Error Codes: C0900, and C0040

have the OBD scanned for any fault codes

, that code is set off when there is various electrical problems in the vehcile and won't clear until the issue is resolved. Hunting down electrical gremlins is the worst.
these links should
assist https://www.autocodes.com/c0900.html this one has videos and manuals to assist http://c0900.enginecodes.net/ GMC Truck Electrical Error Fix

Do yourself a favor an take it to a qualified repair shop . You need a factory scan tool to see these code's
DTC C0040 00 : Right Front Wheel Speed Sensor Circuit No Additional Information
DTC C0040 0F : Right Front Wheel Speed Sensor Circuit Erratic Signal
DTC C0040 18 : Right Front Wheel Speed Sensor Circuit Signal Amplitude Less Than Minimum

DTC C0899: Device Voltage Low
Circuit/System Description
The electronic brake control module (EBCM) monitors the battery voltage level available for system operation. A low voltage condition prevents the system from operating properly.

Circuit/System Testing
Measure the voltage at the battery terminals.
With scan tool, read the battery voltage in the EBCM data list.
? If less than 10 volts, test the ground circuit of the EBCM for high resistance. If the circuit test normal, test or replace the EBCM.
Go to Battery Inspection/Test and Charging System Test .

It is the right front (RF) ABS sensor that this codes corresponds to nothing to do with the rear.. Are you using a GM TECH 2 scanner? If not that is the problem.

What was the original code ? Did it have the work circuit anywhere in the code description ? Circuit low or circuit high ? An what are the seven codes you pulled now?

Low voltage code. Since you don't have the other codes associated with it. I would suspect a wiring issue to the SRS system. Needs to go on a scanner to locate the problem area.

c0040 indicates right front wheel sensor, so problem is more than likely sensor or wiring to it. c0245 indicates wheel speed sensor frequency error. This confirms wheel sensor or wiring.

Hello: You need to have your car's ads system interrogated with a scanner to see what codes show, then from the codes you can diagnose and repair the problem.

Wow these are hard to find!!

Diagnostic codes for 1987 Pontiac

13 Open Oxygen Sensor Circuit
14 Coolant Temperature Sensor (High Temp Indicated)
15 Coolant Temperature Sensor (Low Temp Indicated)
21 Throttle Position Sensor (Signal Voltage High)
22 Throttle Position Sensor (Signal Voltage Low)
23 Manifold Air Temperature (MAT) Low Temp Indicated
24 Vehicle Speed Sensor (VSS) Circuit
25 Manifold Air Temperature (MAT) High Temp Indicated
32 Exhaust Gas Recirculation (EGR) Vacuum Control
33 Mass Air Flow Sensor (MAF) (GM/SEC High)
34 Mass Air Flow Sensor (MAF) (GM/SEC Low)
41 Cam Sensor Circuit
42 C3-I EST or Bypass Circuit Failure
43 Electronic Spark Control (ESC) Circuit
44 Oxygen Sensor Circuit (Lean Exhaust Indicated)
45 Oxygen Sensor Circuit (Rich Exhaust Indicated)
51 PROM Error
52 CALPAK Error
55 ECM Error
If you have your owners manual they should be in there. I couldn't find one online but did find this.

Future reference
http://autorepair.about.com/library/ts/obd-i/bl-dtcs-28.htm?once=true&

