The difference between ratio and oxygen sensors

Although technicians perform diagnostics and install replacement components when an original part fails, DENSO believes it’s beneficial for you to understand the differences between similar parts. This helps ensure the correct component is supplied for the repair.

Despite the rise in popularity of pure electric powertrains, VMs are still under intense pressure to improve the ecological performance of their combustion engines to comply with ever stricter emissions targets. As a result, monitoring the engine’s exhaust emissions takes central stage and the air/fuel ratio sensor is a special type of oxygen sensor that can support more sophisticated emission control processes.

To comply with current and future emissions targets, engine management technology never rests, as OE component manufacturers, such as DENSO, continue to develop more sophisticated sensors to satisfy the needs of the VMs. As a result, the trusted oxygen – more commonly referred to as Lambda – sensor has had to be supported with the addition of many other sensors, including air/fuel ratio sensors.

Although both these sensors serve a similar purpose – to monitor the exhaust gases and communicate this data to the vehicle’s engine management system (EMS), to optimise the fuel and air mixture coming into the engine’s combustion chambers, as well as the engine’s ignition timing etc – the air/fuel ratio sensor provides a higher level of sensitivity than the traditional oxygen sensor. This allows the EMS to respond to the needs of the engine with greater accuracy, which enables the fuel to burn more efficiently, therefore producing fewer emissions and increasing the vehicle’s fuel economy.

This difference in sensitivity can be demonstrated by the fact that the oxygen sensor produces a voltage signal that suddenly changes on either side of the ‘Lambda ratio’ – defined as 14.7 parts air to one part fuel, which is the most efficient air/fuel mixture for a combustion engine – 0.8V if the mixture is too rich or 0.2V if it’s too lean.

In contrast, the signal output of an air/fuel ratio sensor is not a voltage but a current. This current, measured in mA, changes in proportion to the amount of oxygen left in the exhaust gases. In other words, the ECU can understand, not only if the mixture is rich or lean, but by precisely how much.

This quantitative measurement allows the EMS to respond to the immediate air/fuel mixture requirements of the engine more quickly and accurately. This provides a greater degree of control over the combustion process, and as a result, the efficiency of the catalytic converter can be improved significantly. Also, in case of a cold start, the engine can reach a closed loop control much quicker, which reduces the emission of unburnt hydrocarbons.

Request like-for-like

It goes without saying that the signals that these two types of sensors produce are very different from each other; therefore, they cannot be exchanged. In fact, there are many different varieties of air/fuel ratio sensors. Some differences are visible from the outside, whereas others vary electronically, but most differences are hidden inside the composition of the ceramic recipe or the protection coatings on the sensor’s elements. Therefore, it is very important that workshops request a replacement of the exact same OE design specifications when changing an air/fuel ratio sensor.

One further point to note: at DENSO, we have witnessed several examples of imitation sensors appearing on the market and our laboratory and vehicle tests of these products have revealed inferior performance levels and lifetime, as well as over-consolidation of vehicle applications, which would certainly lead to premature failure and reoccurring engine warning lights on the dashboard, should workshops fit one to a customer’s vehicle.