Transmission codes P0743 and P0755

Tiny
NICKBAR87
  • MEMBER
  • 1999 FORD F-150
  • 5.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 219,000 MILES
I am having trouble codes p0743 and p0755. The truck is having a hard time accelerating even if I have the pedal to the floor. The fluid is full and is clean. The truck revs up just sometimes gets stuck for a few seconds around 3000 rpm's and then drops. What are possible causes for these issues? I am at a loss here.
Tuesday, March 13th, 2018 AT 8:30 AM

11 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
Good afternoon.

Does the transmission shift gears?
Also, it sounds like you have or may have a clogged catalytic converter. You can confirm by either removing the upstream O2 sensors and drive it or drop the front pipe and drive it and see if the rpm's goes higher than 3000.

Let me check the codes.

Roy
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Tuesday, March 13th, 2018 AT 9:13 AM
Tiny
NICKBAR87
  • MEMBER
  • 30 POSTS
I honestly do not know if it shifts gears anymore because it has such a horrible take off that it takes forever to even get going. But I would assume so because I have not gotten up faster than about twenty five mph and it did not whine like it did not shift to second gear.
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Tuesday, March 13th, 2018 AT 9:17 AM
Tiny
NICKBAR87
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  • 30 POSTS
Both secondary catalytic converters have been removed, one of them was very clogged up.
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Tuesday, March 13th, 2018 AT 9:18 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
Okay, 743 is for the torque converter solenoid circuit either being open or shorted. The pan would have to be dropped to inspect the harness and do voltage tests to the solenoids to be sure. The solenoid needs to be checked for about five ohms across the terminals.

The 755 is for the shift motor for the four wheel drive system.

Roy
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Tuesday, March 13th, 2018 AT 9:22 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
What about the catalytic converters up front by the engine?

Roy
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Tuesday, March 13th, 2018 AT 9:27 AM
Tiny
NICKBAR87
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Okay as far as 755 code I can understand that one and why. For the 743 code, could that circuit have been fried with a transmission wiring harness issue I had, melted to exhaust? But has been repaired.
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Tuesday, March 13th, 2018 AT 9:27 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
It may have damaged the solenoid and the inside harness, yes.
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Tuesday, March 13th, 2018 AT 9:28 AM
Tiny
NICKBAR87
  • MEMBER
  • 30 POSTS
The catalytic converters by the engine are still in place. I could not tell a difference in exhaust flow with the exterior catalytic converters removed. Would that solenoid damage cause the issue with no power at take off from standing still?
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Tuesday, March 13th, 2018 AT 9:33 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
The torque convector will cause that if it is stuck applied. You need a full blown scan tool to monitor it.

As far as the catalytic converters, you cannot tell from amount of exhaust. You need to verify by a back pressure test.

Roy
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Tuesday, March 13th, 2018 AT 9:37 AM
Tiny
NICKBAR87
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How do you check the solenoid once it is removed to make sure it is still working correctly?
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Tuesday, March 13th, 2018 AT 1:17 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
You need to ohm out the two terminals with an ohm meter.
You will also need to see if you have twelve volts on one of the wires to the solenoid with the key on.

Then you will need a scan tool to turn the solenoid on and off to be sure it can be activated.

Roy
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Tuesday, March 13th, 2018 AT 1:23 PM

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