not a chevy expert, but if it goes into Failure Mode Strategy, that is difficult to diagnose, but usually an easy fix when you find it. The engine module very seldom is the cause.
here is what Ford book says:
On Board Diagnostics II System
The California Air Resources Board (CARB) began regulation of On Board Diagnostics (OBD) for vehicles sold in California beginning with the 1988 model year. The first phase, OBDI, required monitoring of the fuel metering system, exhaust gas recirculation (EGR) system, and additional emission related components. The malfunction indicator lamp (MIL) was required to light and alert the driver of the malfunction and the need for service of the emission control system. The MIL must be labeled "CHECK ENGINE" or "SERVICE ENGINE SOON." Associated with the MIL was a fault code or diagnostic trouble code (DTC) identifying the specific area of the fault.
The OBD system was proposed by the CARB to improve air quality by identifying vehicles exceeding emission standards. Passage of the federal Clean Air Act Amendments in 1990 has also prompted the Environmental Protection Agency (EPA) to develop on board diagnostic requirements. CARB OBD II regulations will be followed until 1999 when the federal regulations will be used.
The OBD II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a DTC will be stored and the MIL will be illuminated.
Fault detection strategy and MIL operation are associated with trips and drive cycles. Each monitor has requirements for setting and clearing DTCs and for controlling the MIL. These processes, DTC and MIL operation, descriptions of the monitors and the definition of trip and drive cycles are discussed in detail within this section.
The diagnostic executive is the computer program in the powertrain control module (PCM) that coordinates the OBD II self-monitoring system. This program controls all the monitors and interactions, DTC and MIL operation, freeze frame data and scan tool interface.
Freeze frame data describes stored engine conditions, such as state of the engine, state of fuel control, spark, RPM, load, and warm-up status at the point the first malfunction is detected. Previously stored conditions will be replaced only if a fuel or misfire malfunction is detected. This data is accessible with the scan tool to assist in repairing the vehicle.
Powertrain Control Module
The center of the OBD II system is a microprocessor called the powertrain control module (PCM). The PCM has a single 88 Pin connector. The PCM receives input from sensors and other electronic components (switches, relays, etc.). Based on information received and programmed into its memory (keep alive memory [KAM], etc.), the PCM generates output signals to control various relays, solenoids and actuators.
Keep Alive Memory (KAM)
- The powertrain control module (PCM) stores information in keep alive memory (KAM), a memory integrated circuit, about vehicle operating conditions, and then uses this information to compensate for component variability. KAM remains powered when the vehicle ignition key is OFF so that this information is not lost.
Fail Safe - This system of special circuitry provides minimal engine operation should the powertrain control module (PCM), mainly the Central Processing Unit or EEPROM, stop functioning correctly. All modes of Self-Test are not functional at this time. Electronic hardware is in control of the system while in fail safe operation.
Fail Safe Condition
Idle Air Held To Full Open.
Fuel injection volume fixed according to driving conditions. Fuel is injected simultaneously into all cylinders once per crankshaft revolution. Timing for the injection is based upon the camshaft position sensor signal.
EGR Valve and EVAP Canister Control Solenoid Closed.
Ignition Timing Fixed.
High Fan Control Relay Energized.
Low Fan Control Relay Unenergized.
A/C Relay Unenergized.
Malfunction Indicator Lamp On.
Fuel Pump Control Relay Energized (Engine Running).
* Federal emissions
Adaptive Fuel Control Strategy
The adaptive fuel control strategy is designed to compensate for variability in the fuel system components. If, during normal vehicle operation, the fuel system is detected to be biased rich or lean, the adaptive fuel control will make a corresponding shift in the fuel delivery calculation.
Whenever an injector or fuel pressure regulator is replaced, keep alive memory (KAM) should be cleared. This is necessary so the fuel strategy does not use the previously learned adaptive values.
To clear KAM, refer to PCM Reset in Section 2B
, Diagnostic Methods.
Failure Mode Effects Management
Failure mode effects management (FMEM) is an alternate system strategy in the powertrain control module (PCM) designed to maintain vehicle operation if one or more sensor inputs fail.
When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value and continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the normal engine running strategy.