P0408 Exhaust Gas Recirculation Sensor B Circuit High

P0408 indicates a high input condition in Exhaust Gas Recirculation Sensor B circuit. The controller is seeing EGR sensor B feedback above the expected range.

Code details

Generic OBD-II · P0

System: Emissions
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the EGR valve until the sensor B signal, reference, ground, and mechanical movement have been tested.

Symptoms of P0408

Check-engine light, often with no obvious change in how the car drives.
A failed emissions test or incomplete readiness monitors.
Sometimes a faint fuel-vapour smell (on EVAP codes).

Common causes of P0408

A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
Exhaust leaks, fuel-trim problems, or a fault in a related monitor.

Useful checks before replacing parts

Compare EGR sensor B voltage, commanded EGR position, actual feedback, MAP or MAF response, freeze-frame load, and related EGR codes.
Inspect for short-to-voltage faults, open grounds, connector corrosion, stuck EGR movement, carbon buildup, and harness damage near hot exhaust parts.
Check whether the high signal remains with key on, changes during commanded EGR movement, or appears only under vibration or heat.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.