Heated Oxygen Sensors (Ho2S)

The HO2S allow the ECM to measure the oxygen content of the exhaust gases, for closed loop control of the fuel:air mixture and for catalytic converter monitoring.

A pre catalyst HO2S is installed in the outlet of each exhaust manifold, which enables independent control of the fuel:air mixture for each cylinder bank. A mid catalyst HO2S is installed in a central position on the side of each catalytic converter and a post catalyst HO2S is installed in each catalytic converter outlet pipe in the exhaust system; these enable the performance of the catalytic converter to be optimized and monitored.

HO2S need to operate at high temperatures in order to function correctly. To achieve the high temperatures required, the sensors are fitted with heater elements that are controlled by a PWM (Pulse Width Modulation) signal from the ECM. The heater elements are operated after each engine start, once it has been calculated that there is no moisture in the exhaust (between 0 and 2 minutes delay), and also during low load conditions when the temperature of the exhaust gases is insufficient to maintain the required sensor temperature. The PWM duty cycle is carefully controlled to prevent thermal shock to cold sensors. A non-functioning heater delays the sensor's readiness for closed loop control and increases emissions.

The pre catalyst HO2S produce a constant voltage, with a variable current that is proportional to the lambda ratio. The mid and post catalyst HO2S produce an output voltage dependent on the ratio of the exhaust gas oxygen to the ambient oxygen.

The HO2S age with mileage, increasing their response time to switch from rich to lean and lean to rich. This increase in response time influences the ECM closed loop control and leads to progressively increased emissions. Measuring the period of rich to lean and lean to rich switching monitors the response rate of the pre catalyst sensors.

Diagnosis of electrical faults is continually monitored in pre, mid and post catalyst sensors. This is achieved by checking the signal against maximum and minimum threshold, for open and short circuit conditions.

If a HO2S fails:

• The ECM defaults to open loop fueling for the related cylinder bank
• The Carbon Monoxide (CO) and emissions content of the exhaust gases may increase
• The exhaust may smell of rotten eggs (hydrogen sulphide H2S).

With a failed HO2S, the engine will suffer from reduced refinement and performance.