FIELD OF THE INVENTION: This invention relates to an automotive sensor. BACKGROUND OF INVENTION: Engines involve a large number of components with a limited service life. It can be difficult to judge when service life is near expiry and replacement of a component is required. For example, two or three wheeled automotive vehicles typically have an air cleaner or an air intake filter for cleaning the air before it enters the engine. Without an air cleaner or filter, dirt will enter the engine and degrade the engine over time. Normally, the air filter consists of a filtering element which restricts entry of dust/dirt particles and allows clean air to enter into engine. Many air cleaners and filters over a period of time due to dirt/dust entrapped or clogged into the filter or cleaning element, decrease the amount of air that can enter into the engine, thus causing the engine's power output to decrease. This also causes lower fuel mileage and higher emissions causing environmental pollution. Therefore it is important to replace the filtering element over a period of time. Known air intake filters include, foam, oiled foam, oiled fabric, and dry paper as filter element. Dust and dirt clog the pores in the filter element, decreasing the airflow and decreasing the engine performance as mentioned above. It is very difficult for the rider or owner of the vehicle to judge the performance and operability of the air filter. Also, it is difficult to remember the replacement frequency of air filter element as recommended by manufacturer. Even if rider checks the filter, it is difficult to judge whether the filter element is in good condition or it is required to be replaced. Hence the rider or owner of vehicle may continue with an inefficient filter clogged with dirt and dust thereby causing the above-mentioned problems or may replace the filter element thereby incurring unnecessary cost for it. The same sorts of problems may arise with other engine components or elements. OBJECT OF INVENTION : It is the object of the present invention to provide an automotive sensor which assists assessment of operability of engine components, such as air filters. STATEMENT OF INVENTION: With this object in view, the invention provides an automotive sensor for measuring operability of an engine element comprising: (a) a light transmitter for directing light at the engine element; (b) a light receiver for receiving light transmitted through, or reflected from, the engine element; (c) an electrical circuit for relating an amount of transmitted or reflected light to a degree of operability of the engine element; and (d) an output means connected to the electrical circuit; Preferably, the output means is an indicator means connected to the electrical circuit, wherein the indicator means provides a signal corresponding with the measured degree of operability of the engine element. Preferably, the output means provides signal to the electronic engine control system corresponding to the measured degree of operability of the engine element. Preferably, the electronic engine control system regulates the fueling rate, to correct the air fuel ratio for proper engine operation. Preferably, the fuel delivery system is a simple fuel injector, the operation of which is controlled by engine control system. The sensor may be applied to determination of degree of physical operability of the engine element. For example, the sensor may be applied to determine whether an air filter element has become too obstructed to operate effectively. The intensity of reflected light, rather than transmitted light, may be preferred as a basis for determining the degree of operability or serviceability of the engine element. For example, where the engine element is an air filter element, a clean filter may be reflective whereas a clogged or dirty filter is distinctively less reflective. This difference in the degree of reflected light, which may be converted to a voltage signal, allows determination to be made as to the degree of operability or serviceability of the air filter. If reflectance is used, light transmitter and emitter may be located on the same side of the engine element, for example, the air filter. This allows simpler and more compact arrangement. If light transmission is used instead, both emitter and receiver would need to be located on either side of the engine element. The electrical circuit preferably comprises a comparator to compare the values of light signals received by the light receiver, with a predetermined range of values and activate an indicating device observable by an operator in case of the value of said received signal exceeds or goes beyond a predetermined range of values. That is, the indicator device may provide information as to whether the engine element is operable or not operable. The indicator device may also provide indications as to the remaining service life of the engine element or the next likely replacement date. For example, the indicator device may take the form of a warning light on a vehicle dash panel that illuminates when degree of operability falls below an acceptable level. Alternatively, the indicator device may take the form of a light which remains illuminated as long as the sensor determines that the engine element is in serviceable condition. The indicator light may be coupled with an audible alarm for greater impact. The light receiver, of photosensitive material, may communicate with the comparator through at least one amplifying means. The comparator may be pre-calibrated with respect to color of a clean air filter element and color of a clogged filter element due for replacement, these colours being related to different levels of light reflectance. The circuit may be arranged to correlate the amount of light received to the degree of operability of the engine element along a continuum of the comparator. -The transmitter may be an infrared light emitter, such as a light emitting diode, and the receiver may be a photo sensitive receiver. -Preferably, the transmitter and receiver are located in one unit though