Why the sensor needs a heater
A narrowband sensor's voltage-to-mixture relationship is only accurate at one working temperature. Too cold and the output is sluggish or wrong, which is why an unheated sensor is useless for the first few minutes after a cold start. Four-wire narrowbands have a heating element built in. When the ECU controls that heater, the sensor reaches temperature quickly and stays there, so closed-loop O2 control can start almost as soon as the engine is running.
The Nexus runs the heater by switching its ground. The heater has 12V on one side, and the ECU output pulses the other side to ground. The percentage of time it's grounded sets how much heat goes in.
The three settings areas
Frequency
The main O2 Heater page sets how fast the heater outputs pulse. The frequency options available depend on how many outputs you've enabled on the Wiring page, so set up the wiring first.
Heater Duty Cycle table
Each heater output gets its own duty cycle table, which is how you hold the sensor at the right temperature as conditions change. At idle and light load, the exhaust is relatively cool and can't keep the sensor hot on its own, so the table supplies more heater duty. Under heavy load, the exhaust does the heating for you, so you can reduce the duty or drop it to zero. That also protects the sensor from overheating.
Two things can go wrong here. Water that condenses in a cold exhaust can crack a hot sensor element, so on sensors far downstream, like post-cat ones, it's common to keep heater duty low until the engine is warm. Higher-wattage heaters also draw more current, so don't command more duty than the output and the heater are comfortable with.
Wiring
| Setting | What it does | Set it to |
|---|---|---|
| Number of outputs | How many O2 sensor heaters the ECU controls | One per heated sensor |
| Connections | Which physical output drives each heater | Usually a DPO (digital pulsed output) |
| Active State | Whether the output is "on" when grounded | Low |
| Pull Up Voltage | An internal pullup on the output | Disabled |
Checking it works
From a cold start, log the narrowband voltage. With the heater working, the sensor should start switching cleanly within a short time. If it stays flat for minutes, check the 12V feed to the heater, the output assignment and the duty table.
If you're tuning, you'll want a wideband for the actual mixture numbers. See Wideband O2. The narrowbands are set up under Narrowband O2.
