Engine Code P0174 - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 11-27-2014
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Engine Code P0174

My check engine light came on recently and I took it to get scanned at AutoZone. They gave me the printout from their diagnostic scanner and it showed a fault code of P0174 (Fuel trim bank two condition). The possible causes listed on the print out showed: *If bank one and two are set together suspect a fuel pressure condition of MAF sensor fault, *Failed H02S21 (Heated Oxygen Sensor - Bank 2 Sensor 1), *Ignition misfire condition, *Fuel injector problem, or *Engine mechanical condition.

Is there anyway that I can narrow the possibilities down through troubleshooting? I obviously don't have a diagnostic scanner and want to avoid taking it to the dealership. I'm not sure it this could be related to the issue, but prior to this fault code, I replaced my thermostat, Engine Temperature Control sensor, and radiator. Since my check engine light has been on, I have noticed an intense vibration when driving my truck between 50-60-ish MPH. When I pass above 60-ish MPH, the vibrations become less noticeable. When I slow down, the vibrations intensify again until I slow down below 45-ish MPH where they will become less noticeable again.
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Old 11-28-2014
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Was P0174 the only code?

I will assume it was.

Bank 2 is the drivers side of engine, so cylinders 4, 5, and 6
Sensor 1 is the O2 sensor on the exhaust pipe near the exhaust manifold
(FYI, sensor 2 would be an O2 sensor behind the Cat Converters, used to test the converters only)

P0174 means the Bank 2 O2 sensor data is showing too much air(oxygen), lean condition, in the exhaust.
To set this code it means the computer has added more fuel to that bank but O2 data still shows too much oxygen, the computer has a preset limit for how much fuel it can add at a given RPM and load, when it reaches that limit it will set that code to notify you.

If it was a dirty MAF sensor I would expect both Banks to be effected.
Or a vacuum leak in upper intake would effect both banks as well.

So I would look at drivers side system specifically.

O2 sensors can only look at oxygen levels, they can't "see" fuel, so if a cylinder on bank 2 is misfiring then no oxygen in that cylinder is being consumed by burning with the fuel, so all that oxygen is pumped out into exhaust pipe and O2 sees that as too much oxygen.

Is your engine misfiring?
i.e. idling rough or missing under load
This could be bad spark plug or dirty injector.
Your vibration could be a misfire, but I would expect a code for that telling you which cylinder was misfiring, and CEL would flash during the misfiring.

Do you have pinging(pre-detonation) when accelerating or driving up a hill?
This is a sign of a lean fuel mix.
Low fuel pressure but I would expect to see that reported on both banks.
But a dirty injector could do that.

The above would be symptoms of an actual lean condition so O2 sensor is correct in reporting it.

If you have no symptoms of actual lean condition then I would suspect the O2 sensor, they do wear out, recommended replacement is 80-100k miles.
You can swap O2 sensors and see if code moves to Bank 1, confirming bad O2 sensor.
I would replace both O2 sensors, they are called upstream sensor 1's

Sensor 2 O2 sensors rarely wear out since they see cleaner exhaust, so don't need to be replaced unless you get a specific code for them.
All O2 sensors on the vehicle are the same kind, no difference in sensor 1's and 2's

Last edited by RonD; 11-28-2014 at 12:46 PM.
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Old 11-28-2014
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Quote:
Originally Posted by RonD View Post
Was P0174 the only code?

I will assume it was.

Bank 2 is the drivers side of engine, so cylinders 4, 5, and 6
Sensor 1 is the O2 sensor on the exhaust pipe near the exhaust manifold
(FYI, sensor 2 would be an O2 sensor behind the Cat Converters, used to test the converters only)

P0174 means the Bank 2 O2 sensor data is showing too much air(oxygen), lean condition, in the exhaust.
To set this code it means the computer has added more fuel to that bank but O2 data still shows too much oxygen, the computer has a preset limit for how much fuel it can add at a given RPM and load, when it reaches that limit it will set that code to notify you.

If it was a dirty MAF sensor I would expect both Banks to be effected.
Or a vacuum leak in upper intake would effect both banks as well.

