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What is an oxygen (O2) sensor?

Read this information page by Rick Kirchoff (edited by Kyle Hamar) for general O2 sensor information. Also, NGK maintains an oxygen sensor FAQ page.

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Why do people monitor the O2 sensor?

The oxygen sensor reading is one of the primary indications of the operating condition of the engine. People monitor it to try and make certain that their engine is getting adequate fuel. Too little fuel and engine damage can occur. See the ECU Primer for the details as to why this occurs.

Caution must be exercised by anybody that is depending on an A/F meter for engine tuning. There is evidence that the oxygen sensors in DSMs are not designed to be accurate except at their 'switch point' of roughly 500 mV. In other words, different sensors may give different A/F readings under the same conditions. There are also many other variables that can affect what is a 'safe' O2 reading for any given car.

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I have a [DSM] with [mods]. What is a safe O2 reading to run?
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There have been many 'safe' O2 sensor readings reported for DSMs. However, there is a growing consensus that O2 readings alone are not enough to guarantee safe operation of the engine.

DSM oxygen sensors should be thought of as more akin to oxygen 'thermostats'. They are designed to 'switch' states, from high to low, very rapidly, around the oxygen level that corresponds to a stoichiometric 14.7:1 air/fuel ratio. As long as they do this, there is no reason for them to be accurate anywhere else.

The principle of monitoring the A/F ratio is to check what the oxygen sensor is reading at A/F ratios that are greatly different from the switch point. However, the oxygen sensors may not be accurate at these levels, so any readings that are taken must be treated with caution.

Evidence to this effect is growing thanks to the introduction of the TMO datalogger, which gathers information about engine operation directly from the ECU. There have been many cases where owners have used the datalogger on their car, only to find (to their utter surprise) that despite sky-high A/F meter readings, they are losing power from not having enough fuel in the air/fuel mixture.

Another problem is that the O2 sensor reading shown by the A/F gauge may not exactly correspond to the O2 sensor reading inside the ECU. This is due to differences in the grounding points of the two devices, and can easily lead to a 0.1V difference, making the A/F meter reading 0.1V higher than the ECU reading. Thus, an owner might think that they are running a safer A/F ratio than, in fact, they are.

This does not mean that A/F meters are useless. Their fast reaction time and simplicity make them an excellent choice for monitoring relatively safe, early-stage modifications to DSMs. They simply have limitations that make them less-than-ideal for precision engine tuning, and new users need to be aware of them.

Having said all of that, authorities in the field have stated that 0.85V is the absolute minimum you can run. Most people prefer 0.90V or 0.95V, but running these levels is simply a guideline - it is not a guarantee that your engine is safe, or producing maximum power. This is because differences in engines, altitude, barometric pressure, gasoline, and other conditions all contribute towards varying this number.

The only sure method by which anyone can state that they must run a certain minimum O2 reading is if they have determined the perfect level for their individual car through experimentation. This type of experimentation is time-consuming. Many racers spend years perfecting their setups.

More information on the limitations of A/F meters and oxygen sensors may be found in this post by Eric Typpo, this one by Greg Denne, and this one by Bill Walls.

The Last Word: Don't be a moron. There are lots of better tools than an A/F gauge these days.

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My O2 sensor reading jumps around like mad while I'm driving.  Is there a fix?
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No - this is normal.  You are viewing the ECUs attempts to supply exactly the right amount of fuel to the engine to achieve stoichiometric operation (equal masses of fuel and air).  The cycling also means your O2 sensor is healthy.

For more information on how the oxygen sensor is supposed to behave, read the The Essential Primer on the DSM ECU.

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My O2 sensor reading stays constant all the time.  Is there a fix?
My O2 sensor reading jumps all the time, except when I accelerate.  Is there a fix?

If this occurs at idle, or during prolonged periods of idling, the oxygen sensor is likely too cold to cycle properly. Warming up the engine somewhat will raise the sensor temperature into a normal operating range. While DSM oxygen sensors are equipped with a heater to aid in keeping the sensor hot, many people find the heater is either broken or simply not adequate to the task of keeping the oxygen sensor hot.

If this occurs during normal cruising speeds, your oxygen sensor may be on its last legs.  Poor cycling is often an early symptom of impending sensor failure.  If it persists long enough, the ECU will throw the code for the O2 sensor, but the ECU is pretty conservative on this;  the sensor has to really be DEAD dead for the ECU to notice.

On the other hand, the O2 sensor reading is supposed to peg high under acceleration. This is because the ECU no longer cares about the oxygen sensor reading, and supplies extra fuel to keep the engine cool. This is known as open-loop operation. The method of changing fuel delivery based on the oxygen sensor signal (which causes O2 readout cycling) is called closed-loop operation.

