An EGR sensor signal that climbs close to 5 volts can only mean an electrical fault. A P0408 code means the EGR sensor B signal is above its allowed range.
This guide explains what the sensor measures, what pushes its signal high and how to test it.
P0408 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 4 | Auxiliary emission controls | The fault involves emission control systems such as EGR, secondary air, EVAP or the catalytic converter. |
| 08 | Specific fault | Exhaust Gas Recirculation Sensor B Circuit High Input |
What the P0408 Code Means
The SAE J2012 standard defines the P0408 code as Exhaust Gas Recirculation Sensor B Circuit High Input. EGR sensor B is a second EGR sensor. Some engines, especially diesels, use two, such as a position sensor and a temperature or pressure sensor, or one sensor for each EGR valve. Check your service manual to see what sensor B is on your engine.
Most EGR sensors get a 5-volt reference and a ground from the powertrain control module (PCM) and send back a signal between about 0.5 and 4.5 volts. On a position sensor, the voltage changes as the valve opens. On a DPFE sensor, it rises as exhaust flows through the EGR pipe.
The PCM uses the reading to confirm that the EGR valve opens and flows as commanded. When the signal is out of range, it usually stops using EGR and sets a code.
High input means the signal voltage rose above the highest value a working EGR sensor B should ever send. That usually points to an open ground, a signal wire touching the reference or another voltage source, or a failed sensor.
Symptoms of a Bad EGR Sensor B
Many vehicles run nearly normally. Common signs of a P0408 code include:
- Check engine light on
- Slight pinging under load
- Rough idle on some vehicles
- Failed emissions test
- EGR flow codes set at the same time
Because the PCM often turns EGR off when it cannot trust the sensor, the engine may feel fine. The main risk is higher combustion temperature and a failed emissions test.
What Causes the P0408 Code?
High input faults almost always come from an electrical problem:
- Open sensor ground. Without a ground, the signal floats up toward the reference.
- Signal wire shorted to the 5-volt reference. Damaged insulation lets the wires touch.
- Signal wire shorted to battery voltage. Less common, but possible.
- Corroded connector. Resistance on the ground pin raises the signal.
- Failed EGR sensor B. An internal fault locks the output high.
- PCM fault. Rare.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool that can command the EGR valve
- Hand vacuum pump for vacuum-operated valves
- Digital multimeter
- Carbon cleaner and a small scraper or brush
- Wiring diagram for your engine
How to Track Down the P0408 Code
Find out what sensor B is on your engine. Then check it at the connector before you replace anything.
Check Live Data
With the key on and engine off, look at the EGR sensor B voltage. Then command the EGR valve, if your scan tool allows it, and watch whether the reading changes smoothly.
Test the Ground First
Back-probe the sensor ground with the key on and measure against the battery negative post. It should read close to 0 volts, ideally under 0.1 volt. A higher reading means a poor ground.
Check for a Short to Voltage
Unplug the EGR sensor B with the key on and watch the signal in live data. If it stays high with the sensor unplugged, the signal wire is shorted to the reference or another voltage source. If it changes, the sensor itself is likely at fault.
Check DPFE Hoses on Ford Vehicles
If sensor B is a DPFE sensor, check its two hoses for cracks and blockage, and make sure they are on the correct ports. A melted or plugged hose can push the reading out of range.
Repairs That Clear the P0408 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Repair the connector or wiring.
- Replace EGR sensor B if it fails tests. On many electric EGR valves, the position sensor is part of the valve.
- Replace cracked DPFE hoses on Ford vehicles.
- Repair a shorted reference shared with other sensors.
After the repair, clear the codes and check that the sensor reading changes smoothly when the EGR valve is commanded. Then drive at a steady cruise so the PCM can run its EGR flow test and confirm the fix.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Replacing the sensor without testing the ground first
- Overlooking a short to voltage inside a wiring bundle
- Replacing the EGR valve when only the sensor or wiring is bad
- Ignoring cracked DPFE hoses
Typical Repair Costs
The cost to fix a P0408 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| EGR passage cleaning | $10 to $30 | $150 to $400 |
| EGR valve | $60 to $300 | $250 to $600 |
| DPFE or EGR pressure sensor | $30 to $120 | $120 to $300 |
| EGR vacuum solenoid | $20 to $80 | $100 to $250 |
| Vacuum hose | $5 to $20 | $50 to $120 |
A DPFE sensor or separate position sensor is inexpensive. If the sensor is built into an electric EGR valve, the whole valve must be replaced, which costs more.
Is It Safe to Keep Driving with a P0408 Code?
Usually, yes. Most engines run normally, though EGR may be turned off. Avoid heavy loads if the engine pings.
Get it fixed before your next emissions test.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0407 | Exhaust Gas Recirculation Sensor B Circuit Low Input |
| P0401 | Exhaust Gas Recirculation Flow Insufficient Detected |
| P0403 | Exhaust Gas Recirculation Circuit Malfunction |
| P0404 | Exhaust Gas Recirculation Circuit Range/Performance |
| P0405 | Exhaust Gas Recirculation Sensor A Circuit Low |
Fixing EGR Sensor B
A P0408 code means EGR sensor B voltage is too high. Lost grounds, shorts to voltage and failed sensors cause most cases.
- Find out what sensor B is on your engine.
- Check reference, ground and signal.
- Inspect DPFE hoses if fitted.
- Replace the sensor only after the wiring tests good.
Frequently Asked Questions
Can a Bad Ground Cause the P0408 Code?
Yes. Without a good ground, the sensor signal floats up toward the 5-volt reference. Check the ground pin and wire before replacing the sensor.
What Is a DPFE Sensor?
DPFE stands for differential pressure feedback EGR. Ford and some other makers use it to measure EGR flow by comparing pressure on both sides of a small restriction in the EGR pipe. It connects to the pipe with two hoses.
Where Is the EGR Valve?
On most gasoline engines, it bolts to the intake manifold or throttle body area, with a pipe from the exhaust manifold. On diesels, it is often near the intake with an EGR cooler beside it. Your service manual shows the location.
How Long Until the Code Comes Back After a Repair?
The PCM usually runs its EGR test during steady cruising or deceleration, which may take a few drives. If the repair worked, the code should stay away and the readiness monitor should show complete. If the fault is still there, the code can return within a few days.
Do Diesels Have the Same EGR Problems?
Diesels often have worse EGR problems, because their exhaust carries more soot. Diesel EGR systems also use an EGR cooler, which can clog or leak coolant. Soot buildup in the intake is common on high-mileage diesels that do a lot of short trips.
Can a Bad EGR Sensor Affect Fuel Economy?
Slightly. When the PCM cannot trust the sensor, it may turn EGR off. Without EGR, combustion runs hotter and the PCM may retard timing to prevent knock, which can cost a little power and fuel economy.