Shift solenoid P0756 Shift Solenoid B (No. 2) Performance Or Stuck Off

you can check the transmission fluid if its ok and also the transmission pressure

DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient
Probably plugged passage's , not the EGR valve .
What type code reader are you using to read these codes ? I have factory service info. Doesn't show P0842 or P0642 , no such codes . You don't show what engine yours has , I'am assuming you have 3.5 L !
P0604
Control Module Random Access Memory (RAM)
PCM
DTC P0601, P0602, P0603, P0604, P0605, P0606, P0607, P1600, P1621, P1627, P1680, P1681, P1683, or P2610 in Engine Controls - 2.2L (L61)
or
DTC P0601, P0602, P0603, P0604, P0605, P0606, P0607, P1600, P1621, P1627, P1680, P1681, P1683, or P2610 in Engine Controls - 3.5L (LX9)

P0717
Input Speed Sensor Circuit Low Voltage
PCM
DTC P0717 in Automatic Transaxle - 4T40-E/4T45-E
P0741
Torque Converter Clutch (TCC) System - Stuck Off
PCM
DTC P0741 in Automatic Transaxle - 4T40-E/4T45-E
P0742
Torque Converter Clutch (TCC) System - Stuck On
PCM
DTC P0742 in Automatic Transmission - 4T40-E/4T45-E
P0748
Pressure Control (PC) Solenoid Control Circuit
PCM
DTC P0748 in Automatic Transmission - 4T40-E/4T45-E
P0751
1-2 Shift Solenoid (SS) Valve Performance - No First or Fourth Gear
PCM
DTC P0751 in Automatic Transmission - 4T40-E/4T45-E
P0752
1-2 Shift Solenoid (SS) Valve Performance - No Second or Third Gear
PCM
DTC P0752 in Automatic Transmission - 4T40-E/4T45-E
P0753
1-2 Shift Solenoid (SS) Control Circuit
PCM
DTC P0753 in Automatic Transmission - 4T40-E/4T45-E
P0756
2-3 Shift Solenoid (SS) Valve Performance - No First or Second Gear
PCM
DTC P0756 in Automatic Transmission - 4T40-E/4T45-E
P0757
2-3 Shift Solenoid (SS) Valve Performance - No Third or Fourth Gear
PCM
DTC P0757 in Automatic Transmission - 4T40-E/4T45-E
P0758
2-3 Shift Solenoid (SS) Control Circuit
PCM
DTC P0758 in Automatic Transmission - 4T40-E/4T45-E
P0815
Upshift Switch Circuit
PCM
DTC P0815 in Automatic Transmission - 4T40-E/4T45-E
P0816
Downshift Switch Circuit
PCM
DTC P0816 in Automatic Transmission - 4T40-E/4T45-E
P0826
Up and Down Shift Switch Circuit
PCM
DTC P0826 in Automatic Transmission - 4T40-E/4T45-E
P0973
1-2 Shift Solenoid (SS) Control Circuit Low Voltage
PCM
DTC P0973 in Automatic Transmission - 4T40-E/4T45-E
P0974
1-2 Shift Solenoid (SS) Control Circuit High Voltage
PCM
DTC P0974 in Automatic Transmission - 4T40-E/4T45-E
P0976
2-3 Shift Solenoid (SS) Control Circuit Low Voltage
PCM
DTC P0976 in Automatic Transmission - 4T40-E/4T45-E
P0977
2-3 Shift Solenoid (SS) Control Circuit High Voltage
PCM
DTC P0977 in Automatic Transmission - 4T40-E/4T45-E

Sometimes with code readers they get confused an just throw any old codes . Have it checked by someone with GM Tech 2 scan tool .

Hi,there could be a problem with the sift solinoid,if you look on u-tube there is a vidio there showing you how to test it,good luck hope this was some help.

