Thursday January 23 2014

What is wrong with my check engine light

I drive a Jeep Grand Cheerokee with a 4.0 litre petrol engine. Recently, my fuel consumption suddenly increased and the engine power has reduced as it hesitates when I accelerate. The engine fault light shows and will not go off even after I have just replaced the spark plugs, air cleaner and fuel filter. What should we do next?

Francis Kasule.

The 4.0 litre Jeep engine’s reduced performance, poor fuel economy and check engine light despite the engine service suggests that it has faulty oxygen sensors. Malfunctioning oxygen (O2) sensors are the most common cause behind a check engine light. Oxygen sensors have been fitted in almost every vehicle since 1981. They are designed to keep engine performance and efficiency at peak levels at all times while also keeping emissions in check. The Oxygen sensor is part of the emission control system fitted in the exhaust pipe. A petrol powered engine relies on a precise ratio of air and fuel to burn efficiently.

After combustion, the O2 sensor detects how much unburned oxygen is present in the exhaust gases to determine if the mixture was too rich or too lean and adjusts accordingly. The relay of information from the sensor to the computer is a result of a chemical reaction of the sensor which generates an electrical signal relayed to the engine computer. When there is too much air in the mixture, it is too lean, and too little air leads to a rich mixture. When the O2 sensor reports a lean mixture, the computer alters the ratio to allow more fuel.

This changes the ratio to be richer, and the sensor will indicate the switch, causing the ratio to change back again. This constant monitoring and adjustment enables the catalytic converter to reduce emissions as much as possible. When the oxygen sensor fails, the car’s engine management computer is forced to guess the appropriate mixture ratio, which can lead to inefficiency (higher fuel consumption) and poor performance. A computer diagnosis at any car diagnostic garage will confirm this. You may have to import this sensor if it is not available at the nearest Jeep parts dealer.