Check engine light staying on...HELP!!
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New User
Joined: May 2005
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From: Sarver, PA
Check engine light staying on...HELP!!
93 XLT 4X4 w/4.0L engine, 5spd manual, 162k miles. Got the check engine light. I got the truck hooked up to a MAC tools code reader. It spit out a 157 MAF fault. Replaced with a unit from the good folks at Millenium Ford in Seattle. Installed the MAF sensor, disconnected battery for over a half hour. CEL light came back on. Truck seems to run fine with an occassional period of rough running. Hooked up the truck again to a MAC code reader. Cleared all engine faults. Started truck up again and the CEL light cam on after the truck ran for about 5-10 seconds. Ran the gamut of tests with the Code reader and it spit out a 157 MAF code again along with a 998 FMEM/RETEST. the MAC code book describes the 998 fault as "Hard fault present, Failure Mode Effects Management, Retest
What exactly does that mean? If anyone has an idea, please let me know your thoughts.
What exactly does that mean? If anyone has an idea, please let me know your thoughts.
Post Fiend
Joined: Jan 2003
Posts: 9,748
Likes: 16
From: Connecticut
DTC 157 indicates that the Mass Air Flow (MAF) sensor signal went below 0.4 volt sometime during the last 80 warm-up cycles. Possible causes:
- Open MAF SIG circuit.
- Open MAF RTN circuit to MAF sensor.
- Open VPWR circuit to MAF sensor.
- Open PWR GND circuit to MAF sensor.
- MAF SIG circuit shorted to ground.
- Damaged Powertrain Control Module (PCM).
- Damaged MAF sensor.
- Air leak before or after MAF sensor.
- MAF sensor disconnected.
- Idle Air Control (IAC) system failure (at closed throttle position).
So it might be a good idea to test for opens and shorts in the MAF circuit.
Battery power (VPWR) to the MAF is the Red (RD) wire and PWR GND is Black (BLK).These two wires come from splices, since the MAF shares its power and ground circuits with other components.
The other two wires are the MAF SIG (LB/R) and MAF RET (T/LB). These run straight to the PCM (pins # 14 and 15, respectively).
When the PCM determines that the MAF circuit is malfunctioning, it will cause the EEC system to go into "failure effects mode" which results in the PCM using substitute values from tables stored in memory rather than using "live" signal feedback from the MAF sensor.
- Open MAF SIG circuit.
- Open MAF RTN circuit to MAF sensor.
- Open VPWR circuit to MAF sensor.
- Open PWR GND circuit to MAF sensor.
- MAF SIG circuit shorted to ground.
- Damaged Powertrain Control Module (PCM).
- Damaged MAF sensor.
- Air leak before or after MAF sensor.
- MAF sensor disconnected.
- Idle Air Control (IAC) system failure (at closed throttle position).
So it might be a good idea to test for opens and shorts in the MAF circuit.
Battery power (VPWR) to the MAF is the Red (RD) wire and PWR GND is Black (BLK).These two wires come from splices, since the MAF shares its power and ground circuits with other components.
The other two wires are the MAF SIG (LB/R) and MAF RET (T/LB). These run straight to the PCM (pins # 14 and 15, respectively).
When the PCM determines that the MAF circuit is malfunctioning, it will cause the EEC system to go into "failure effects mode" which results in the PCM using substitute values from tables stored in memory rather than using "live" signal feedback from the MAF sensor.
Last edited by Rockledge; Mar 30, 2006 at 12:02 AM.
. pemnam, please post the driveability problem/concern/issue.
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New User
Joined: May 2005
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From: Sarver, PA
More info on problem
First off, thanks for the replies, good info provided. Found the harness sorta wedged against fender well, don't think it was the problem. Checked supply and gnd. wires. Good solid voltage reading on the supply wire and a good gnd also. The truck seems to be running OK, but my son said it ran rough on him once. I took it out for a drive and I ran fine for me. Today I disconnected the battery ground for about 1/2 hr. but the check engine light is still on. I tapped into the sensor wires and checked with a voltmeter while running. I get about .98 volts idling and if the engine is revved to 3000 rpm the voltage goes up to about 2.9 volts. The MAF appears to be working, correct? Since the ECM spit out the 998 failure effects code, do I need to hook up a diagnostic tool to reset that code? Could the ECM be bad or is the new MAF defective? I haven't disconnected the harness at the ECM. I don't want to disturb that connector if I don't have to. Any suggestions 
Posting Guru
Joined: Jul 2003
Posts: 1,978
Likes: 1
From: Alvin, Texas.
You should get .2 to 1.5 volts at idle and it should rise as revs rise. Good there.
You could pull the ECM fuse and pop it back in and pray the code was only for the MAF and won't come back. I hate suggesting that, though, but if it works.....
You could pull the ECM fuse and pop it back in and pray the code was only for the MAF and won't come back. I hate suggesting that, though, but if it works.....
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New User
Joined: May 2005
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From: Sarver, PA
Broken wire found
It was the harness. There is a second acc. battery mounted behind the right headlight. Apparently the sensor signal wire was intermittent. I little closer look and some jiggling of the harness and the wire broke. After repair the MAF code and Failure effects management cleared. After driving a while however, the check engine light can back on. After shutting off truck and starting again the CEL stays out until the truck pulls a hill. It seems that any time the engine becomes loaded, i.e. large throttle opening with lower engine RPM, the CEL comes on. Any thoughts what it could be? I haven't had the chance to hook up a code reader but I'm thinking maybe its an Oxy. sensor. Has this ever happened to your Ranger?
Thanks for the advice, its highly accurate and greatly appreciated.
Thanks for the advice, its highly accurate and greatly appreciated.
Post Fiend
Joined: Feb 2003
Posts: 6,940
Likes: 2
From: Eastern WA
Need to know the code # and which of the three types of codes it is. Yeah, I had this happen on my old 94 w/4.0. Codes first; any less is gambling. I'll assume you have already done a thorough and complete visual under the hood for any further wiring and/or vac line issues and it's 125% confirmed that none exist.
