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P0721 code problems

Tiny
THATGEODUDE
  • MEMBER
  • 1999 SATURN SL1
  • 1.9L
  • 4 CYL
  • AUTOMATIC
  • 170,000 MILES
Hello, I have been having problems with my car recently and have a few questions.

My speedometer has been giving me problems whenever I start to slow down to around twenty to thirty mph, it will start to fluctuate like crazy and my car does a slight jump, as if the transmission is not changing gears properly.

I used my dad's scanner and the P0721 code popped up saying "output speed sensor out of range" I want to know what the problem is and how to fix it. I have replaced one of the speed sensors from under the car near the transmission but maybe I replaced the input speed sensor because the problem still persists.

Any advice, or even diagram of where I can find the output speed sensor would be great.

Thank you.
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Tuesday, August 22nd, 2017 AT 1:01 AM

12 Replies

Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Did you change the sensor by the screw on transmission filter or the one by your axle?
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Tuesday, August 22nd, 2017 AT 10:31 AM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
The one by your transmission filter is the input speed sensor and one by the axle is the output speed sensor both sensors are the same part just used for different inputs.
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Tuesday, August 22nd, 2017 AT 10:32 AM
Tiny
THATGEODUDE
  • MEMBER
  • 6 POSTS
I looked under my car earlier today and definitely saw a sensor near the filter. I did not change that one out at any point. The one I changed out must have been the one near the axle.

If that is the case why am I still getting the P0721 code?

Thank you for your replies.
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Tuesday, August 22nd, 2017 AT 1:02 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
I posted the trouble tree for the code that is for signal noise on the signal line for the speed sensor signal to the power-train control module. I have never seen that code on those cars in the over sixteen years I worked for Saturn. Let me know what you find following the trouble tree.
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Tuesday, August 22nd, 2017 AT 5:44 PM
Tiny
THATGEODUDE
  • MEMBER
  • 6 POSTS
I will be honest, my experience with a scanner is very minimal. Can only find and clear codes.

The output sensor that I had recently replaced a month ago came from a auto parts yard. Maybe it was faulty as well. I ordered a new one to come in tomorrow and I will see if that fixes it.

My uncle who is a mechanic said that if the new output sensor does not fix it that it is probably the transmission or the PCM.
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Tuesday, August 22nd, 2017 AT 8:46 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
I highly doubt it is the transmission and as far as the used sensor being bad doubt it is bad those sensors have a low failure rate. I only remember ever replacing those for leaking transmission fluid into the connector.
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Tuesday, August 22nd, 2017 AT 8:52 PM
Tiny
THATGEODUDE
  • MEMBER
  • 6 POSTS
If the new sensor does not fix it then what could be causing the code?
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Tuesday, August 22nd, 2017 AT 9:08 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Have you read the entire trouble tree I posted? It gives possible causes etc with some tips, PCM, ignition power, wires etc, causing electromagnetic interference. Come to think of it that has happened on those Saturn S models.
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Tuesday, August 22nd, 2017 AT 9:20 PM
Tiny
THATGEODUDE
  • MEMBER
  • 6 POSTS
Yes I did. I was not sure how to navigate the scanner to find the trans-axle information or a access the VSS. Could be that the scanner my dad has is old?

I apologise in advance for my lack of experience with a scanner.
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Tuesday, August 22nd, 2017 AT 9:33 PM
Tiny
SATURNTECH9
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What kind is the scanner your dad has? Brand model etc?
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Tuesday, August 22nd, 2017 AT 9:35 PM
Tiny
THATGEODUDE
  • MEMBER
  • 6 POSTS
Brand is Snapon.

Model is modis.
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Tuesday, August 22nd, 2017 AT 9:42 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
That may have some live data there the code is for noise in the output speed signal so your just going to have to follow that trouble tree.
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Wednesday, August 23rd, 2017 AT 1:06 PM

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