in different compartments, each being provided with a guiding channel. However, the transmitter and receiver may be located in different vertical planes to prevent cross - signaling and interference. The automotive sensor may be mounted on a housing of the engine element. For example, the sensor may be mounted on the air filter housing. Advantageously, the housing in which the sensor is to be mounted and/or the particular orientation of the sensor, is selected such that if the engine element is not present, a low operability signal is generated by the sensor. In the example of the engine element being an air filter, if the air filter was accidentally removed or left out of its housing, the colour of the housing and/or the location of the senor would be such that a low reflectivity signal (that is, a fully clogged or blocked filter) was generated and thereby alert the operator to a problem. Further, the engine element being an air filter, may be provided with a wire mesh of suitable colour distinguished from that of the air filter element, so that the sensor would advantageously measure the operability of the engine element. While the sensor is located to determine the operability of one engine element, the signals from the sensor may be used to diagnose other possible engine operating conditions and problems of serviceability of other engine components. For example, early clogging of an air filter may perhaps indicate problems with other engine components. The output means may provide a signal from the automotive sensor to be used as a control signal which may be in turn be used as an input for a control unit for an engine. Sensing of the operability of the engine component, for example an air filter, may - by providing early warning of potential failure or un-serviceability - prevent engine malfunctions or performance degradation. In another aspect of the invention, there is provided a method for measuring operability of an engine element comprising the steps of: (a) directing light at the engine element from a light transmitter; (b) receiving light transmitted through, or reflected from, the engine element by a light receiver; (c) measuring the amount of transmitted or reflected light; and (d) relating the amount of transmitted or reflected light to a degree of operability of the engine element; and providing a signal output corresponding with the measured degree of operability of the engine element. The method may be applied to determine operability of a number of engine elements. In one embodiment, the method is applied to determination of the operability of an air filter. The method may be used as part of an engine control strategy or as a diagnostic to alert a vehicle operator to potential engine malfunctions. In this way, the operator may take requisite service action to avoid the malfunction. The signal output may be used as an input to an engine management system. Fueling computations in a typical engine management system are based on certain necessary or essential inputs, some of which are: 1. Throttle position and engine speed or 2. Throttle position and intake manifold pressure together with engine speed or 3. Intake manifold pressure and engine speed. It is possible to achieve further refinements and accuracy based on inputs such as intake air temperature, engine temperature, roll over sensor, air pressure sensor or air fuel ratio sensor. In the present embodiment of the invention, the inputs from the sensor which will vary depending upon the condition of the air filter foam, may be used to further refine fuelling rate. This will be useful in typical engine control systems where the calibration for fuelling rate based on intake manifold air pressure and engine speed cannot be fine tuned owing to memory or software constraints. While the engine control system computes interpolation for the intermediate calibration points, the input from the sensing element, as described in this invention, may be used in refining the fuelling rate. Thus, as signal varies with operability of the engine element, an engine management strategy may be correspondingly vahed. For example, as an air filter becomes contaminated, it may lead to a reduced air flow rate to the engine and to an over rich air fuel ratio. Thus the fueling rate delivered to the engine may be reduced to correct the air fuel ratio for proper engine operation. The automotive sensor and methods of the invention are applicable to a variety of engines. It is applicable to air breathing internal combustion engines whether used in 2, 3 or 4 wheeled vehicles. The reference to the "light" is intended to include a reference to visible and/or non-visible light. DETAILED DESCRIPTION OF INVENTION: The following is detailed description of a preferred embodiment of the invention made with reference to the accompanying drawings, in which: Figure 1 illustrates a block diagram of a sensor for sensing operability of an air filter element according to one embodiment of the invention. Figure 2 illustrates a mounting arrangement of sensing/detecting device for sensing operability of the air filter element. Figure 3 illustrates details of sensing elements of the sensor device of Figure 2. Figure 4 illustrates a block diagram of a sensor for feeding output to an engine control system to regulate fueling rate. Referring to Figure 1, there is shown an automotive sensor 30 used to assess degree of operability of an air filter 40, comprising an air filter element 12. The sensor 30 uses infra red light as diagnostic medium. To this end, sensor 30 includes an infrared light transmitter 1 connected to a power supply source battery 2 through a power supply regulator 3. Transmitter 1 may be in the form of a light emitting diode (LED). A regulator 3 may be provided with a short circuit protection device. The infrared transmitter 1 is also connected to a pulse generator device 4 to emit infrared signals intermittently at a predetermined frequency. An infrared photosensitive receiver 5 is connected to a signal shaping circuit 6, which shapes the signal to a square wave eliminating spil