Im working on a friends 98 olds intrigue...MIL on,has 2 codes missfire detected cyl.3 and system lean bank 1.......swaped plug wires around and miss moved so i replaced #3 coil now engine runs good no missing......cleared codes and drove.....MIL back on with same codes...now i cant feel miss any more (seems to be running good, 100% better than before)...any suggestions on missfire? and on the O2 sensor is bank 1 the up-stream and bank 2 the down-stream? there are only 2, 1 before cat and 1 after.....

Make sure the code is cleared, it may be history.

If the engine was run a long time with a misfire, the O2 sensor my be fowled up. But before swapping in a new sensor, be sure to reset the code. Unplug the negative battery cable for 20 minuets. That should clear anything on the PCM.

And yes, 1 is before the cat, and 2 is after.

Lastnite,replaced O2 senser (customers request)..disconnected batt. For 30 min.. Test drove, runs good no MIL..just now, got call light came on this morning, but car running good...going to ck with scanner this eve...

Check that code. Bet it'* a P0171

Has three codes P0171,P0303,P1404 & two pending codes P1404,P0171.... Runs good, alittle ruff @ idle but will clear up after 60 seconds..

P0171 is a real pain to track down. The P0303 is a misfire in cylinder 3. Start with that. Might be a bad plug or plug wire. Might be a bad coil but that generally effects 2 cylinders instead of just one.