221 = mixture too lean at part throttle
check for an air leak, low fuel pressure, faulty sensors

231 = mixture too lean or too rich at part throttle and the
ECT adaptive adjustment limit has been exceeded

Check for a vacuum leak first. This takes some patience.
I traced my 231 to a flame trap hose that had fallen off the
oil breather box under the intake manifold.

The manual also suggests checking the exhaust system for leaks ahead
of the O2 sensor. It points to the union between the downpipe and
the exhaust manifold.

The final check is the Air Mass Meter. Start by cleaning the
connector. With the ignition off, check to verify ground at
terminal 1. With the engine running and the AMM connected verify
~2.3 volts at terminal #3. Make sure the multi-meter is high impedance
or you risk harming the ECT.

The last check is for fuel pressure and residual pressure.

After you find the problem and correct it, you'll need to reset the adaptive trim. It's reset by pulling the fuse to the ECT.

Good luck