P0408 — Exhaust Gas Recirculation Sensor B Circuit High

P0408 — Exhaust Gas Recirculation Sensor B Circuit High. A fault in the exhaust gas recirculation (EGR) system. The car is usually drivable while you plan the repair. Typical causes: faulty EGR solenoid or wiring, carbon buildup blocking the EGR valve or passages, ECM control circuit fault.

powertrain (engine/transmission)

Is it safe to drive?

Caution — diagnose promptly P0408 can cause the controller to reduce or disable EGR, increasing emissions and potentially exhaust temperature. It is not normally an immediate stop condition, but driving should be limited if the vehicle also has severe misfire, overheating, or unstable idle.

Typical symptoms

The check-engine light illuminates with P0408 stored in the powertrain controller
Rough idle, hesitation, or a mild stumble may occur when EGR operation is reduced or disabled
Fuel economy may decrease because the controller alters EGR flow
Exhaust-gas temperature may rise if EGR is not functioning correctly
An emissions test may fail even though the engine otherwise appears to run normally

Common causes

A faulty EGR sensor B with an internally high or erratic output signal
A short in the EGR sensor B signal circuit to 5 volts or another electrical source
Corrosion, water intrusion, or a loose terminal in the EGR sensor B connector
An open or high-resistance connection at the sensor ground or shared reference circuit
Incorrect wiring, pin damage, or an improperly installed replacement sensor
A less-common PCM reference-voltage or internal circuit fault affecting the EGR sensor B input

How to diagnose with an OBD scanner

Confirm P0408 and capture the operating data Connect an OBD scanner such as ELM327 with AIScanAuto, read stored and pending codes, and save freeze-frame data before clearing anything. If supported, monitor EGR sensor B and commanded EGR position while the fault is active. Confirm P0408 rather than assuming an EGR-related symptom has the same cause.
Inspect the sensor, connector, and protected wiring Locate EGR sensor B using the vehicle wiring diagram because its bank or pipe location can differ. Inspect the connector for green corrosion, backed-out pins, water, heat damage, and oil contamination. Carefully examine the signal and ground wires for chafing, pinholes, crushed sections, or a short to a power circuit.
Measure the circuit against manufacturer specifications With the ignition on and the engine off, back-probe the EGR sensor B connector and measure reference voltage, ground voltage, and signal voltage. Compare each reading with the wiring diagram and sensor specification. A signal held near supply voltage points toward a short, bad ground, or failed sensor rather than a simple EGR valve restriction.
Isolate the sensor from the wiring harness Disconnect EGR sensor B with the ignition off and check the signal voltage at the harness connector. If an abnormally high reading remains, investigate a harness short, connector problem, or controller reference circuit. If the harness reading returns to the specified range while the code identifies a high signal from the connected sensor, the sensor is the stronger suspect.
Repair, clear P0408, and verify the result Repair the identified wire or terminal, replace the sensor with the correct application-specific part, or address a confirmed controller circuit fault. Do not replace the EGR valve solely because P0408 is present. Clear the code, complete the required drive cycle, and confirm that P0408 does not return and that EGR sensor B data remains plausible.

Approximate repair cost

A typical independent-shop estimate for diagnosing and repairing P0408 is approximately $180–$700 USD, depending on whether the fault is the EGR sensor B, a connector or wiring defect, or related EGR service. Figures are estimates and vary by region, vehicle, and local labor rates.

Common questions

Does P0408 mean the EGR valve is definitely clogged?

No. P0408 identifies a high signal from EGR sensor B, so the valve may be unaffected. A carbon-restricted EGR valve can coexist with the code, but the electrical fault must be verified first.

Can the vehicle be driven with P0408?

Usually for a short distance, but the fault should be diagnosed promptly. Avoid extended driving if the engine is misfiring, running unusually rough, producing excessive heat, or approaching overheating.

Will cleaning the EGR valve clear P0408?

Cleaning may correct a separate flow restriction, but it will not normally repair a high sensor signal. Confirm the wiring, connector, reference voltage, and sensor before spending money on valve cleaning.

What should an OBD scanner show for this code?

The scanner should confirm P0408 and may provide freeze-frame or EGR sensor B data. A high live signal with wiring intact supports a sensor fault, while an abnormal harness reading points to wiring or grounding.

When your OBD2 scanner pulls code P0408, the engine control unit is reporting that the exhaust gas recirculation sensor B circuit is reading a voltage that exceeds the normal operating range. The official definition reads "Exhaust Gas Recirculation Sensor B Circuit High." This falls under the powertrain category, meaning it directly affects engine operation or emissions control.