For more information on open and closed-loop operation, read the The Essential Primer on the DSM ECU.

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I can't get a reading from the oxygen sensor when the car is idling!   Is there a fix?

If you just started your car, the oxygen sensor is cold, and will not give any reading for a little while. This can also happen if the car has been idling for a long period of time.

Also, if you have removed the lower honeycomb from your MAS, your oxygen sensor reading at idle will likely drop to zero (or almost zero) at idle. This is a common side-effect of removing the lower honeycomb, and does not represent a problem. This effect is only affects O2 sensor readings at idle, and will not change the sensor or car behavior while cruising or while accelerating.

For more information on how the ECU handles fuel, read the The Essential Primer on the DSM ECU.

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My O2 sensor might be dead, but I'm not getting a 'Check Engine' light! How can I tell?
I'm sure my O2 sensor is dead, but I'm not getting a 'Check Engine' light! Why not?

The ECU only checks the oxygen sensor under specific circumstances. If it doesn't get to check the sensor, it can't tell if the sensor is dead.

According to Todd Day of Technomotive:

" Your car must undergo a "cruise" session above 45 MPH for 30 minutes or two "cruise" sessions for 20 minutes (depending on [model] year) before the ECU will flag a dead O2 sensor. At least for the 1Gs - this probably got a lot tighter for 2Gs. This is why a lot of people won't ever see a code get thrown for O2 in their daily commute driving."

The best way to determine if an O2 sensor is dead is to monitor it with an air/fuel (A/F) gauge.

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Has anybody ever used a one-wire / 1-wire oxygen sensor on my [DSM]?
Can I use a one-wire / 1-wire oxygen sensor on my [DSM]?
How do I use a one-wire / 1-wire oxygen sensor on my [DSM]?

According to some DSMers, you can use a one-wire oxygen sensor on your car instead of the four-wire OEM version. However, there may be some restrictions on their use.

Stock DSM oxygen sensors include a heating element that allows them to heat up to operating temperature faster, especially in cold weather. It is almost certain that a large number of owners have oxygen sensors with broken heaters. They don't notice the lack because the heater is not essential for O2 sensor operation. At worst, the oxygen sensor will take some additional time to heat up to operating temperature, and gas mileage might drop a little bit. So two of the four wires on the O2 sensor may certainly be considered optional, especially for those living in warmer climates.

The one wire on the non-DSM sensor is the oxygen sensor signal. Since there is no ground wire, the sensor must use the mounting point as ground. There is a small possiblity that this point might not be a good ground on some cars. Cars with upgraded downpipes might be suspect, as there is a grounding strap on the OEM downpipe that is frequently removed during the upgrade. This may affect the ground reference of the oxygen sensor to some extent.

Even in the worst-case scenario, this is highly unlikely to affect the operation of a stock or near-stock DSM. The precise reading of the oxygen sensor is not important, and is not used by the engine computer, so the ECU will not 'see' any shift in ground potential on the single-wire O2 sensor.

Owners of upgraded cars who use the O2 sensor for tuning purposes might have to be a little more careful. In many cases, owners rely on their oxygen sensors providing a consistent (if not accurate) reading. A shift of 0.100V might be enough to make their tuning more difficult. Thus, individuals who switch to a 1-wire sensor may have to spend some time re-learning their tuning methods to compensate for any differences in the new setup.

It must be noted, however, that tuning by OEM oxygen sensor is quite possibly the worst method of tuning a DSM. Owners with upgraded cars will hopefully have better and more reliable methods than relying on their O2 sensor.

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I have a [DSM] with [mods]. What is a 'safe' EGT to run?
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This depends a lot on where the EGT probe is mounted, as pre-turbo EGTs are always higher than post-turbo EGTs. It also varies somewhat from car to car.

In the past, HKS has recommended that downpipe installations not exceed 1380 degF, and pre-turbo (exhaust manifold) installations not exceed 1550 degF.

For more information, see Brad Baur's EGT Gauge Installation FAQ and Tom Stangl's Westech EGT Install FAQ. According to Tom, 1650 degF (900 degC) is the ideal point at which to finish a 1/4 mile run.

The Last Word: Do yourself a favor and get a datalogger. EGTs are nice, but they're not the be-all of engine monitoring - not by a long shot.

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Last edited 09/01/06

Maintained by Sean Costall. Changes and suggestions are welcomed!  If you have any information on the answers to any of these questions or wish additional questions, please mail me.

This page is an extension of Club DSM .