GM Prizm/Nova 1985-1993 Repair Guide

Diagnostic System 1988 VIN Code 5 Engine



The Check Engine light is the device providing communication between the Electronic Control Module (ECM) and the driver. The ECM controls the electronic fuel injection, the electronic spark control, the diagnostic function and the fail-safe or default function.

The ECM receives signals from various sensors indicating changing engine operating conditions. These signals are utilized by the ECM to determine the injection duration (amount of time each injector stays open) to maintain the optimum air/fuel ratio under all conditions. The conditions affecting the injector duration are:

Exhaust gas oxygen content
Intake air mass
Intake air temperature
Coolant temperature
Engine rpm
Electrical load
Air conditioning on/off

The ECM is programmed with data for optimum ignition timing under any and all operating conditions. Using data provide by the sensors, the ECM triggers the spark within each cylinder at precisely the right instant for the existing conditions.

The ECM detects any malfunctions or abnormalities in the sensor network and lights the Check Engine light on the dash panel. At the same time the trouble is identified by circuit and a diagnostic code is recorded within the ECM. This diagnostic code can be read by the number of blinks of the instrument light when both check engine terminals are shorted under the hood. For the VIN code 5 engine in 49-state trim, there are 13 different codes; California cars produce 14 codes.

In the event of an internal computer malfunction, the ECM is programmed with back-up or default values. This allows the car to run on a fixed set of rules for engine operation. Driveability may suffer since the driving conditions cannot be dealt with by the faulty computer. This back-up programming allows the computer to fail with out stranding the car, hence the nickname fail-safe. No computer is safe from failure, but a back-up system helps make the best of the situation.

With the exception of the oxygen sensor (discussed earlier in this Section) the testing and replacement of the various sensors is discussed in Fuel System .