Mr Smith's 1.6-litre Toyota Corolla was using about 10l/100km in urban driving, well above the the 8l/100km it was meant to be using.
A trip to the garage showed the problem lay with a tiny component installed in the exhaust system that most people have never heard of, and its replacement could improve his car's fuel economy.
Total Automotive owner Kevin Telford said the lambda sensor, more commonly known as an oxygen or O2 sensor, was installed on most vehicles that ran on petrol.
The sensors make modern electronic fuel injection and emission control possible, measuring whether the air-fuel ratio in exhaust from a petrol engine is rich (with unburned fuel vapor) or lean (with excess oxygen).
If the sensor failed, the vehicle's catalytic converter could be damaged, fuel consumption could rise by "several litres per 100km" and mechanical problems could result.
"Most of the time, the car will run not too badly, but it [sensor failure] can greatly change the way the car performs and the fuel economy," Mr Telford said.
Southern Motor Court service manager Paul McMillan said a faulty sensor could be detected by an electronic scan, and more modern vehicles might light an alert on the instrument panel.
Mr Telford said he often had to explain the sensor's function when asking the car owner whether they want a $70 to $300 (depending on the type of sensor and the fuel injection application) replacement fitted.
Mr Smith had never heard of the sensor, but after doing some research he decided to get a replacement, hoping it would lower his car's running costs.
"It will be interesting to see how it improves it," he said.
