CEL codes 172, 176, 177

[rumblecloud]

Got a CEL tonight driving home from dinner out. After about 5 minutes truck lost power. Would not accelerate at all. Pedal to the floor seemed to make it worse. Pulled over let it rest for about 5 minutes. Restarted and it ran ok for about 5 minutes - everything normal. CEL came on again. 5 minutes later, the same thing happened. Repeat the process all the way home. It seemed to be ok at about 20-25 mph, but no acceleration - just bogged down.
Codes pulled were:

172 HO2S Sensor Fault/Lean

176 HO2S Sensor Fault/Always Lean
177 HO2S Sensor Fault/Always Rich

Any advice?

The 172 is new to me. I've gotten the other two before and thought I had eliminated those issues, but this is different. Truck has been running pretty good for quite some time now getting decent mileage in the 14 to 15 range.

Thanks
r

 

[RonD]

172 (R,M) Oxygen sensor not switching – system is or was lean Right
176 (M) Oxygen sensor not switching – system is or was lean Left
177 (M) Oxygen sensor not switching – system was rich Left

1994 4.0l has a left and a right upstream O2 sensor, no downstream O2s

How old are they?
O2s last 100k miles or 12 years after that they start running out of the chemical coating that detects Oxygen in the exhaust
O2s generate their own voltage, and it's not much
0.1volt means high oxygen which translates as "lean" exhaust
0.9volt means low oxygen which translates as "rich" exhaust

We tend to think of lean and rich as a FUEL mix, this is not that
If engine is Pinging/Knocking it has a lean fuel mix
If engine is blowing smoke out the tail pipe its a rich fuel mix

But the computer WILL add more fuel on that bank of the engine if O2 has lower voltage(lean), and it will add less fuel if O2 has a higher voltage(rich)
Computer tries to keep O2 voltage at 0.4volt average, as O2 gets older it generates less voltage so............Lean codes

The "not switching" means computer is adding more fuel but O2 on that side is still showing low voltage/high oxygen, regardless of what computer does, so it sets a Lean code

And computer is adding less fuel to one side if O2 voltage is high/low oxygen, if voltage doesn't drop it sets a Rich code

This all relies on the O2s working perfect and their wires to the computer also being perfect, at 0.1v to 0.9v any corrosion will cause lean codes, i.e. low voltage

So first thing is "are the O2s trustworthy", lol, if not then you will be chasing your tail trying to find air leaks or fuel leaks

O2s also have an internal heater, 2 of the 4 wires are 12v and ground for that heater, 1994 doesn't have a monitor for these heaters, 1995 and up do, so you could test one of the O2 connectors for 12v, light blue/orange wire, key on
Check fuse 18 in cab fuse box, 15amp, that powers these heater

Only time I ever had Lean on one side and Rich on the other is when a shop reversed my O2 sensors on my 1994 4.0l
Same plugs so honest mistake, O2s use universal plugs
And engine ran like crap after warm up, so not that in your case

Lean codes and rich codes can be partially clogged exhaust, i.e. Cats or Muffler is breaking up internally

Lean codes can be air leaks/vacuum leaks
Dirty MAF sensor
Low fuel pressure

Rich codes are fairly limited in cause
Bad O2 sensor
Bad/leaking injector on that side

I would clear the codes and see what comes back because the Rich code is an anomaly

 

[rumblecloud]

Thanks RonD.
The O2s do have some miles on them in the 2,000 mile range (from Rock Auto at about 213,3??, currently at 215,926). If I suspected one, it would be on the passenger side. I have some of your previous help solutions saved in my Ranger work file as well. Between that and what you have just posted I should be able to figure it out.

I was thinking the throttle body only because that's about the only thing I haven't replaced

I really appreciate your response(s)

 

[RonD]

If O2s are newer then they will be easy to swap around, if a code moves to the other bank(follows the O2) then replace that O2

Also check the heater fuse, fuse 18 in cab fuse box

 

[rumblecloud]

If O2s are newer then they will be easy to swap around, if a code moves to the other bank(follows the O2) then replace that O2
Also check the heater fuse, fuse 18 in cab fuse box

Managed to to do a little more work yesterday and today.
Fuse 18 is good
Cleaned the MAF
Checked vacuum at the PCV and booster
Unplugged the O2 connector on the right side at the top of the bell housing (which was probably a mistake ). Checked the voltage and it was getting 12V

1994 4.0l has a left and a right upstream O2 sensor, no downstream O2s
0.1volt means high oxygen which translates as "lean" exhaust
0.9volt means low oxygen which translates as "rich" exhaust

You mentioned that O2s give voltage. I reconnected the sensor and checked for that voltage, but I'm not sure I did it correctly. If I did,
it gave me 1 volt. Here's how I did it and what the O2 sensor looks like:
View attachment 82983
Please correct me if I am wrong.

Oh...they do have an internal heater...ouch!

 

[RonD]

O2s generate voltage only after they are heated above 600degF, their chemical coating reacts with the Oxygen in the Exhaust, but inverse reaction, high oxygen = lower volts, low oxygen - higher volts

So it can only be tested LIVE i.e. running engine that's warmed up
That's usually not practical on OBD1

With OBD2 you can watch the voltage on the Reader

If you have running issues BEFORE engine warms up then its not O2s
Cold start and for the first 4 min or so computer ignores O2s and runs in Open Loop
So rough running at that time points to other issues, not O2 related

Also at WOT(wide open throttle) the computer ignores O2 sensors, goes for MAX power, economy is out the window, lol
So if there is still rough running at WOT even after warm up then its not an O2 sensor issue

 

[rumblecloud]

Well alrighty then...that sensor goes back in and I move on to something else.
Fuel pressure? If so, then I get to buy a new tool .

When I go WOT, I get nothing. It just doesn't do anything. Backing off seems to help. but any kind of acceleration does virtually nothing.

I pressed the schrader valve in after a test drive the on Sunday and it gave a pretty good shot. I know that is not an indicator of good pressure, but when I did it again today after not driving ti since and there was no pressure.

It does start with no issues. After warm up the idle fluctuates between 1100/1200 rpm, down to about 500/600 rpm. There's another post here that I read yesterday with someone having the same issue.

Hey, thanks for your continued help.