P0408: the EGR system's second position sensor is reading too high
EGR feeds a small amount of exhaust back into the engine to lower the temperature inside the cylinder and cut emissions, and some EGR systems carry a second position sensor, called sensor B, alongside the main one. This code means the electrical signal from that second sensor is reading higher than the range the computer will accept. With this signal out of range, the engine control unit can't fully trust what the valve is actually doing.
You might notice
- The engine warning light comes on
- The engine may idle unevenly or hesitate
- EGR
- — a system that feeds a little of the exhaust back into the engine to lower the temperature inside the cylinder and cut emissions
- sensor B
- — a second EGR position sensor some systems carry alongside the main one
What the code itself says
P0408 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
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