General Description Of Secondary Air System Monitoring

After a cold start the secondary air pump starts and the secondary air valves open. Air is forced into the exhaust ports of the engine and mixes with the engine exhaust gases. This results in a lean mixture at the upstream oxygen sensor.

The upstream oxygen sensors will measure the exhaust lambda, which is used to calculate the secondary airflow rate.

The measured secondary airflow rate is compared with the secondary airflow rate model and the quotient is obtained and used as the relative secondary air volume. This volume is compared with the failure secondary air threshold.

After the secondary air phase, the fuel control system begins closed-loop operation. During this phase, the quality of the open-loop A/F ratio is checked to assure the accuracy of the diagnostic.

When the lambda control is between pre-set limits and the secondary airflow rate is measured to be too low, a failure of the secondary air system is diagnosed.