Ok I've seen many listings for this code but none have fully helped but they have been helpful. So here is what I got. Code 157 maf sensor below minimum voltage. Here is what the truck does. When the MAF is plugged in and I step on it, the truck surges and when it drops below 1k rpm it dies but I can start it back up. If I hold the throttle at 1k it stays alive. When I drive with the MAF unplugged it runs better no surging but it still dies. Here is what I have done. Reset codes no change, put new MAF in no change. I have traced all 4 MAF wires checked continuity from inside of plug to inside of plug. All wires checked out good. No cuts breaks or anything. I have checked my battery, starter, and alternator ad all check out fine. I read that someone said if the corresponding transistor in the PCM took a dumb then it would cause not enough voltage to get to the plug because the transistor would dump the voltage from the battery leading to the MAF. I thought about running a direct power source from the battery to the voltage side of the MAF with a switch in the truck to just see if that would solve the issue but I don't know if that is possible or smart. Any help would be greatly appreciated. Thank you

 

Newest update is this. i supplied an external voltage source and still no change. I also probed the back of the connector while the truck was running and every single wire read 14+ volts while the truck was at idle. Then when the truck did nothing changed no voltage drop no nothing

 

What year truck is this on?

Even though the code says maf sensor, the issue may be somewhere else. Since you did a fairly thorough job with the maf, I would think the problem lies elsewhere - have you checked for vacuum leaks? How's the IAC valve?

 

it's on a 1993 4.0 5 speed manual 4x4. I have tried tons of different things to no prevail I did check for vacuum leaks and I didn't have any. I did another code check with the maf unplugged with the truck off and on and while the truck was off I got a 212 code which is Ignition module circuit failure/SPOUT circuit grounded. I check my spout wire because I remember nicking a wire with my knife while doing the continuity test on the maf so after retracing that wire it was my spout wire. i re-wrapped the wire and the code went away. but i"m not sure if the ignition module is going out and just not throwing the code. I'm not sure if the code 157 is just making other issues or not but its the only code that pops right now. I tried reading the voltage at the back of the plug and I couldn't get a reading which the book says the maf is at fault but I already had a new maf in the truck and still couldn't get a reading. I literally had every wire read 14 volts on the back of the pigtail where you should read .2-1.5 with the maf signal and maf sig rtn wires probed. I'm at a loss. My last option if the pigtail doesnt solve the issue is to try a new ecu and hope that what I have read about a dropped transistor dumping the voltage is my problem. but if you have any other ideas please let me know.

 

almost forgot. If I hold the throttle at 1k rpm the truck will stay on but as soon as I let out it will slowly back off the rpms hit 500 and die. I have checked the TPS and it checks out fine at the voltage readings it should. i even went so far as to bump my throttle stop to keep my throttle open more to try to keep the truck alive but that does not work either

 

Welcome to FTE.

You may have more than one problem. Did these problems come about suddenly after some event????

Good trouble shooting so far & I like Furyus1 IAC idea, as its responsible for maintaining idle speed & won't always throw a trouble code when acting out. So, did you have an idle problem Before the MAF code woes???? Maybe try removing & cleaning it with a plastic safe spray & see how idle speed goes. Cleaning the IAC may not last, it didn't for me on my 99, but was a good trouble shooting step.

I like your idea that you may have a wiring, or ECM problem on the P0157, MAF below minimum voltage trouble code.
What sort of voltage readings do you get at the connector with it Disconnected from the MAF sensor???
With it disconnected, does the Sig Rtn wire seem to be shorted to the power lead, or grounded????
If the MAF harness & connector check out ok electrically, maybe suspect the ECM, so do you have a friend with a like vehicle that you could try an ECM swap on???? If not, maybe a salvage yard would let you try a replacement before buying.

Some thoughts for pondering, let us know what you find.

 

The code just showed up one day while I was driving. My truck bucked on my like a surge and then I heard a pop from the exhaust and then the check engine light came on. I need to check the IAC still. Um here is the weird thing. When I checked the voltage with the ignition on I got 14 volts at every wire. now for some reason I can't figure out which wire is which. I assume that the red is power black is ground since the other two lead to the ecm. Now when I probe just the other two wires i get no voltage whatsoever truck ignition on or the truck running. I know that says the maf would be my issue but I replaced the maf and got the same thing. I have checked all the wires for cuts or grounds and continuity and everything checked out fine. I did clean my maf with QD electrical cleaner and that didn't change anything. I will try the IAC and see where that leads me. I just need to read the book to refresh my memory on where it is. This is my first ford, I'm a Chevy guy but i wanted a solid truck and the ranger is one solid truck. I unfortunately don't have any friends and the salvage yards don't have any either. So my goal is to eliminate everything else before buying a new ECM. I let you know what I find on the IAC. Thanks for the help

 

Yup the IAC is a common problem part when we have idle woes, so most folks remove & clean it to see how it goes. It's on the intake air tube, just in front of the throttle body. A couple of T-20 torx fastners & one electrical connector to remove it.

For sure the computer isn't happy with the rtn/signal its getting from the MAF sensor, so if the new MAF sensor, signal return wiring back to the ECM & return signal to the computers firewall connector checks out, that leaves the ECM on your suspect list.

I'm not clear on the MAF voltage tests you posted about. Were those tests done with the MAF sensor connected & the leads back probed????
Have you done the MAF connector disconnect test yet & if so, was B+ on the correct pin & was the ground lead actually grounded???? Have you back probed the Sig Return lead with the engine running????

Do you have a repair manual to work from????