862 Secondary Air Injection

The aim of secondary air injection into the exhaust system is to reduce CO and HC emissions in the period after start-up and during the warm-up phase. The extra oxygen that is introduced assists the catalytic converter with the oxidation process. Figure 8.17 shows a system used by Volvo cars.

Sailor Ditty Bag Pattern
Fig. 8.17 Secondary air injection system

With reference to Fig. 8.17, the solenoid valve (4) is under the control of the ECM, as is the relay (2) that provides the electrical power for the air pump (3). When the ECM starts or cuts off the secondary air, the solenoid valve (4) controls the application of manifold vacuum to the combined air and check valve (6) which opens and closes the valve that passes air to the exhaust injection port at (7). The secondary air flow is monitored by the pre-catalyst oxygen sensor and if the flow rate is not within the limits set by the manufacturer, a fault will be recorded.

The following precis of the self-diagnostic routine for the ECM of the Volvo system gives an insight into the diagnostic power of modern systems.

1. The idling and part load fuel functions are inhibited and the EVAP valve is closed. The ECM checks the signal from the oxygen sensor and if this shows an unchanging maximum value, the air pump is running continuously and the secondary air valve is leaking. This will cause a fault code to be recorded for the pump and valve.

2. The secondary air valve is closed and the air pump is started. The oxygen sensor signal (as determined by the ECM) should remain steady. If the oxygen sensor signal (at the ECM) exceeds a certain value within 6 s, the secondary air valve is leaking and a fault code for this valve will be recorded.

3. The secondary air pump runs continuously and the secondary air valve is opened. In this case the oxygen sensor signals (as assessed by the ECM) should exceed a specified limit within 6 s. If this does not happen, the secondary air pump is not running or the secondary air valve is not opening. This will cause the ECM to record a fault code for the secondary air pump.


Whenever the ECM detects the conditions that cause it to record a diagnostic trouble code (fault code), the sensor readings (variables) and the vehicle operating conditions that exist at the time of the occurrence of the fault, are stored in a section of ECM memory. The format in which this information is presented at the diagnostic tool is known as a 'freeze frame'. The information contained in the freeze frame is read out by the scan tool and is of use in diagnosis. Freeze frames can be overwritten if a diagnostic trouble code of higher priority, in the same system, occurs later.


One of the problems that has confronted independent garages that wish to repair and maintain vehicles equipped with computer controlled systems, has been the lack of information about diagnostic trouble codes (fault codes). To some extent this lack of DTC information is overcome by the companies listed in the Appendix who publish volumes of DTCs for many different types of vehicles. A quick glance through these books will show that there is a lack of uniformity in the codes and yet they are effectively reporting faults on components that are identical (or very similar) on many vehicles.

OBD II overcomes this problem because it stipulates the use of standardized fault codes. It achieves this standardization by making use of SAE standards such as SAE J 1930 and SAE J 2012. The SAE J 1930 standard provides a system for naming the component parts of a computer controlled automotive system and SAE J 2012 details the DTC descriptions. It is understood that the European OBD standards for DTCs are likely to be very similar to the SAE ones and a sample of these is given in the Appendix.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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