I hope these helped you

Hello, It should actually be an oil sending unit, and is located on the back of the intake manifold to the left of the distributor. *The P0523 is Oil Pressure Sending Unit Voltage too high *P1693 Generic fault indicating a fault in the bus or PCM/ECM interface *P1682 - I can't find this code Anyways, I will post here all the error codes for future reference. Dodge Cummins Check Engine Error Codes Cycle Key on-off-on-off-on to read codes in odometer display P0002 ECM fuel calibration error P0016 Crankshaft/Camshaft Timing Misalignment/Out-of- Sync P0071 Ambient Temperature Sensor Performance / Rationality P0072 Ambient Temperature Sensor Low P0073 Ambient Temperature Sensor High P0088 Fuel Rail Pressure Too High P0106 Manifold Absolute Pressure Sensor Performance / Rationality P0107 Manifold Absolute Pressure Sensor Low P0108 Manifold Absolute Pressure Sensor High P0111 Intake Air / Charge Temperature Sensor Performance / Rationality P0112 Intake Air / Charge Temperature Sensor Low P0113 Intake Air / Charge Temperature Sensor High P0116 Engine Coolant Temperature Sensor Performance / Rationality P0117 Engine Coolant Temperature Sensor Low P0118 Engine Coolant Temperature Sensor High P0121 Accelerator Position Sensor (APPS) Signal Voltage Too Low P0122 Accelerator Position Sensor (APPS) LowAPPS Voltage Input Too Low P0123 Accelerator Position Sensor (APPS) Signal Voltage Too High P0128 Cooling System Error / Thermostat Rationality P0148 Fuel Delivery Error P0168 Fuel Temp Too High P0169 Water-in-Fuel Detected For Too Long P0192 Fuel Rail Pressure Sensor Low P0193 Fuel Rail Pressure Sensor High P0201 Fuel Injector 1 Circuit P0202 Fuel Injector 2 Circuit P0203 Fuel Injector 3 Circuit P0204 Fuel Injector 4 Circuit P0205 Fuel Injector 5 Circuit P0206 Fuel Injector 6 Circuit P0216 Fuel Injection pump Timing Malfunciton [new injector pump time] P0217 Coolant Temperature Too High P0219 Engine Overspeed P0230 Transfer pump circuit out of range P0234 Turbocharger Overboost Condition P0235 Turbocharger Boost Sensor A Circuit P0236 Turbocharger Boost Pressure Sensor Performance / Rationality P0237 Turbocharger Boost Pressure Sensor Low P0238 Turbocharger Boost Pressure Sensor High P0251 Fuel Injection Pump Mechanical Failure Fuel Valve Feedback Circuit P0252 Injection Pump Fuel Metering Control "A" Range/Performance P0253 Fuel Injection Pump Fuel Valve Open Circuit P0254 Fuel Injection Pump Fuel Valve Current Too High P0300 Multiple Cylinder Misfire P0301 Cylinder 1 Misfire P0302 Cylinder 2 Misfire P0303 Cylinder 3 Misfire P0304 Cylinder 4 Misfire P0305 Cylinder 5 Misfire P0306 Cylinder 6 Misfire P0335 Crankshaft Position Sensor Circuit P0336 Crankshaft Position Sensor Performance P0337 Crankshaft Position Sensor Circuit Low P0338 Crankshaft Position Sensor Circuit High P0340 Camshaft Position Sensor Circuit P0341 Camshaft Position Sensor Performance / Rationality P0370 Timing Reference High Resolution Signal "A" P0380 Intake Air Heater #1 Control circuit P0381 Wait-to-Start Lamp Inoperative P0382 Intake Air Heater #2 Control circuit P0460 Fuel Level Sensor A Circuit P0461 Fuel Level Sensor 1 Performance / Rationality P0462 Fuel Level Sensor 1 Low P0463 Fuel Level Sensor 1 High P0477 Exhaust Pressure Control Relay Low P0478 Exhaust Pressure Control Relay High P0480 Cooling Fan 1 Control Circuit/Open (Low Speed Fan Relay) P0483 Cooling Fan Speed P0500 Vehicle Speed Sensor 1 Circuit, or No Vehicle Speed Reference Signal 1 P0514 Battery Temperature Sensor Performance / Rationality P0516 Battery Temperature Sensor Low P0517 Battery Temperature Sensor High P0521 Oil Pressure Sensor Performance / Rationality P0522 Engine Oil Pressure Sensor/Switch Circuit Low Input P0523 Oil Pressure Sending Unit Voltage too high P0524 Oil Pressure Too Low P0532 A/C Pressure Sensor Low P0533 A/C Pressure Sensor High P0541 Intake Air Heater Relay 1 Low P0542 Intake Air Heater Relay 1 High P0562 Battery Voltage Low P0563 Battery Voltage High P0572 Brake Switch 1 Low P0573 Brake Switch 1 High P0576 Cruise Control Input Circuit Low P0580 Speed Control Multiplexed Switch 1 Low P0581 Speed Control Multiplexed Switch 1 High P0602 Control Module Programming Error/NotProgrammed P0604 Internal Control Module RAM Integrity Failure P0606 Internal ECM Processor P0607 ECU Internal Performance P0622 Generator Field Control Circuit P0628 Fuel Pump Relay Circuit Low, or Fuel Lift Pump Control Circuit Low P0629 Fuel Pump Relay Circuit High, or Fuel Lift Pump Control Circuit High P0630 VIN Not Programmed in PCM P0633 SKIM Key Not Programmed in PCM P0642 Sensor Reference Voltage 1 Circuit Low P0643 Sensor Reference Voltage 1 Circuit High P0646 A/C Clutch Relay Circuit Low P0647 A/C Clutch Relay Circuit High P0652 Sensor Reference Voltage 2 Circuit Low P0653 Sensor Reference Voltage 2 Circuit High P0698 Sensor Reference Voltage 3 Circuit Low P0699 Sensor Reference Voltage 3 Circuit High P0700 Transmission Control System (MIL Request) P0743 Torque Converter Clutch Circuit Electrical P0753 Shift Solenoid "A" Electrical P0837 Four Wheel Drive (4WD) Switch Performance / Rationality P0838 Four Wheel Drive (4WD) Switch Circuit Low P0839 Four Wheel Drive (4WD) Switch Circuit High P1488 Auxiliary 5 volt supply output is too low P1492 Ambient/batt temp sensor volts too high P1493 Ambient/batt temp sensor volts too low P1596 Speed Control Switch always high P1597 Speed Control Switch always low P1652 J1850 Communication Bus P1687 No Cluster BUS Message or No Messages from the Mechanical Instrument Cluster P1688 Internal Fuel Injection Pump Controller Failure P1689 No Communication Between ECM & Injection Pump Module P1690 Fuel injection pump CKP Sensor Does Not Agree With ECM CKP Sensor P1691 Fuel Injection Pump Controller Calibration Failure P1693 Generic fault indicating a fault in the bus or PCM/ECM interface P1757 Governor pressure is above 3 PSI when 0 PSI is requested P1762 Governor Press Sensor Offset Volts Too Low or Too High P1765 Transmission 12 volt supply relay control circuit P2121 Pedal Position Sensor 1 Performance / Rationality P2122 Pedal Position Sensor 1 Low P2123 Pedal Position Sensor 1 High P2127 Pedal Validation Switch 2 Low P2128 Pedal Validation Switch 2 High P2146 Fuel Injector Group 1 Supply Voltage Circuit P2149 Fuel Injector Group 2 Supply Voltage Circuit P2266 Water in Fuel Sensor Circuit Low P2269 Water in Fuel Condition P2502 Charging System Output Voltage P2503 Charging System Output Low P2504 Charging System Output High P2509 ECM/PCM Power Input Signal Intermittent P2607 Intake Air Heater Relay 2 Low P2608 Intake Air Heater Relay 2 High P2609 Intake Air Heater System Performance Hope this will help you. Cheers! Ryan