Minor and Major Engine systems faults

The following lists some of the possible combinations of exhaust gas values and the most likely causes.

 

CO CO2 HC O2

A/F Ratio : possible problems, conditions or causes

Low to Moderate Low Low to Moderate Low Major Engine Faults:
  • Compression Low,
  • Camshaft lobe lift insufficient
Low to Moderate Low Low to Moderate Low

Minor Engine Faults:

  • Ignition Timing over advanced

  • Spark Plug/s wire/s grounded or open

  • ECM trying to compensate for a vacuum leak

Low High Low High

Injector misfire, catalytic converter operating correctly

High Low High Low Thermostat or coolant temperature sensor faulty - "cold running engine"
Low High Low Low Thermostat or coolant temperature sensor faulty - "hot running engine"
Low Low Low High Exhaust leak after the catalytic converter
High High High High Combination of rich mixture and vacuum leak, Injector misfire, Cat. converter not working.
Low High Low Low Good combustion efficiency and catalytic converter working properly!

 

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