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Narrowband O2 Sensor

Ian C

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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)



It allows accurate closed loop fuelling



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.


You can try and fix an old o2 sensor by removing it and soaking it in lemon juice overnight - this eats away the carbon deposits on it, cleaning up the sensor. If that doesn't work, they do eventually wear out so it's probably time to replace it.


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, or the sensor has worn out/broken/is dirty.

Any solid figure under 0.4v means it's running too lean and the ECU can't compensate, or the sensor has worn out/broken/is dirty.


The usual failure mode is to emit less than 0.4v. This makes the ECU think the engine is running too lean, so it adds fuel all the way up to its maximum adjustment (around 10 to 15%). When this happens, the economy of the car will plummet to around 15mpg and there may be a rough low idle and a propensity to stall when coming off the throttle.


If you have a VVTi car, Toyota saw fit to remove quite 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.

The O2 sensor location on the downpipe.jpg

Diagnostics port.jpg

Edited by Ian C (see edit history)
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  • 7 years later...

Hi, just want to check something here.


Last week I measured the OX1 pin(against GND) just to see if the voltage would be like in the above range. 0.4V - 0.8V

I got a value starting (cold) around 0.4V, but once warmed up, it went up to 1.8V on idle with the occasional drop to 1.0V.


My question is whether a wideband O2 sensor would have different readings? (Car has piggyback ecu) Or would the values just be lower and higher (0.1V to 1.2V) for example?

Don't know whether or not my car already has a wideband sensor, that's why.


Any info would be greatly appreciated.

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  • 2 years later...
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