ST165 Code 12 with Wideband O2 Sensor

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ST165 Code 12 with Wideband O2 Sensor

Postby Maet » Wed Sep 15, 2021 12:34 am

I just started getting a code 21 after I swapped out my O2 sensor for a wideband which will eventually connect to a standalone ECU, but in the meantime is there anyway to get rid of the check engine light? I don't have an O2 heater so it's throwing a code.
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Re: ST165 Code 12 with Wideband O2 Sensor

Postby alltracman78 » Wed Sep 15, 2021 7:15 pm

Your wideband has a heater circuit too, but it works different.
It's the wrong kind of sensor.
You need to have a regular O2 sensor hooked up to the factory ECU.

A regular O2 sensor creates voltage depending how much oxygen is available and sends that voltage to the ECU.
An A:F (wideband O2) sensor changes the amount and direction of current flowing through it depending how much oxygen is available. The ECU sends a specific voltage to the sensor and monitors the current flow through the sensor.

So an ECU set up for a regular O2 sensor can't read the signal from an A:F (wideband O2) sensor.
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Re: ST165 Code 12 with Wideband O2 Sensor

Postby Maet » Wed Sep 15, 2021 7:39 pm

I'm more looking into a way for the ECU to trust the data from the new sensor depite it having no heater connected. I read somewhere that you can put a 5.5K resistor (what the O2 sensor usually has) somewhere to get rid of the light.
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Re: ST165 Code 12 with Wideband O2 Sensor

Postby 93celicaconv » Mon Sep 20, 2021 1:10 am

You can't connect a wide-band O2 sensor to an ECU designed to operation with the older narrow band O2 sensor and expect it to work. It can't.

First, a narrow-band O2 sensor operates between 1.0V to 0.05V, and it "switches" voltage at stoic (14.7:1 air:fuel ratio for gasoline engines). A wide-band O2 sensor operates from 1.7V to 3.0V, and it gradually increases voltage as the air:fuel ratio goes from lean towards rich. The attached chart shows how the narrow-band O2 sensor and wide-band O2 sensor outputs voltage based on actual air:fuel ratios. Totally different. A narrow-band based ECU is looking for information in the operating range of the narrow-band O2 sensor, and all that ECU knows is if the ratio is lean or rich (the article says it knows stoic, but it really doesn't - it just knows on which side of stoic the O2 sensor information is - at stoic, generally the narrow-band O2 sensor switches frequently between 1.0V and 0.05V, which the ECU interprets to be stoic). The wide-band O2 sensor is linear as air:fuel ratio changes, so that ECU is programmed to know exactly where the air:fuel ratio is, and if the engine is under heavy load, light load, coasting, etc., so it can adjust the air:fuel ratio target for the operating condition. Below is a good write-up.

Narrow Band vs Wide Band o2 Sensors

A narrow band o2 sensor is an oxygen sensor that is only calibrated to know three things. Rich, stoic, and lean. What I mean by this is that it only has a narrow window that it see’s the air fuel mixture through. The sensor can tell the computer when it’s stoic. If it’s not stoic, it can tell the ECU that it’s either Rich, or Lean, but that’s it. It doesn’t really output any particular value other than that. How rich the car is the sensor has NO IDEA about. Same with how lean the car is. All it does know is that it’s not stoic. The ecu with a narrowband o2 sensor, when in closed loop mode, will then lean on the fuel map if it is receiving a rich signal from the o2 sensor till its stoic. The opposite would be true if the sensor was telling the ECU that it was currently running lean, it would richen the fuel map till the o2 sensor was reading stoic.

A wideband o2 sensor is much more sophisticated than a narrowband sensor, and can be relied upon to be used as a tuning tool. Wideband sensors not only are a lot faster acting in the reading, but can tell you the exact a/f ratio that the motor is currently at. So instead of just telling the ECU that the motor is running rich, it will read a voltage that correlates to an actual value, like 11.2. One thing to remember with a wideband sensor is that it has a heating element that needs to be heated up before the sensor will be accurate in its readings. This usually only takes a few seconds, but just remember that for those first few seconds the gauge is not useful. Wideband sensors give the ECU the ability to tune exact A/F value’s to a tenth of a decimal instead of just richening and leaning the mixture till a stoic value is seen.

What is Stoic, Rich, and Lean?

Stoic, rich, and lean are terms that we use to describe the burn ratio of fuel to air in the combustion chamber. The perfect ratio for good combustion of every fuel is called stoic. A stoic value will always give the motor a good clean burn while taking gas mileage into account as well.

Rich refers to conditions in the air fuel value were there is more fuel present than is ideal to the air content. Rich conditions will cause the burn to use more fuel, but can also offer more power at the same time.

Lean refers to conditions in the air fuel value were there is more air than fuel. This causes the combustion to occur rapidly and burn very hot. While this is great for gas mileage, often the vehicle will be down on power and become dangerous to the motors internals.
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Re: ST165 Code 12 with Wideband O2 Sensor

Postby Maet » Mon Sep 20, 2021 2:44 am

93celicaconv wrote:You can't connect a wide-band O2 sensor to an ECU designed to operation with the older narrow band O2 sensor and expect it to work. It can't.

