I have a 1996 Eddie Bauer with a 5.8 that won't pass an emission check. The check engine light stays on. I'm getting codes 155 and 174. We've replaced all three o2 sensors, plugs, wires, cap, rotor, and filters. It keeps telling me bank 2 sensor 1. We switched sides with the front sensors and get the same reading. We checked the wires from the sensors to the main wire harness with a voltmeter and they read ok. We've checked for vacuum leaks with carburator cleaner and found none. The pcv valve and fuel pressure check out ok also. Truck runs well and idles fine, but still will not pass emission. Anybody have any ideas?? I appreciate any help.

 

Which brand O2 sensors did you purchase?

If no vac leaks, I'd perform a wiggl test....if codes still persist, I'd empty the codes and recheck wth the OBD tool.

What are the smog results? All of them...

idle o2, Co2, CO%, HC's NOX at speed o2, Co2, CO%, HC's and NOX

would rather use the propane method of check for exh leaks...

update:
155 is the O2sensor on bank 2...
174 is bank 2 running too lean (redundant code to tell you the O2sensor isn't working)

If you aren't using the motorcraft units....reconsider.

 

All of these sensors are linked together in some way to allow the emissions system to work properly. They work in unison sending info to the computer to calibrate the sending units (solonoids, pumps etc.) Did your vehicle ever throw a code for bank 2 running lean or too rich? If not then your sensor may not be the problem. That bank or side of your motor may not be getting enough fuel thus sending a higher air to fuel ratio out the exhaust on that side of your motor which could possibly be the cause of still getting a fault on the oxygen sensor. By chance did you get a 4-wire sensor??? These vehicles have very specific sensors and are determined by 2, 3 and 4-wire oxygen sensors. So check all your sensors that relate to the emissions. If all seems well use an obd 2 scanner and do an erase codes and then set it up to do a running diagnostic function set to only start recording data when a code is thrown so you can see exactly what is happening when the code is thrown. I would also just erase the codes and take the time to do a 20-30 minute scenic drive (after erasing the codes) This sometimes works. The computer needs to rework new data being sent from possible problem areas it has hard coded in order to fix them and needs to be driven to simulate actual conditions of engine load. I also will ask you another question...Is your exhaust hot or warm out the exhaust pipe?
174-running too lean. This may also be relative to your fuel rail on bank 2 check the vacuum on the rail and use a stethoscope to listen to the injectors as the engine runs. An injector(s) may be bad. I had this same problem with a 1996 ford econoline 150 with a 5.8 liter.

 

life would be so much easier without emissions

 

We put Bosch sensors in, which are the same part number as the ones we removed (Bosch were o.e.). We didn't get any smog details, they plug into the computer on the 1996. We have cleared the codes and run several diagnostics. It was suggested to us that it could be the mast air flow sensor had dirt in it, so we are going to check that also.

 

That is another good place to check. Be careful about that Mass air flow sensor though. With problems dealing with emissions control you can expect it to be anything from air in to exhaust. All of those sensors work in unison. Is your exhaust warm or hot? You could also check the air temp. sensor on the lower intake manifold. Get yourself a tube of dielectric grease too, use this on all of the electrical connections as you go through them.

 

Exhaust leaks will also cause O2 codes because they pull in extra oxygen wich appears to be a lean condition for the sensors.
Still kinda sounds like a lean misfire condition or what kind of spark plugs are you runnin.