A narrow-band exhaust gas oxygen (EGO) sensor is very accurate at air/fuel ratios near 14.7:1 (for petrol). At richer or leaner are/fuel ratios there is a very small output signal change for large changes in air/fuel ratio. This makes the narrow-band sensor very good at detecting either a lean or rich condition, but not very useful for detecting how lean or rich. For this reason, a narrow-band sensor is not recommended for tuning purposes. However a narrow band sensor is suitable for running Closed Loop Lambda (CLL). CLL allows the ECU to self tune at cruise to improve economy and emissions.
There are many variations of EGO sensors although most are identical in terms of the output signal. The principal differences are mostly physical involving lead tolerance, heated or unheated, mounting methods, and whether or not a signal ground wire is supplied.
Lead Tolerance
The lead additives used in most high-octane fuel (aviation gas or race gas) will reduced the lifespan of the sensor considerably. Some probes are shielded and are more tolerant than others.
Heating
Many probes incorporate an electrical heating element, which is powered by the vehicles 12 Volt supply. These heaters allow the probe to be mounted in cooler portions of the exhaust system and significantly improve the probe performance at idle and during warm up phases of operation. This is because the probe temperature must exceed 300ºC before accurate readings are possible.
An Auxiliary Output can be used to control the heating of the oxygen sensor if required. To do this, ground the sensors heater wire through any of the ECUs auxiliary channels.
Mounting
Most probes have an M18 x 1.5 metric thread designed to screw into a mating boss. Some variants use a bolted flange arrangement, but these are relatively uncommon.
Almost all EFI engines will have a probe installed as original equipment in the exhaust manifold or turbo housing.
If the vehicle does not have a factory fitted EGO sensor, it will be necessary to manufacture a sensor mount according to engine type and layout.
The ideal mounting position of the sensor in the exhaust can vary depending on the application. Most of the time the preferred position is in the exhaust manifold collector on a naturally aspirated engine or after the turbocharger on a turbocharged engine. However, a location further down the exhaust is acceptable provided the probe is adequately heated. Note that it is also possible to get an EGO sensor too hot which also causes an inaccurate reading. Therefore in applications with particularly high exhaust gas temperatures (e.g. turbo engines, rotary engines) it may be necessary to either use an unheated sensor or move a heated sensor further down the exhaust.
Caution: EGO sensors use ceramic material internally and are susceptible to impact damage. Handle probes carefully to avoid impacts at all times.
Narrow Band Oxygen Sensor Wiring
Typically narrow-band EGO sensors can be recognised as having one, two, three or four wires. These have the following functions.
·Single Wire Sensor - The wire is the signal output and should be connected directly to an Analog Voltage Input.
·Two Wire Sensor - One wire for the signal output (to Analog Voltage Input). The other is the signal ground (Signal Ground (green)).
·Three Wire Sensor (Heated) - One wire for the signal output (to Analog Voltage Input). Two wires for the heater. One of the heater wires should be connected to an ignition switched 12V supply. The other heater wire can be connected to a convenient ground (or controlled using an auxiliary output). Heater polarity is not important.
·Four Wire Sensor (Heated) - As for three wire sensor, but with an extra wire for the signal ground which must be connected to Sensor Ground (green).
Recommended Narrow Band Oxygen Sensor
The recommended narrow-band EGO sensor is a Bosch 3-wire lead-tolerant unit with Part Number 0 258 003 070. As with other Bosch 3 wire sensors, the wire colours are:
2 white wires = heater (18 watts)
1 black wire = output signal