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Eric, Automotive Repair Shop Manager
Category: Buick
Satisfied Customers: 29723
Experience:  20+ yrs. experience as repair shop manager and technician.
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Why does the Service Engine Soon light come on and off?

Resolved Question:

Why does the "Service Engine Soon" light come on and off?
Submitted: 7 years ago.
Category: Buick
Expert:  Eric replied 7 years ago.



What is the year and model of your Buick? Any problems with how the engine runs right now?

Customer: replied 7 years ago.
1991 Buick Century 4 cylinder- and the car has been very dependable until that light started coming on.
Expert:  Eric replied 7 years ago.



When you say the car has been very dependable until that light started coming on - do you mean that the engine is acting up since the light started coming on, or just that the light itself is coming on?

Customer: replied 7 years ago.
I'm sorry, no it is still running good, i just am worried about the light. I have checked all the fluids, but still coming on.
Expert:  Eric replied 7 years ago.



The service engine soon (SES) light only comes on when the on-board computer detects a fault in engine driveability or performance, transmission control system or transmission, or the emissions system, and a fault code is set and stored in the computer. Faults that can set the SES light can range from a vaccuum leak, engine mis-fire, low engine compression, faulty sensor such as O2 sensor or other engine sensor, hydraulic pressure loss in transmission, transmission shift solenoid, open/short in wiring harness, emission evaporative system leak, or one of dozens upon dozens of reasons. Not all them will cause outward symptoms.


The only way to find out what is setting off the SES light is to read the onboard computer for stored DTCs(diagnostic trouble codes). On a 1991 Buick this can be done with either a diagnostic scanner at a shop, or you can retreive the codes yourself.


Below is how to retreive the codes yourself and what they mean:


Diagnostic Trouble Codes - 1991

13 Open Oxygen Sensor Circuit
14 Coolant Temperature Sensor (High Temp Indicated)
15 Coolant Temperature Sensor (Low Temp Indicated)
16 System Voltage High
17 RPM Signal Problem
21 Throttle Position Sensor (Signal Voltage High)
22 Throttle Position Sensor (Signal Voltage Low)
23 Manifold Air Temperature (MAT) Circuit (Low Sig.)
24 Vehicle Speed Sensor (VSS) Circuit
25 Manifold Air Temperature (MAT) High Temp Indicated
26 Quad-Driver (QDM) Circuit
31 Park/Neutral Switch Circuit
34 Mass Air Flow Sensor (MAF) (GM/SEC Low)
36 Transaxle Shift Control Problem
38 Brake Input Circuit
39 Torque Converter Clutch (TCC) Circuit
42 Electronic Spark Timing (EST) Circuit
43 Electronic Spark Control (ESC) Circuit
44 Oxygen Sensor Circuit (Lean Exhaust Indicated)
45 Oxygen Sensor Circuit (Rich Exhaust Indicated)
46 Power Steering Pressure Switch (PSPS) Circuit
48 Misfire Diagnosis
51 Mem-Cal Error
58 Personal Automotive Security System Circuit.
61 Cruise Vent Solenoid Circuit.
62 Cruise Vacuum Solenoid Circuit.
65 Cruise Servo Position Circuit.
66 Low A/C Refrigerant Charge.
67 Cruise Switches Circuit.
68 Cruise System Problem.
69 A/C Head Pressure Switch Circuit.

To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.

Ground terminals A and B with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.

After 12 is displayed, any stored trouble s will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

ALDL Connector (Located Underdash)

The ALDL diagnostic connector is located Front seating area, driver side, under dash, mounted near fuse panel



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