P0408

Universal

Exhaust Gas Recirculation Sensor B Circuit High

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The engine computer has detected a voltage or signal that is higher than expected in the EGR Sensor B circuit. EGR systems recirculate some exhaust gas to help control emissions, and the sensor provides feedback used to monitor the system. This code points to a circuit reading, not proof that the EGR valve or sensor needs replacement. Depending on the vehicle, the code may cause the check-engine light, emissions-test failure, or drivability changes; some vehicles may show no noticeable symptoms.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the engine stalls, runs very poorly, or loses power enough to make driving unsafe, stop in a safe place and arrange assistance. If the vehicle otherwise runs normally, seek diagnosis soon and follow any dashboard warnings. Avoid touching hot exhaust components.

Common Symptoms

  • The check-engine light may be on because the engine computer detected a high signal in the EGR Sensor B circuit.
  • There may be no noticeable change in how the vehicle runs, especially if the fault is intermittent or the control system compensates.
  • Rough running, hesitation, or reduced performance may occur if the faulty feedback interferes with EGR control; these symptoms can also have other causes.
  • The vehicle may fail an emissions inspection while the code or related emissions fault is present.

Common Causes

  • A damaged wire or short to voltage may make the Sensor B circuit signal read high.
  • A loose, corroded, contaminated, or damaged connector may interrupt the sensor circuit or produce an incorrect reading.
  • A fault in the sensor or its associated circuit may cause an abnormally high signal.
  • A power, ground, or shared-circuit problem may affect the signal; the correct circuit arrangement depends on the vehicle.
  • An EGR-system or mechanical issue could affect sensor feedback on some designs, but this code alone does not establish that the EGR valve has failed.

Diagnostic Steps

  1. Note when the check-engine light appeared and whether the engine runs differently; this information can help guide diagnosis.
  2. With the engine off and cool, visually inspect accessible EGR Sensor B wiring and connectors for damage, loose fit, corrosion, or contamination. Do not pull on wires or touch hot exhaust parts.
  3. If a scan tool is available, record this code and any related codes before clearing anything; related faults may help identify a shared wiring or system issue.
  4. Use the vehicle-specific service information to identify the correct sensor and safe inspection procedure. Do not guess at terminals or apply power to the circuit.
  5. If the connector or wiring appears damaged, or if electrical testing is needed, have a qualified technician test the circuit before replacing components.

When to Call a Professional

A technician can compare EGR Sensor B scan data with commanded EGR operation, inspect the connector and harness, and test the circuit using the vehicle-specific wiring diagram and service procedure. Checks should distinguish a sensor signal fault from wiring, power, ground, connector, or EGR-system issues before parts are replaced.

Compare component listings

Confirm diagnosis and vehicle fitment before buying. A listing does not prove that a part needs replacement. We may earn a commission from qualifying purchases.

Search Amazon Search eBay

Commonly Asked Questions

Does P0408 prove the EGR valve is bad?

No. The code identifies a high signal in the EGR Sensor B circuit. Wiring, connectors, the sensor, or another circuit or system issue may be responsible, so diagnosis should come before replacement.

What does “circuit high” mean here?

It means the engine computer detected a signal higher than expected in the Sensor B circuit. It does not, by itself, identify the cause or confirm that the sensor is faulty.

Can I drive with P0408?

If the vehicle runs normally, arrange diagnosis soon and monitor its behavior. Stop driving if it stalls, runs very poorly, or loses power enough to make the trip unsafe.

Can the code be caused by a wiring problem?

Yes. A damaged wire, short to voltage, or connector problem can produce a high circuit signal. A technician can check the wiring and sensor using the vehicle-specific diagram and procedure.

Browse Available Code Guides

Previous and next available guides in database record order, across makes. Adjacent entries may share a code number and do not necessarily describe related faults.