P2178Powertrain
System too rich off idle (bank 1)
Where to start
A rich mixture under load. The opposite of P2177: too much fuel. Look at rail pressure, the return line and the air flow sensor — an over-reading meter makes the unit add fuel it does not need.
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
We have a detailed piece on thisLean and rich mixture: P2177, P2187, P2195 vs P0171
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code, second group
Which part of the engineFuel and air metering
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Mixture: lean and rich»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.