Yet more codes
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
Yet more codes
Finally got around to replacing my rear tank, pump and sender. Was so excited to be able to get further than 200ish miles between fill ups. Then the CEL started.
171-(M)
Oxygen sensor not switching – system was at adaptive limits – Single, Right or Rear HO2S – Fuel control
179-Fuel system was rich at part throttle Single, Right or Rear HO2S – Fuel control
634-Park/Neutral Position (PNP) or Clutch Pedal Position (CPP) circuit fault Electronic shift transmission – Manual Lever Position (MLP) sensor out of range in Park-Transmissions
637-TOT sensor is/was high or open – Transmissions
send help.
171-(M)
Oxygen sensor not switching – system was at adaptive limits – Single, Right or Rear HO2S – Fuel control
179-Fuel system was rich at part throttle Single, Right or Rear HO2S – Fuel control
634-Park/Neutral Position (PNP) or Clutch Pedal Position (CPP) circuit fault Electronic shift transmission – Manual Lever Position (MLP) sensor out of range in Park-Transmissions
637-TOT sensor is/was high or open – Transmissions
send help.
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
94 f150 4.9
i think after doing some digging it may be related to the regulator being weak after replacing the pump.
the other codes kinda hint at some issues for the transmission hiccup I’ve been experiencing
i think after doing some digging it may be related to the regulator being weak after replacing the pump.
the other codes kinda hint at some issues for the transmission hiccup I’ve been experiencing
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
When were codes 634 and 637 displayed? Key On Engine Off (KOEO) or Stored Codes (aka Continuous Memory) or both?
Codes 171 and 179 are Stored Codes so I highly suggest to clear them. Drive the truck for a few cycles then re-run the KOEO tests and Stored Code display. If you get System Pass, then move onto the Key On Engine Running (KOER) tests.
Codes 171 and 179 are Stored Codes so I highly suggest to clear them. Drive the truck for a few cycles then re-run the KOEO tests and Stored Code display. If you get System Pass, then move onto the Key On Engine Running (KOER) tests.
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
When were codes 634 and 637 displayed? Key On Engine Off (KOEO) or Stored Codes (aka Continuous Memory) or both?
Codes 171 and 179 are Stored Codes so I highly suggest to clear them. Drive the truck for a few cycles then re-run the KOEO tests and Stored Code display. If you get System Pass, then move onto the Key On Engine Running (KOER) tests.
Codes 171 and 179 are Stored Codes so I highly suggest to clear them. Drive the truck for a few cycles then re-run the KOEO tests and Stored Code display. If you get System Pass, then move onto the Key On Engine Running (KOER) tests.
Hotshot
Joined: Jun 2015
Posts: 12,870
Likes: 2,443
What has been happening with the transmission?
I really should add some back story here. I literally just installed a new rear fuel tank, pump and filter. Previously I would get an occasional check engine light to appear for about 10 seconds then disappear. Yesterday was the first day I’ve used the rear tank running 55-65 for extended periods. At about 45 the CEL would illuminate and stay on until I was slowing down/ foot off the gas. This leads me to believe the path of the fuel regulator. BUT I do intend on the tests you’ve recommended. I’m going to fuel up this afternoon and run the front tank and see what transpires as well.
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
What do you mean by a “weak” regulator? What fuel pressure do you get from each tank? How would that cause a rich condition?
What has been happening with the transmission?
What codes did you get before the gas tank replacement and what did you do to fix the problem? The “test” that was suggested is to clear the codes and then drive the truck for a while to see if the codes return.
What has been happening with the transmission?
What codes did you get before the gas tank replacement and what did you do to fix the problem? The “test” that was suggested is to clear the codes and then drive the truck for a while to see if the codes return.
a failed regulator allowing too much volume to pass is how it would cause a rich condition. Front tank idles at 55 psi. Rear tank idles at 95 psi. Just a blip of the throttle and it pegs out of my 100 psi mechanical fuel pressure gauge.
Codes cleared, codes returned. KOER test only showed 111, but I imagine if I drove my normal hour long commute route I’d have something there.
the transmission problem I’m referring to is the one you dismissed in my post several months ago. That’s not really important right now.
at this point I’m digging in to the fuel system to see what I’m repairing next.
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Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Two different pressure readings for each tank is not right. There is only one fuel pressure regulator on the return line. Pressure readings are way too high.
That is the reason for Codes 171, 179.
That is the reason for Codes 171, 179.
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Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
this is why I assumed the regulator. Ain’t it supposed to open to allow additional fuel to return?
it’d be my luck its actually the new fuel pump not allowing the return to flow.
Hotshot
Joined: Jun 2015
Posts: 12,870
Likes: 2,443
a failed regulator allowing too much volume to pass is how it would cause a rich condition. Front tank idles at 55 psi. Rear tank idles at 95 psi. Just a blip of the throttle and it pegs out of my 100 psi mechanical fuel pressure gauge.
Codes cleared, codes returned. KOER test only showed 111, but I imagine if I drove my normal hour long commute route I’d have something there.
the transmission problem I’m referring to is the one you dismissed in my post several months ago. That’s not really important right now.
Codes cleared, codes returned. KOER test only showed 111, but I imagine if I drove my normal hour long commute route I’d have something there.
the transmission problem I’m referring to is the one you dismissed in my post several months ago. That’s not really important right now.
Only way to know is to drive the truck a few days and recheck for codes
I’m not sure what transmission problem I dismissed. I looked through your threads and didn’t see any transmission thread or other thread where I did that.
Good possibility since the rear tank is so much higher than the front
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
Wouldn’t a failed regulator allowing too much fuel to return to the tank cause a lean condition and lower pressure? I’d try another gauge.
Only way to know is to drive the truck a few days and recheck for codes
I’m not sure what transmission problem I dismissed. I looked through your threads and didn’t see any transmission thread or other thread where I did that.
It is supposed to restrict flow.
Good possibility since the rear tank is so much higher than the front
Only way to know is to drive the truck a few days and recheck for codes
I’m not sure what transmission problem I dismissed. I looked through your threads and didn’t see any transmission thread or other thread where I did that.
It is supposed to restrict flow.
Good possibility since the rear tank is so much higher than the front
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
it could be a restriction. I’m wondering if the internal shuttle valve on the new pump isn’t operating.
Fleet Mechanic
Joined: Oct 2025
Posts: 1,834
Likes: 399
Did you have the factory fix for the fuel transfer on the lines? The external check valve’s on the lines at the pump? If so, did you check that they operated? One of mine was rust locked shut with zero chance of fixing.
Thread Starter
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Mountain Pass
Joined: Mar 2013
Posts: 232
Likes: 6
it’s likely just the pump at this point. I’ve had pretty decent luck with the fuel pumps from oreillys in the past, this one was a precision fuel pump, and am considering returning it as it clearly isn’t operating quite right. Before I get to that point I’m going to check the line for obstructions. I put a Delphi in the front tank in September ish when I needed to get it running and it’s worked flawlessly.
I did check the codes again today and only got 179 and 634 KOEO and a system ok for the KOER test. So really at this point it’s either the line or the pump, but don’t expect the line to be blocked. I’d *assume* 95 psi would be enough to blow a small blockage out but I’ve been surprised before. And for what’s its worth, subford had a great explanation on this post.