Have you check fluid level ? Is your check engine light on ? You have a bad shift solenoid inside the transmission ,more then likely . Have codes checked ,you mite have P0700 + code

P0750 - Shift Solenoid A

P0751 - Shift Solenoid A Performance or Stuck Off

P0752 - Shift Solenoid A Stuck On

P0753 - Shift Solenoid A Electrical

P0754 - 1-2 Shift Solenoid Intermittent

P0755 - Shift Solenoid B

P0756 - Shift Solenoid B Performance or Stuck Off

P0757 - Shift Solenoid B Stuck On

P0758 - Shift Solenoid B Electrical

P0761 - Shift Solenoid C Performance or Stuck Off

Possible causes of OBD code P0756
- Low transmission fluid level - Dirty transmission fluid - Faulty shift solenoid B valve - Shift solenoid B valve harness or connectors - Shift solenoid B valve circuit is open or shorted The Error code is generally activated on detection of the following conditions: The P0756 code is detected when the ECM has detected a mechanical fault with the solenoid energized, valve stuck off hope this helps

The P0756 means Shift Solenoid "B" Performance or Stuck Off, and the P0758 means Shift Solenoid "B" Electrical. What you should do is clear the fault code with your tech 2, If the code still on, I think you should take your car to car repair shop.

P0700 Transm.Contr.System Malfunction 17084
P0702 Transm.Contr.System Electrical 17086
P0703 Torque Converter/Brake Switch B Circ Malfunction 17087
P0705 Transm.Range Sensor Circ.(PRNDL Inp.) Malfunction 17089
P0706 Transm.Range Sensor Circ Range/Performance 17090
P0707 Transm.Range Sensor Circ Low Input 17091
P0708 Transm.Range Sensor Circ High Input 17092
P0710 Transm.Fluid Temp.Sensor Circ. Malfunction 17094
P0711 Transm.Fluid Temp.Sensor Circ. Range/Performance 17095
P0712 Transm.Fluid Temp.Sensor Circ. Low Input 17096
P0713 Transm.Fluid Temp.Sensor Circ. High Input 17097
P0715 Input Turbine/Speed Sensor Circ. Malfunction 17099
P0716 Input Turbine/Speed Sensor Circ. Range/Performance 17100
P0717 Input Turbine/Speed Sensor Circ. No Signal 17101
P0721 Output Speed Sensor Circ Range/Performance 17105
P0722 Output Speed Sensor Circ No Signal 17106
P0725 Engine Speed Inp.Circ. Malfunction 17109
P0726 Engine Speed Inp.Circ. Range/Performance 17110
P0727 Engine Speed Inp.Circ. No Signal 17111
P0730 Gear Incorrect Ratio 17114
P0731 Gear 1 Incorrect Ratio 17115
P0732 Gear 2 Incorrect Ratio 17116
P0733 Gear 3 Incorrect Ratio 17117
P0734 Gear 4 Incorrect Ratio 17118
P0735 Gear 5 Incorrect Ratio 17119
P0740 Torque Converter Clutch Circ Malfunction 17124
P0741 Torque Converter Clutch Circ Performance or Stuck Off 17125
P0748 Pressure Contr.Solenoid Electrical 17132
P0750 Shift Solenoid A malfunction 17134
P0751 Shift Solenoid A Performance or Stuck Off 17135
P0752 Shift Solenoid A Stuck On 17136
P0753 Shift Solenoid A Electrical 17137
P0756 Shift Solenoid B Performance or Stuck Off 17140
P0757 Shift Solenoid B Stuck On 17141
P0758 Shift Solenoid B Electrical 17142
P0761 Shift Solenoid C Performance or Stuck Off 17145
P0762 Shift Solenoid C Stuck On 17146
P0763 Shift Solenoid C Electrical 17147
P0768 Shift Solenoid D Electrical 17152
P0773 Shift Solenoid E Electrical 17157
P0790 Normal/Performance Switch Circ Malfunction 17174

