Update 1/26:
We now offer a plug-and-play wideband controller, which you can find in our store here: https://vasttuning.com/product/plug-and-play-wideband-controller
Wideband lambda control (sometimes called wideband regulation) is when a wideband sensor’s signal is used by the ECU to control closed-loop fueling. M4.3/M4.4 equipped cars—such as the 850 and the 1998 V70, C70, XC70, and S70—came from the factory with a narrowband oxygen sensor. The purpose of the oxygen sensor is to provide feedback to the ECU about the air-fuel ratio (AFR) the engine is running.
In closed loop, the ECU constantly compares the sensor feedback to a target mixture and makes small real-time corrections to injector pulsewidth to hit that target. This is how the ECU compensates for differences in fuel quality, small air leaks, injector flow variation, sensor aging, and other real-world deviations that would otherwise make the engine run lean or rich.
A narrowband sensor can only reliably tell the ECU whether the AFR is leaner or richer than 14.7:1. 14.7:1 is the stoichiometric ratio for gasoline (often shortened to “stoich”), and it’s the desired AFR at idle and cruising because it results in the lowest emissions and allows the catalytic converter to work efficiently.
However, when you get heavy on the throttle and boost rises, the desired AFR is much richer (more fuel), typically around 11.7:1. One reason for this is charge cooling: as fuel evaporates (changes phase from liquid to vapor), it absorbs heat and lowers the in-cylinder charge temperature. Under boost and heavy load, a cooler charge reduces the likelihood of knock (uncontrolled combustion), which is one of the fastest ways to damage a turbocharged engine. So at full throttle, we intentionally command a richer AFR.
With the factory narrowband oxygen sensor, the ECU is essentially “blind” at these richer AFRs. In normal closed-loop operation, the ECU actively targets ~14.7:1 and makes frequent small adjustments to maintain that target. But because a narrowband sensor can’t accurately report how rich (or how lean) you are once you move away from stoich, the ECU typically stops using narrowband feedback at high load and full throttle (open loop) and relies instead on the fuel tables.
With wideband lambda regulation, the ECU can know the actual mixture across a wide range of AFRs—not just “richer than stoich” or “leaner than stoich.” That means it can continue making corrections even when running richer mixtures under boost. This makes tuning inherently safer, because the ECU can add fuel on the fly to help counteract a lean condition. It also improves drivability: the factory narrowband sensor is relatively slow to respond, while a wideband is much quicker. With faster, more accurate feedback, fueling corrections can be more responsive, leading to a more stable AFR and a smoother driving car.
Interfacing a wideband sensor with M4.4 is, fortunately, quite straightforward. Most wideband gauge/controllers on the market include an “analog voltage output.” The controller reads the wideband sensor, calculates AFR (or lambda), displays it on the gauge (if equipped), and also outputs a corresponding 0–5V signal on the analog output wire. On M4.4, the ECU’s oxygen sensor input circuitry is capable of being repurposed to accept and interpret a 0–5V signal—meaning the wiring path that originally carried the narrowband signal can be reused for the wideband controller’s analog output.
Because a wideband sensor requires a dedicated control strategy (heater control, pump cell control, calibration, and signal processing), you cannot wire a wideband sensor directly to the ECU and expect it to work. You must use a wideband controller to run the sensor, then send the controller’s analog output to the ECU. The AEM X-Series 30-0300 is one of the best consumer-grade wideband gauge/controllers available and is the wideband gauge we recommend.
If you’re wondering how it’s possible to interface a wideband with an engine management system designed long before widebands were common, it may help to think of an ECU input a little differently. Rather than thinking of the oxygen sensor wire explicitly as “the oxygen sensor wire,” think of it as a general-purpose, analog voltage input that the ECU just happened to have originally used to interpret a narrowband signal. In principle, any sensor or device that outputs a 0–5V signal can be read by the ECU—it’s “just” a matter of writing the correct software so the ECU knows how to convert voltage into a meaningful value.
The oxygen sensor input isn’t the only 0–5V channel the ECU can read. The M4.4 ECU has multiple 0-5v inputs – like the MAF, TPS, tank pressure sensor, outside temperature, barometric pressure and vertical acceleration sensor. In theory, the ECU code could be modified so that any of these channels could be used to receive a wideband signal.
Update 6/25
Unfortunately, newer AEM X-Series gauges may ship with an FAE brand oxygen sensor rather than a Bosch sensor. When the X-Series is used with the FAE sensor, the analog output can become inoperable. Since the analog output is how the gauge sends the wideband signal to the ECU, the gauge must be paired with a genuine Bosch LSU 4.9 sensor. The X-Series can also be purchased without a sensor: part number 30-0300NS.
When sourcing a Bosch LSU 4.9 sensor, be wary—there are a lot of Chinese knockoff sensors on the market. As a general rule, if the sensor is less than $65, it’s very likely to be counterfeit. The cheapest place to get a genuine Bosch sensor is often RockAuto: https://www.rockauto.com/en/moreinfo.php?pk=11750320
However, you can also get the same sensor at a local auto parts store. There are multiple LSU 4.9 part numbers with the correct connector but different wire lengths. If a local shop is out of stock of one part number, check whether they have another LSU 4.9 variant available.
