Failed Smog Test (Cylinder misfire & Oxygen sensor error codes) - Is it worth fixing?

Looking at misfired first,bad to find out problem,start with fuel filter,replace also the front O2 sensor.About $500 to start,clear codes,perform drive cycle and see if monitors complete,if not,fix what the computer stores as pending code (s).Good luck.

Replace both the pre-cat O2 sensors. They're around $80-$100 each. Get the ones with the BMW specific connectors already attached, not a universal sensor. Splicing the wires is a false economy, and can create problems down the road. Buy the sensors online from someone like bimmerparts.com or rmeuropean.com.

The rear sensors only monitor the performance of the front sensors and the cats. Typically they last at least twice as long because they're running in a much cleaner and cooler environment. Their function also does not affect the operation of the car.

You'll need a special O2 sensor socket to remove then easily. Autozone will actually lend them to you, or buy one for about $15.

After the sensors have been replaced, clear the codes. You can do that yourself, too, with a code reader from someone like Peake Research. It's available direct from peakereasearch.com and from multiple other online sources for around $150, which is about what you'll pay for labour to have a pro do it.

Drive the car, and see which codes, if any, reoccur. My guess is that the problems are in the O2 sensors, especially the one that's failed to ground, and that it's causing the other faults. It also makes sense to tackle them first to rule them out as the main cause. Finding the misfire, if it exists, will be much more difficult, so look for this second, only if the code repeats after O2 sensor replacement.

You can do this yourself, if you have a jack and jack-stands, for about $350, and have a code reader handy for the next time you want to read codes, or reset the service lights after an oil change.

O2 sensors can be expensive, around $100 each. Don't know how many you have (my Crown Vic has 4) but it sounds like all of them may need replacing. Bad O2 sensors will cause you to fail emissions and will hurt fuel economy and acceleration. Your misfires are likely old codes that are no longer a problem, because you would certainly feel it if all of your cylinders were misfiring. I'd say replace the O2 sensors, get your codes cleared, and keep your BMW. No sense in throwing out the baby with the bathwater.

Sometimes, 02 sensor and mixture faults can be caused by a faulty mass air flow meter. Especially since you have misfire recordings on all 6 cylinders. If you can find another car to borrow the MAF meter from, I'd clear the faults and try it. Although I do see 02 sensors go bad, usually they post a fault of "aging" not necessarily shorted etc. Also, if your mixture adaptations are "too rich" i'd definitely suspect a MAF, if they are too lean, then you likely have a vacuum leak. Under load you might not notice, but how does the car run at idle?

Try unplugging your mass air flow meter and see if it runs better. It wont run perfect, but the computer will substitute estimated values for the disconnected sensor. If it does seem to run better, I'd suspect the meter to be bad.

These cars do not have a distributor cap. They are direct coil ignition. As far as fluids go, factory BMW oil is basically re-badged Castrol Syntec 5w30. So that would be my recommendation for oil. For spark plugs I recommend only using original BMW parts for those. Idle air control valves do get dirty and stuck. Usually the symptom of that is an erratic idle (up and down constantly) Also cars tend to stall when the valve gets stuck closed.

E39 O2 Sensor

Fix the misfire code before doing anything else. You may want to start by checking the crankshaft position sensor.

A misfire will make an O2 sensor read lean.

NOW READ THIS!!! All these items are failing because your idle air control valve and throttle body are DIRTY!!!! Figure out what these things are and clean them to show room new!!! In 1999, i believe these things still had distributor caps. Remove it and examine the inside for signs of sparking, and center terminal presence. Replace if necessary with new Bosch units. Power wash the engine. There is nothing wrong with this engine, it is just neglected. De-sludge the crankcase with Butyl cello-solve, and use only ELF oil or Mobil One 0W-40 oil, for the rest of it's life. Change it when it gets dirty or thick. Remove the spark plugs, Make sure they are the right plugs and are GERMAN or JAPANESE in manufacture. Use ONLY "top tier" fuels. (read about it in internet). Examine ALL fuses. Make sure they all make contact. Examine all engine intake system rubber components for signs of wear. Replace anything that looks like it is decomposing. Good Luck!!