So I would look at drivers side system specifically.

O2 sensors can only look at oxygen levels, they can't "see" fuel, so if a cylinder on bank 2 is misfiring then no oxygen in that cylinder is being consumed by burning with the fuel, so all that oxygen is pumped out into exhaust pipe and O2 sees that as too much oxygen.

Is your engine misfiring?
i.e. idling rough or missing under load
This could be bad spark plug or dirty injector.
Your vibration could be a misfire, but I would expect a code for that telling you which cylinder was misfiring, and CEL would flash during the misfiring.

Do you have pinging(pre-detonation) when accelerating or driving up a hill?
This is a sign of a lean fuel mix.
Low fuel pressure but I would expect to see that reported on both banks.
But a dirty injector could do that.

The above would be symptoms of an actual lean condition so O2 sensor is correct in reporting it.

If you have no symptoms of actual lean condition then I would suspect the O2 sensor, they do wear out, recommended replacement is 80-100k miles.
You can swap O2 sensors and see if code moves to Bank 1, confirming bad O2 sensor.
I would replace both O2 sensors, they are called upstream sensor 1's

Sensor 2 O2 sensors rarely wear out since they see cleaner exhaust, so don't need to be replaced unless you get a specific code for them.
All O2 sensors on the vehicle are the same kind, no difference in sensor 1's and 2's
Yes, the P0174 was the only code. Thank you for your very descriptive response! I am having a friend take a look at it tomorrow with me.
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Old 12-18-2014
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Update for my P0174 Code Troubleshooting

So I cleaned the MAF sensor and replaced the drive's side manifold O2 sensor hoping that it would clear the fault, but no success. I took is to AutoZone and was told it is most likely a vacuum line. I had already figured it was since I am not getting a code for both fuel banks being lean and I had already replaced the #2 upstream O2 sensor, with no luck. Anyone have any suggestions on which vacuum line(s) I should check. The guy at AutoZone suggested spraying started fluid on the vacuum lines and if the engine revs high, then that vacuum line has a leak.
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Old 12-18-2014
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With engine warmed up unplug the IAC Valve, idle should drop down to about 500rpms, or engine may even stall either is good.
If idle doesn't drop down that low then yes there is a vacuum leak.
With engine idling this way unplug one vacuum hose at a time and plug port with your finger, if idle drops or engine stalls that is the leaking line.

BUT, a vacuum hose leak would usually effect both banks, since upper intake is used by both banks, it could be effecting both banks and bank 2 is just the first to show it.
If you do a live scan and watch the Fuel Trims it would tell you if both banks are effected.
You can get Blue Tooth OBD II scanners that work with a Smart Phone, $30-$50

Lower intake manifold leak could effect just one bank.
Or exhaust leak on bank 2 exhaust manifold, exhaust manifolds have negative pressure when engine is running and will suck in air cause O2 to show Lean.

Now on the other side of the equation is not enough fuel, dirty injector on bank 2.
Do you notice any misfires?
Doesn't need to be much

And low fuel pressure can show up as Lean on one bank first, Lean code is set because computer has reach it's limit for how long it can open a fuel injector.
O2 sensor reports lean(too much oxygen)
Computer opens injectors on that bank longer, adding more fuel
O2 still shows Lean
Computer opens injectors even longer
O2 still reports Lean
Computer sets code P0174 because something is wrong, injectors are open too long for Lean O2 reading, this is a preset limit in computer memory



As far as spraying starting fluid around, the best way to do that is with engine cold and with "fan belt" unhooked.
A cold engine can run a couple of minutes without alternator or water pump, but only a couple.
Remove serpentine belt from crank pulley, this will stop the fan from blowing around the starting fluid, it will also make engine bay very quiet so you can hear changes in RPM or pinging the starting fluid will cause if sucked in.
Start engine and start spraying around lower intake, then move up to upper intake.

Spraying flammable vapors around a warm engine is NOT a well thought out plan.

