Narrowband O2 Sensor

[Ian C]

Name:
Narrowband O2 sensor

Also known as:
Oxygen sensor
Lambda sensor
HEGO sensor (Heated Exhaust Gas Oxygen)

What is it?
It’s a sensor that sits in your exhaust downpipe and it measures the amount of oxygen in the exhaust gas. From this, the ECU can know if you are running rich or lean. However – this is a “narrow band” sensor, meaning it’s accurate over a small range of air-fuel ratios (AFRs), and these are sited around, brace yourself, the stochiometric AFR. That word is a fancy way of saying “the perfect ratio”. Without going into too much detail, the perfect AFR at idle and cruise is 14.65:1 air :fuel. (Warning – when under power and boost, your AFRs need to be closer to at least 12.5 and more like 11.7 for safety. The narrow band sensor is out of range for this, you’ll need a wideband O2 sensor for tuning)

Pros:
It allows accurate closed loop fuelling

Cons:
It can get old and go wrong. To check if it’s going wrong, or indeed if you have another fuelling problem, you need to access the reading from the O2 sensor via the diagnostics port.

Diagnostic port reading procedure:
Your diagnostic port is a small black box on the side of the intake, with "diag" written on the flippy top lid. Put the negative probe on the battery ground and the positive probe on the pin OX1 (the pins are labelled on the underside of the flippy lid). You should see, at warm idle, a hunt between 0.5v and 0.7v.
Any solid figure between 0.8 and 1.0v is running too rich and the ECU can't compensate.
Any solid figure under 0.4v means it's running too lean and the ECU can't compensate.

If you have a VVTi car, Toyota saw fit to remove aquite a few useful pins out of the diagnostics port for reasons unknown. If you need to check your O2 sensor readings, here is a handy thread on how to find the right signal wire.