P0400: the exhaust recirculation system has a flow fault
EGR feeds a small amount of exhaust back into the engine to lower the temperature inside the cylinder and cut emissions. This code means the engine control unit has detected the flow isn't behaving the way it expects, without narrowing it to specifically too much or too little. It sits alongside the more specific insufficient-flow and excessive-flow codes as the general EGR flow fault.
You might notice
- The engine warning light comes on
- You may notice a faint pinging sound from the engine under load, or slightly uneven idling
- EGR
- — a system that feeds a little of the exhaust back into the engine to lower the temperature inside the cylinder and cut emissions
What the code itself says
P0400 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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