You will need to pull codes to to determine the possibilities. Here is how to pull codes.
When the Check Engine warning light has been reported on, turn the ignition on, move the front passenger seat to the rear position and observe the LED on the front of the ECU. The LED indicates a system failure code by blinking frequency. The ECU LED can indicate any number of simultaneous component problems by blinking separate codes, one after another.
Diagnostic Trouble Codes - 1991
1 Primary Oxygen Sensor (HO2S) (Sensor 1).
2 Rear Heated Oxygen Sensor.
3 Manifold Absolute Pressure Sensor (MAP).
4 Crankshaft Position Sensor (CKP).
5 Manifold Absolute Pressure Sensor (MAP).
6 Engine Coolant Temperature (ECT Sensor).
7 Throttle Position Sensor.
9 No. 1 Cylinder Position Sensor (CYP).
10 Intake Air Temperature (IAT) Sensor.
12 Exhaust Gas Recirculation (EGR) System.
13 Barometric Pressure (BARO) Sensor
14 Idle Air Control (IAC) Valve.
15 Ignition Output Signal.
16 Fuel Injector.
17 Vehicle Speed Sensor (VSS).
18 Ignition Timing Adjustment.
23 Left Knock Sensor (KS).
41 Left Heated Oxygen Sensor (HO2S) Heater.
42 Right Heated Oxygen Sensor (HO2S) Heater.
43 Left Fuel Supply System.
45 Left Fuel Metering.
46 Right Fuel Metering.
53 Right Knock Sensor (KS).
Wednesday, April 14th, 2010 AT 6:08 PM