P0138Powertrain
Downstream O2 sensor circuit high voltage (bank 1)
Where to start
The sensor after the catalyst constantly reports a rich mixture. This comes either from a short in the wiring or from the sensor being fouled by long driving with misfires. First make sure there are no other codes — for fuel trim and for misfires.
What to watch in live data
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
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
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
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.