Code 171 and 174 are for bank 1 and 2 running too lean. The engine has compensated as rich as it can go, and it is still to lean. This does not necessarily mean your mixture is bad, but something (a sensor) is telling the ECM to correct lean. The oxygen sensors alert the ECM to how lean/rich the exhaust is, and then the ECM trips a code. Maybe you have a bad oxy. sensor, but since there are 4 of them (2 per bank), I don't think they could all go at once.