More Info – Smog Pre Test

More Things to Know About Smog Tests


  • Gas Cap Leakage Detected
  • Catalytic Converter Failures
  • Drive Cycle Needed/Failures

Gas Cap Detected Leakage 

One of the common smog test failures is a  gas tank pressure error. The car’s computer  reads sensors to verify there is no gas tank pressure loss that could result in gas vapors being released into the environment. If it detects a loss of pressure, it trips the error code  which can be as a result of:

  • Leak at the gas cap or cap incorrectly closed.
  • Defective Gas tank Pressure sensor
  • Other possible leak in the gas tank or gas lines

Catalytic Converter Failures

Another expensive error code can occur if the Catalytic Converter shows incompletely burned gas escaping.

All cars are now equipped with Catalytic Converters. They are designed to increase to the quality of the air at the exhaust by doing a transformation of incompletely  burned gas when passing through the Catalytic Converter.

To work well, these converter needs to be hot and to verify their efficiency there is one Oxygen sensor before the converter and another one just after it. The car’s computer checks each sensor and computes the difference to assess the converter’s effectiveness. Failures can occur, for example, when:

  • The O2 sensor located in the flow before the converter is defective
  • The O2 sensor located after the converter is defective
  • The Catalytic convertor itself is defective.
  • The converter is not at the optimal temperature to be efficient.
  • There are an inadequate number of measurements stored. This is generally due to an incomplete “Driving Cycle”.

Drive Cycle Needed/Failures

The car’s computer requires a certain amount of driving in order to collect enough information to assess the vehciles operation. This process is called a drive cycle and generally (differs by vehicle) means allowing the car to:

  • Idle for a certain lengh of time
  • Driving the car while its warm
  • A certain amount of city driving
  • A certain amount of freeway driving (for example, 20 minutes of driving at or below 55 mph)

The two main reasons for requiring a new Driving Cycle are : the car’s computer error code has been reset or if the battery was disconnected or discharged recently.