First, a narrow-band O2 sensor operates between 1.0V to 0.05V, and it "switches" voltage at stoic (14.7:1 air:fuel ratio for gasoline engines). A wide-band O2 sensor operates from 1.7V to 3.0V, and it gradually increases voltage as the air:fuel ratio goes from lean towards rich. The attached chart shows how the narrow-band O2 sensor and wide-band O2 sensor outputs voltage based on actual air:fuel ratios. Totally different. A narrow-band based ECU is looking for information in the operating range of the narrow-band O2 sensor, and all that ECU knows is if the ratio is lean or rich (the article says it knows stoic, but it really doesn't - it just knows on which side of stoic the O2 sensor information is - at stoic, generally the narrow-band O2 sensor switches frequently between 1.0V and 0.05V, which the ECU interprets to be stoic). The wide-band O2 sensor is linear as air:fuel ratio changes, so that ECU is programmed to know exactly where the air:fuel ratio is, and if the engine is under heavy load, light load, coasting, etc., so it can adjust the air:fuel ratio target for the operating condition. Below is a good write-up.

Narrow Band vs Wide Band o2 Sensors

A narrow band o2 sensor is an oxygen sensor that is only calibrated to know three things. Rich, stoic, and lean. What I mean by this is that it only has a narrow window that it see’s the air fuel mixture through. The sensor can tell the computer when it’s stoic. If it’s not stoic, it can tell the ECU that it’s either Rich, or Lean, but that’s it. It doesn’t really output any particular value other than that. How rich the car is the sensor has NO IDEA about. Same with how lean the car is. All it does know is that it’s not stoic. The ecu with a narrowband o2 sensor, when in closed loop mode, will then lean on the fuel map if it is receiving a rich signal from the o2 sensor till its stoic. The opposite would be true if the sensor was telling the ECU that it was currently running lean, it would richen the fuel map till the o2 sensor was reading stoic.

A wideband o2 sensor is much more sophisticated than a narrowband sensor, and can be relied upon to be used as a tuning tool. Wideband sensors not only are a lot faster acting in the reading, but can tell you the exact a/f ratio that the motor is currently at. So instead of just telling the ECU that the motor is running rich, it will read a voltage that correlates to an actual value, like 11.2. One thing to remember with a wideband sensor is that it has a heating element that needs to be heated up before the sensor will be accurate in its readings. This usually only takes a few seconds, but just remember that for those first few seconds the gauge is not useful. Wideband sensors give the ECU the ability to tune exact A/F value’s to a tenth of a decimal instead of just richening and leaning the mixture till a stoic value is seen.

What is Stoic, Rich, and Lean?

Stoic, rich, and lean are terms that we use to describe the burn ratio of fuel to air in the combustion chamber. The perfect ratio for good combustion of every fuel is called stoic. A stoic value will always give the motor a good clean burn while taking gas mileage into account as well.

Rich refers to conditions in the air fuel value were there is more fuel present than is ideal to the air content. Rich conditions will cause the burn to use more fuel, but can also offer more power at the same time.

Lean refers to conditions in the air fuel value were there is more air than fuel. This causes the combustion to occur rapidly and burn very hot. While this is great for gas mileage, often the vehicle will be down on power and become dangerous to the motors internals.



Yes I am aware of the differences between narrow v. Wideband. AEM gets around this but having multiple positions on their wideband allowing the car to function as it does normally, the one I got is AEM part #30-4110 and in position number 4 it gives the ECU a 0-1 volt, but that's not really what I made this thread about, I just don't want to have a check engine light and have the old O2 sensor just hanging around. I'm a complete beginner to auto electronics and I just wanted to get rid of the check engine light because the car is running perfectly fine, I just want the check engine light to be reserved for something important and not always on.
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Re: ST165 Code 12 with Wideband O2 Sensor

Postby 93celicaconv » Mon Sep 20, 2021 10:55 am

I think you may be misunderstanding output differences between the two though.

If you look at the attachment from my previous response, which is an output from the narrow-band O2 sensor, at stoic, the output switches from from about 1.0V to about 0.1V. When tested on an operating engine at idle, this output fluctuates between these two voltage values at a frequency of about 8 to 10 times within 10 seconds, or about 1 Hz. This is what your original ECU is looking for.

If you look at the attachment to this response for your setup, you will see at setting #4, your wide-band O2 sensor output is a steady 0.410V. No fluctuation. No frequency output.

Your original ECU will activate the check engine light and set code 21 when, during the air:fuel ratio feedback, the voltage output from the O2 sensor does not exceed a set value on the lean side and the rich side continuously for a certain period. A normal O2 sensor that has a good frequency of 8 or more cycles within 10 seconds with amplitudes that exceed the lean and rich set point values passes the test. Your wide-band O2 sensor with a steady voltage output of 0.410V that does not fluctuate widely at about a 1 Hz frequency will not pass the test and with set code 21.

Hard to explain all this in words. But no, your wide-band O2 sensor set at setting #4 does not provide the original ECU with the type of signal it needs to control properly. When code 21 is set, your car is no longer in closed loop control regarding the fuel map - it is on open loop control. I'm sure it idles and feels just fine - but the fueling of your engine will not be influenced by the wide-band O2 sensor you are trying to use.
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