Last edited by RonD; 12-18-2014 at 01:50 PM.
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Old 12-18-2014
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Ok, I'm kinda stumped now. I just disconnected the IAC Valve and the idle dropped down. I haven't noticed any misfires or anything. The engine feels like it is running well. I did change the spark plugs and wires a few thousand miles ago and just changed the oil and fuel filter last night, along with the #2 upstream O2 sensor. I am not aware of any exhaust leaks in my engine compartment. I haven't seen or smelled any exhaust fumes in the area, while I am working on it. If there another way of diagnosing an exhaust leak? My old Camaro had an exhaust leak at the headers and I could see the gasket , but not on my Ranger. I am kinda stumped with this. Can you please define lower intake and upper intake for me. I have a little bit of auto mechanical skills, but it's limited.
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Old 12-18-2014
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You would hear exhaust leak if it was large enough to set lean code.

If idle dropped down to 500 then I would look at the fuel side of the equation.
Can you borrow or rent a fuel pressure gauge, auto parts stores often loan or rent these.

1996 should have 35psi pressure at idle
Remove Fuel Pressure Regulators vacuum hose and it should jump to 40psi
Put if back on and raise RPMs to 2,000 and hold it there for a minute, pressure should hold steady not slowly drop.
Shut off engine and watch pressure, in may drop to about 30psi and then hold there for at least 10 minutes.


Also run a can of Seafoam in the gas tank to clean out injectors.


Lower intake is where intake bolts to the heads, fuel injectors are in the lower intake
Upper intake bolts to the lower intake, upper intake has the throttle and vacuum hoses


Also you do need to reset computer after any repairs, unhook battery's negative cable for 5 minutes, then reconnect.
This causes computer to recheck all sensors and controls

Last edited by RonD; 12-18-2014 at 04:13 PM.
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Old 12-18-2014
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Ok, so I rented the fuel pressure test kit from AutoZone and hooked it up and performed the steps that you explained. During idle the fuel pressure read 35 PSI. When I unplugged the vacuum line to the Fuel Pressure Regulator it jumped up to about 43 PSI. When I reinstalled the vacuum line, the pressure dropped back down to 35 PSI and remained there while I raised the RPM approximately 2000 RPM. I did not have assistance nor a tachometer. I just manipulated the throttle. When I shut the engine off, the pressure did start dropping until it went down to 0. It didn't even make it 10 minutes until it was down to 0. Any suggestions? I did however find a dry rotted vacuum line. It was a small 4" 90 degree elbow that connects to a 3/8" line that runs to I'm assuming a charcoal canister in the front driver's side of the engine compartment and connects to a 5/16" port at the front of the intake. I replaced that line and had my computer reset. We shall see if that if the issue.
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Old 12-19-2014
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You also mentioned that you changed the spark plugs a few thousand miles ago; which plugs did you use?

On my 2000 3.0, a previous owner (or perhaps the dealership I bought it used from) installed a fancy set of super dooper dual ground electrode platinum plugs. When I removed a few to check the status of the cylinders, I noticed these plugs were very hot looking, almost white. I replaced them with a single ground electrode plug, and my pinging went away. I removed a few of these plugs a week ago, and they look way better; grey and clean, not white and squeaky clean!

Maybe your code is due to an improper heat range plug? However, as RonD points out, this would effect both banks, not just one.
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Old 12-19-2014
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Your fuel pressure numbers are good except for the drop after engine is off.

Was there any fuel around the pressure gauge?
The drop to 0 PSI means fuel is leaking out, if gauge was loose it could be from there, so no worries.
If engine doesn't start the first time every time then fuel pressure leak could be real.

If you don't smell fuel(external leak) then either the FRP is leaking fuel back thru Return line, but vacuum line removal and pressure increase would pretty much rule that out, or check valve(backflow preventer) on in tank pump is leaking, could also be a cracked lifting hose in the tank.

Running engine at 2-3,000rpms does increase fuel demand but not by much since there is no load.
When driving at speed fuel demand is much higher so a leak would cause a pressure drop.

Yes, hopefully that vacuum leak is the source of the P0174
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