The present application relates generally to oxygen sensor systems that are frequently found within the exhaust systems of automotive vehicles and, more particularly, to a dynamic malfunction detector for a heater within a Heated Exhaust Gas Oxygen ("HEGO") sensor assembly. Many automotive vehicles include an internal combustion engine and an exhaust system that provides a conduit for exhaust gas to move away from the engine. The temperature of the exhaust gases ranges from ambient temperature, when the engine has not been in operation recently, to 400.degree. Celsius ("C") or more.
A HEGO sensor assembly includes a sensing element and an associated pair of electrical output leads, as well as the heater. The sensing element is placed in the stream of exhaust gas passing through the exhaust system. The HEGO sensor then detects the oxygen level after equilibration and provides an electrical signal on the pair of output leads. The signal on the output leads may then be used, for example, by the vehicle's fuel delivery system to adjust the air/fuel mixture being provided to the combustion chambers of the vehicle's engine.
The HEGO sensor should detect the oxygen level in the exhaust gas, where the temperature of the gas varies over a wide range. To assist the HEGO sensor in making accurate measurements over a wide range of exhaust gas temperatures, a HEGO sensor assembly generally includes an electrical heater physically adjacent, or near, the HEGO sensor. When actuated, the electrical heater warms the HEGO sensor to enable it to make more accurate measurements and thus lower its sensitivity to the temperature of the exhaust gas.
Prior art systems exist for detecting faults in a HEGO sensor assembly. For example, U.S. Pat. No. 4,724,815, issued to Mieno et al., relates to a system of abnormality detection for an oxygen concentration sensor. The patent discloses a heater malfunction detection system for a Universal Exhaust Gas Oxygen ("UEGO") sensor assembly in which the detection of a malfunction of a heater is based, in part, upon a measurement of voltage between the electrodes of the UEGO sensor.
Since HEGO sensor assemblies are generally mass-produced and put on many cars, even a small savings on one part of the assembly can accumulate to substantial annual savings for a car manufacturer. Moreover, it is important that a HEGO sensor assembly, and the fault detection system within such an assembly, be reliable.
Unfortunately, many presently available systems require the use of additional components to measure a heater's effectiveness, thus increasing the cost and complexity of the HEGO sensor assembly. Other devices only indirectly determine whether the heater of a HEGO sensor assembly is functioning correctly.
Still other devices may not test the operation of the HEGO heater after the engine has been started and running for an extended period of time. As a result, a substantial delay may occur between the actual heater malfunction and the subsequent testing of the heater to detect the malfunction. Moreover, other detectors may spuriously sense a heater's operation and incorrectly signal that the heater is operating properly.