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1992 Ford Explorer XLT engine code 172 - HEGO sensor lean

Canadia72

New Member
Joined
April 9, 2021
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City, State
New York, New York
Year, Model & Trim Level
1992 Ford Explorer XLT
I bought a used 1992 Ford Explorer XLT a few months back which had spent a while out of regular use. Initially it had trouble starting and I needed to replace various components to get it into decent shape. The fuel pump, fuel filter, spark plugs, fuel injectors, and air filter have all been replaced in the last two months. The fuel pump and fuel filter in particular were in awful condition and had clearly not been maintained in a vary long time. The injectors were also quite filthy.

After about 20 minutes of driving, generally only when driving at speed (giving it a decent amount of gas, and traveling over ~35mph or going uphill) the engine light comes on and stays on for the rest of the trip. To check that this wasn't simply a computer issue, and because I had another computer handy anyway, I initially tried simply swapping the computer for a 'fresh' one with no codes. However, after a half hour on the road the engine light was back on again under the same driving conditions.

In the end, I tried a code reader and the computer gave me no KOEO codes, but generated a continuous memory code '172'. [HEGO (HO2S) Sensor fault/lean].

From what I've read there are a variety of potential causes for this code, but I wonder if anyone here can offer advice as to where to start looking for the most likely source(s). Some hints: although the code tells me the engine is running 'lean', I've noticed that a puff of black soot is ejected from my exhaust at startup which indicates to me (perhaps?) that the engine is actually running rich, not lean. I have also noticed that the engine has less power than it seems it should, and has fairly poor fuel economy. I've considered this another hint that the engine is actually running rich, since it seems like it's using more fuel than it ought to. Lastly, the engine does not seem to be very steady at idle.

From what I've read it seems that common causes of this issue include 1) a faulty MAF sensor, 2) a leak in the air/vacuum intake to the engine, 3) faulty O2 sensor(s), 4) fuel pressure issues, or 5) a leak in the exhaust before the O2 sensor causing inaccurate readings.

If the car is running rich (i.e. there is unburnt fuel and not much O2 in the exhaust) but the sensor is telling me that it's running lean (code 172) does that point to a faulty O2 sensor as being the most likely explanation? Any suggestions would be appreciated - thanks.
 
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