My mother has a 2000 Ford expedition the Radiator blew 3-4 months ago and the CEL came on. I put my scanner on it checked it and it showed a P0171 and a P0174 (bank 1&2 lean) so i cleared it and told her to let me know if it came back well the other day she came to me and said it was on (and had been since the day after I had cleared it) well i swung by a buddys shop and he put his big High dollar computer on it and said that looked like Bank 2 down stream O2 was bad, So i changed that Yesterday drove it a few miles nothing came on so i dropped her car off to her and she drove it to work and bout halfway home (aprox 40-50 miles) and it poped a code so i swung by this afternoon and pulled the code excpecting to find P0171 again but instead it was P0174..... Any Ideas why changing bank 2 O2 would clear up the Bank 1 code but not Bank 2? and any ideas what else could be causing the code? Im not off again tell next monday so She is gonna take it to my buddy's shop prolly Friday and see what he can pull then.

And Yes i changed the Driver side down stream O2.

 

Did the radiator spew antifreeze toward the air cleaner? Might want to check air filter and clean MAF sensor. Another possible is if the air intake tube was removed for rad repair, recheck all hose connections and clamps from air filter to throttle body. Any air intrusion from MAF to throttle body may create lean codes.

 

Passengers side of radiator is were it busted. intake has been pulled twice since then an put back cafefully. it just seems weird to me bank 2 o2 fixed bank 1...

 

The down stream 02 sensor(2) monitors the condition of the catalytic convertor
it has no affect on air/fuel management
As Lime1gt states you have air entering the intake that is not being measured by the
mass air flow sensor
Check for vacuum leaks, hoses ect.
I suspect the po171 being fixed was just a coincidence and will return soon

Good luck and keep us posted

 

Might I comment as to why with good equipment the live data is not looked at and considered beyond just the codes?
When the codes are cleared the PCM does all the diagnostics over from scratch.
The same test conditions likely are not the same each time for air temp, baro pressues and humidity.
The bank that passed likely just barely did so inside the limit but is still out of the norm.
You can only see this by looking at the short term and longer term fuel trim values
This is why one needs to go beyond just working with codes to solve some issues that keep coming back.
In this case the fuel trim is out of limits because the OX sensor detected to much OX in the exhaust and shifts the fuel table 'richer' trying to compensate. This is what causes the code "indirectly" because the system has no other way to know what is happening.
As you see, it's pretty good as designed.
Good luck.

 

The live data was looked at. Everything else looked good but the one o2 sensor that was changed it had 2 of the 4 wires were rubbed all but into. now the lights off an has been for around 50 miles. I did not use a scanner to clear them or anything so guess we will see what's going on. I've looked for vacuum leaks with no sucess.