P0700 Transm.Contr.System Malfunction 17084
P0702 Transm.Contr.System Electrical 17086
P0703 Torque Converter/Brake Switch B Circ Malfunction 17087
P0705 Transm.Range Sensor Circ.(PRNDL Inp.) Malfunction 17089
P0706 Transm.Range Sensor Circ Range/Performance 17090
P0707 Transm.Range Sensor Circ Low Input 17091
P0708 Transm.Range Sensor Circ High Input 17092
P0710 Transm.Fluid Temp.Sensor Circ. Malfunction 17094
P0711 Transm.Fluid Temp.Sensor Circ. Range/Performance 17095
P0712 Transm.Fluid Temp.Sensor Circ. Low Input 17096
P0713 Transm.Fluid Temp.Sensor Circ. High Input 17097
P0715 Input Turbine/Speed Sensor Circ. Malfunction 17099
P0716 Input Turbine/Speed Sensor Circ. Range/Performance 17100
P0717 Input Turbine/Speed Sensor Circ. No Signal 17101
P0721 Output Speed Sensor Circ Range/Performance 17105
P0722 Output Speed Sensor Circ No Signal 17106
P0725 Engine Speed Inp.Circ. Malfunction 17109
P0726 Engine Speed Inp.Circ. Range/Performance 17110
P0727 Engine Speed Inp.Circ. No Signal 17111
P0730 Gear Incorrect Ratio 17114
P0731 Gear 1 Incorrect Ratio 17115
P0732 Gear 2 Incorrect Ratio 17116
P0733 Gear 3 Incorrect Ratio 17117
P0734 Gear 4 Incorrect Ratio 17118
P0735 Gear 5 Incorrect Ratio 17119
P0740 Torque Converter Clutch Circ Malfunction 17124
P0741 Torque Converter Clutch Circ Performance or Stuck Off 17125
P0748 Pressure Contr.Solenoid Electrical 17132
P0750 Shift Solenoid A malfunction 17134
P0751 Shift Solenoid A Performance or Stuck Off 17135
P0752 Shift Solenoid A Stuck On 17136
P0753 Shift Solenoid A Electrical 17137
P0756 Shift Solenoid B Performance or Stuck Off 17140
P0757 Shift Solenoid B Stuck On 17141
P0758 Shift Solenoid B Electrical 17142
P0761 Shift Solenoid C Performance or Stuck Off 17145
P0762 Shift Solenoid C Stuck On 17146
P0763 Shift Solenoid C Electrical 17147
P0768 Shift Solenoid D Electrical 17152
P0773 Shift Solenoid E Electrical 17157
P0790 Normal/Performance Switch Circ Malfunction 17174

I have a 1990 Buick lasaber where is the solenoid at

What was the codes exactly that Auto Zone gave you.??? it might just be a simple wire harness that may have gone loose since you did some work on the speed sensor.
Here are the trouble codes for your car.  Good Luck and get the car scanned again for a more define information.

P0751 Shift Solenoid A Performance or Stuck Off

P0752 Shift Solenoid A Stuck On

P0753 Shift Solenoid A Electrical

P0754 Shift Solenoid A Intermittent

P0755 Shift Solenoid B Malfunction

P0756 Shift Solenoid B Performance or Stuck Off

P0757 Shift Solenoid B Stuck On

P0758 Shift Solenoid B Electrical

P0759 Shift Solenoid B Intermittent

P0760 Shift Solenoid C Malfunction

P0761 Shift Solenoid C Performance or Stuck Off

P0762 Shift Solenoid C Stuck On

P0763 Shift Solenoid C Electrical

P0764 Shift Solenoid C Intermittent

P0765 Shift Solenoid D Malfunction

P0766 Shift Solenoid D Performance or Stuck Off

P0767 Shift Solenoid D Stuck On

P0768 Shift Solenoid D Electrical

P0769 Shift Solenoid D Intermittent

P0770 Shift Solenoid E Malfunction

P0771 Shift Solenoid E Performance or Stuck Off

P0772 Shift Solenoid E Stuck On

P0773 Shift Solenoid E Electrical

P0774 Shift Solenoid E Intermittent

P0780 Shift Malfunction