Wideband Resource page
Knowing your Air to Fuel ratio is very important in tuning an engine. This is valuable for any EFI system and even carbureted engines. Modern ECU's and standalone ECU's can run in "closed loop" where it actively changes the fueling based on what the O2 sensors are reading. There fore its important to have good data for the ECU to use.
We sell 2 kinds of Wideband Oxygen sensor's
Bosch 4.2 LSU - Floor It PN: S-WB12
An older design, but very robust. Uses a larger connector then the 4.9, though they are similar in shape. Threads into an M18x1.5 mm bung.
Bosch 4.9 LSU - Floor It PN: S-WB41
The newer variant of the Bosch LSU series. Also threads into an M18x1.5mm bung. This is now more commonly supported by aftermarket ecu's and controllers. AEM X series UEGO's, Haltech WB1/2, LINK CANLAM, Motec LTC, Holley Terminator, etc.
This is a widely contested topic, and there are a few factors that can determine where they will go.
Heat - many aftermarket brands advise installing them further down the exhaust, where the heat is less intense. In OEM applications these are pre-catalytic converter, meaning they are very close to the exhaust ports and with a tune as good as an OEM vehicle, emissions equipment and all these sensors will last a long time there. What fuel you're going to be using, how you're going to be using the car will determine what temperatures the exhaust gas can be expected to be. In my personal experience I haven't had much issues placing them fairly close to the collector or turbo.
Serviceability - Being able to get at the sensor or remove your downpipe/head without removing the sensor makes servicing the vehicle much easier. As with most things in automotive packaging be comes a concern and there's always a series of compromises to be made. You also don't want the sensor to be rubbing anything or wiring extremely close to a heat source.
Accuracy of Data - I have seen many headers with the O2 bung in a fairly comprised location where it will only pick up 2 or maybe even 1 cylinder's exhaust gas accurately. The sensor should be able to measure all 4 or 6 (V engines will likely need 2 sensors for complete accuracy) cylinders in the system. It should be after the merge collector, not in the middle of it.
It is always preferable to have the O2 sensor before a V band or flange joint and even before a flex tube. Reason is that any exhaust leaks before the O2 sensor will result in oxygen entering the system and the sensor will falsely read a lean condition. This can throw off a tune completely or a closed loop condition, leading to poor drivability and or bad fuel consumption.
Moisture - O2 sensors do not like water in them. Proper O2 control wont turn the sensor on until the engine has run for a bit to burn off all the moisture in the exhaust system. The controller should then enable the O2 heater to burn off the sensor itself before taking a reading as water can damage and damage a sensor. It is also important to mount the sensor in a way that water wont pool up or drip past the sensor when the engine is off. You would be surprised how much ambient moisture can get sucked into an exhaust when the tubing is cooling down after the engine running. Always have the sensor on the top half of the tube and at the lightest angle possible
Wiring - The connectors for both the 4.2 and 4.9 sensors are very well labeled on the connector itself. Look at the back near where the wires go in. See below for the excellent pinouts from ECUmaster for each sensor and compare this to your ECU's pinout documentation. Some ECU's will have a pin for 12V for the heater, some may not. Use a switched 12V if powering them externally from the ECU.
Its also important to note that the male connector attached to the sensor itself contains what's called a calibration resistor. O2 sensors are tested during manufacturing and the calibration resistor allows an ECU or controller to calibrate the reading correctly. If this connector or wiring behind it becomes damaged, replace the sensor do not cut out the connector.
Oxygen sensors are somewhat fragile. The tip doing the measuring is porcelain and if the sensor is dropped or shocked the porcelain can crack, rendering the sensor useless. Handle them with care.