes
Potential causes of a P0408 code include:
Possible Solutions
If the vehicle starts and stalls or fails to run with this code present, try unplugging the EGR valve and then restarting the engine. If this does not make a difference, remove the EGR valve and inspect it for debris. Clean the valve and reinstall it. If the problem persists, block off the EGR port and attempt to start the engine. If the engine starts in this case, it indicates that the EGR valve is stuck open and needs to be replaced. If unplugging the PCM controlled EGR valve allowed the engine to restart, there is likely a wiring issue, possibly a short circuit that is keeping a solenoid open.
Using a scan tool, check the EGR position on the datastream with the Key on Engine running and compare it with the desired EGR position. If the reading is normal, suspect an intermittent problem. If it shows 5 volts or higher, check the EGR sensor signal circuit for a short circuit to the 5 volt reference wire or to B+. Repair as necessary. Also ensure that there is a good ground path on the ground circuit. Repair any open circuits or short circuits in the ground circuit.
For a vacuum controlled EGR valve with EGR solenoid: if unplugging the EGR valve vacuum source allows the engine to start, suspect a faulty EGR solenoid that is allowing vacuum to reach the EGR valve constantly. Replace the EGR solenoid as needed. If this code is present and the engine starts and runs normally, suspect an open circuit in the wiring. Inspect and repair any wiring issues. If the wiring checks out, replace the EGR valve. If there are issues with the wiring to and from the EGR valve
Related EGR codes: P0400, P0401, P0402, P0403, P0404, P0405, P0406, P0407, P0409
P0408 Diagnosis Video
Note: We are not affiliated with the producers of the following video. We are including it here as an additional troubleshooting tool, it may or may not apply to your vehicle issue.
