BMW E38 fuel trim fault and high emission readings
What do the fuel trim codes P0170 and P0173 mean in the E38?
The codes tell you that the engine management cannot correct the mixture ratio. The cause is typically a broken air flow meter or an intake leak, for example at the front of the intake manifold or a cracked rubber hose. After the repair the learned values can be reset, so the mixture settles faster.
Why are the oxygen and lambda values too high in the emissions test?
An exhaust leak lets air into the exhaust gas and raises the meter's lambda and oxygen readings. The leak can be, for example, a crack in the muffler, and it is found with a smoke test. An intake leak can also distort the values. The meter's lambda is calculated and does not tell the condition of the lambda sensors.
What can cause too high a CO reading?
High CO means too rich a mixture, so the catalytic converter does not work properly. Causes include the lambda sensors, an exhaust leak before the sensors, a stuck fuel pressure regulator and bad ignition coils. A short circuit to ground or to 12 volts in the sensor wires upsets the fuel trim.