A "circuit high" condition indicates that the signal wire is carrying voltage closer to battery level when it should be at a lower reference value. In the EGR system, this typically means one of three things: the signal wire is shorted to a power source, the ground return path has opened up, or the sensor itself has failed in a way that pulls the circuit high. The engine control module constantly watches this circuit while the vehicle is running. When the abnormal voltage persists across enough consecutive drive cycles to meet the failure threshold, P0408 gets stored and the check engine light illuminates. The freeze frame data captured alongside the code is valuable—it shows you the exact RPM, engine load, coolant temperature, and vehicle speed when the fault first appeared, giving you real-world conditions to reproduce during testing.

The exhaust gas recirculation system reduces nitrogen oxide emissions by routing a measured amount of exhaust gas back into the intake manifold. This lowers combustion temperatures. Sensor B monitors the EGR valve's position or flow, depending on the design, and reports back to the ECU. When that signal goes high unexpectedly, the control unit knows something is wrong and sets P0408 to flag the problem for diagnosis.

Common Symptoms of P0408

The check engine light stays illuminated on the dashboard
Engine power feels slightly reduced during acceleration
You may notice pinging or knocking sounds under load
Idle quality suffers—engine may run rough or stall at idle
NOx emissions increase, which can cause failed emissions testing
Some vehicles experience hesitation or stumble when pressing the accelerator

What Causes P0408?

The signal wire for the EGR sensor B circuit is shorted directly to battery positive voltage
The ground return circuit for the sensor has broken or corroded, leaving the signal floating high
The EGR sensor B itself has failed and is outputting an abnormally high signal voltage
Wiring harness damage—chafing, melted insulation, or corroded connectors in the EGR circuit
Carbon deposits blocking the EGR valve pintle or intake passages, forcing the system to work harder
Vacuum leaks in the EGR control lines or diaphragm, disrupting proper operation
The engine control module has an internal circuit fault affecting the EGR signal input
The EGR valve position sensor or integrated solenoid has worn out or failed electrically

How to Diagnose P0408

Connect your OBD2 scanner and confirm P0408 is stored in the ECU. Save the freeze frame data immediately—it tells you exactly what the engine was doing when the fault triggered, which is the condition you need to replicate during your road test after repairs.
Look for any other stored codes. P0408 often appears alongside other EGR-related codes or upstream sensor faults. Document everything before clearing. Also search for technical service bulletins specific to your vehicle make and model—manufacturers sometimes publish known issues with software updates or updated part numbers.
Perform a thorough visual inspection of the entire EGR wiring harness. Check the connector to the sensor B, look for damaged wire insulation, corroded pins, and make sure the harness is secured away from heat sources like the exhaust manifold.
Disconnect the EGR sensor B connector and probe the signal wire at the harness side with a multimeter. With the key on and engine off, you should see near-zero volts if the circuit is healthy. A reading near battery voltage confirms a short to power in the wiring. If the reading drops to normal with the sensor unplugged, the sensor itself is the culprit.
Use your scanner to command the EGR valve open and closed while monitoring the position sensor feedback. The valve should respond smoothly. Watch the idle quality—stalling or extreme roughness when commanding the valve open points to a mechanical or electrical problem.
Remove the EGR valve and physically inspect it. Look for carbon buildup on the valve seat and in the intake passages. Blockage here can cause the sensor to read outside normal parameters.
Clear the code and drive the vehicle through a complete drive cycle, including conditions matching the freeze frame data. If P0408 returns, the underlying issue has not been resolved.

Repair Options and Costs

Address the most likely causes first to avoid unnecessary spending:

EGR valve replacement: $250–$650 (parts and labor at independent shop rates)
EGR valve and passages cleaning: $120–$350
Vacuum line repair or wiring harness fix: $80–$250

Total Cost to Fix P0408

Depending on what actually failed, your total bill will vary. Replacing the EGR valve outright is the most expensive scenario, typically running $250–$650 when done at an independent shop. Cleaning the valve and passages costs considerably less at $120–$350. If the problem turns out to be a simple wiring issue or leaking vacuum hose, expect to pay between $80–$250.

Most shops will charge one hour of labor for diagnosis on top of any repair work. Spending time with a multimeter and scanner before buying parts is worth it—jumping straight to an EGR valve replacement when the issue is a corroded connector wastes money. The good news is that the most common root causes are also the cheapest to fix.

Can You Drive with P0408?

For short distances, yes. P0408 is classified as a short-term driving safety concern. You may experience rough idle, pinging under acceleration, and elevated exhaust emissions. Some vehicles will enter a reduced-power mode if the ECU detects a severe EGR malfunction. If the engine stalls repeatedly or runs extremely rough, treat it as urgent and get it diagnosed before your next long trip.