What a narrowband sensor tells you
A narrowband oxygen sensor only really answers one question: is the mixture richer or leaner than stoichiometric (about 14.7:1 AFR on gasoline)? Its output voltage swings sharply either side of that point and says very little about how rich or how lean. That's exactly what factory emissions systems need, because a catalytic converter works best when the mixture flips back and forth around stoich, and a narrowband sensor is perfect for driving that oscillation.
It's also why a narrowband sensor isn't a tuning tool. You can't use it to set a full-load mixture of 12.5:1, because it can't tell 12.5 from 11.5. For tuning, fit a wideband such as the WB1 CAN wideband controller kit and see our wideband O2 article.
On a Nexus you can use a narrowband sensor for monitoring and logging, or as the feedback for closed loop O2 control. The ECU accepts up to two narrowband inputs, and these settings apply to any narrowband sensor.
Settings in NSP
Main node
This sets the top of the voltage scale used by any gauges that show narrowband voltage. Narrowband sensors output roughly 0 to 1V, so set the scale to suit that range and your gauges will be readable.
Diagnostics
- Minimum Value: when enabled, a trouble code is set if the sensor voltage falls below this level.
- Maximum Value: when enabled, a trouble code is set if the voltage rises above this level.
- DTC Severity: sets how serious each of those codes is treated.
Voltage limits just outside the normal operating range catch the common failures: a sensor stuck low from an open circuit, or a signal wire shorted to something higher.
Wiring
Narrowband sensors connect to an analog voltage input, and any free AVI will do. Assign it here, and turn the Pull Up off. A narrowband sensor produces a weak, low-voltage signal, and a pull-up resistor would drag it upward and give you false readings. See O2 wiring for more.
Most narrowband sensors today are heated, with three or four wires. The heater gets the sensor up to working temperature quickly and keeps it there at idle. Make sure the heater is powered, or the sensor will give lazy or no readings until the exhaust is hot.
FAQ
Can I tune my car with a narrowband O2 sensor?
Not properly. It can confirm you're near stoich at cruise, but it can't measure how rich you are under load. Use a wideband for tuning.
What's closed loop O2 control for?
It lets the ECU trim fuel to hold the mixture around stoich at cruise and idle, mostly for emissions and catalytic converter health. See narrowband O2 target voltage for how the target is set.
