October 2000 Articles & Features

Launch Tech

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Driveability Corner

By Mark Warren | October 2000

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We've always been taught that an O2 sensor looks at just oxygen in the exhaust. But does it need a little HC to get things in gear? Mark takes to the lab to find out.

Fuel control is a complex interaction of mechanical, chemical and electronic systems. However, broken down into subsystems, the relationships are fairly simple. Or are they? Let's check it out.

First, a little background theory. We know that a common zirconium dioxide oxygen sensor will read low voltage (0 to .2 volt) in a lean exhaust (high oxygen content) and high voltage (.8 to 1 volt) in a rich exhaust (low oxygen content). Because an O2 sensor measures oxygen, any incomplete combustion event should increase the oxygen content in the exhaust, causing a rich fuel command from the PCM.

To check out these theories, we ran some tests on a low-mileage 2000 Honda Odyssey with a 3.5-liter V6, single-bank O2 engine. Assuming an atmospheric oxygen content of 21% and an average exhaust oxygen content of .5% at stoichiometry, then a single-cylinder misfire on this Honda should yield a precatalyst exhaust content of 4.0% [(21.0% / 6) + .5%].

We made some other assumptions as well:

* An ignition misfire on this V6 should peg the precatalyst O2 sensor at a low voltage due to the high oxygen content of the exhaust.

* An injector misfire should react exactly the same way.

* There will be so much oxygen in the exhaust that the precat O2 sensor will always be pegged lean, forcing the feedback system to stay in open-loop.

* The precat exhaust will have higher-than-normal carbon monoxide (CO) and oxygen content. The converter will then oxidize the CO and oxygen, reducing the oxygen to low levels, thus yielding a high postcat O2 sensor voltage. Let's see how things panned out.

Fig. 1 at left is a recording of the pre- and postcat O2 sensors and rpm on the Odyssey, with one ignition coil disabled to create the ignition misfire. Note that the precat O2 sensor is spending most of its time in the low voltage range, as we expected. However, the spikes into the higher voltages (as high as .82 volt) are unexpected, given that no matter how rich the other cylinders are, the precat oxygen levels have to be at least 4.0%. A key question here is: If the O2 sensor is affected by oxygen only, how could it show rich (high) voltages in a high-oxygen environment?

Also note in Fig. 1 that the postcat O2 sensor reacted as predicted, giving a high voltage reading (.85 volt).

Fig. 2 at right is the same test except with an injector disabled on the same cylinder. Note that here the O2 sensor responds in the opposite manner as in Fig. 1. Now it spends more time in the higher voltage ranges. This is the opposite of our prediction, based on the O2 sensor being affected by oxygen only. Once again, the postcat O2 sensor reacted as we expected.

The oxygen levels in both tests are identical, yet the precat O2 sensor reacts very differently. The only difference is the hydrocarbon (unburned fuel) content of the precatalyst exhaust caused by disabling the coil. So, the O2 sensor must also be affected by exhaust HC levels. Quite different from what we've been taught, right?

Searching through engineering manuals on O2 sensor operation yielded one mechanism for HC shifting O2 sensor readings. The O2 sensor is made up of two platinum thimbles (one exposed to the atmosphere, the other to the exhaust) separated by a solid zirconium dioxide electrolyte. The exhaust thimble, being made of platinum, is a catalyst much like a catalytic converter. If excessive CO or HC is present in the exhaust, it can be catalyzed on the thimble, using up the oxygen next to the thimble and causing a low-oxygen (rich) indication despite the fact that a high-oxygen content exists in the exhaust. This explains the mysterious rich O2 indication in my April 2000 column on using pre- and postcat O2 sensor readings to predict ignition vs. fuel sensor failures. Can't find any of your old Motor magazines? Go to and click on the month and Driveability Corner.

Just to reiterate, my April column had to do with a complete ignition misfire on all cylinders giving a rich O2 indication. The example in Fig. 1 here is a single-cylinder misfire. Perhaps the HC spikes in the exhaust from the misfire are what's causing the rich spikes in the precat O2 sensor. Of course, the spiking is not at the same frequency as the cylinder misfire, since an O2 sensor can't react that fast. Nor will a scan tool report at anywhere near the misfire frequency.

The reaction shown in Fig. 2 is more mysterious. Why did an injector misfire cause a generally rich O2 sensor indication? There was no added HC here to cause a reaction like that.

Hydrocarbon levels affecting the O2 sensor have been discussed in iATN's technical discussion forum in the past. To find the discussion (sponsoring members only), use your search feature. For nonsponsoring members, become a sponsor, then use the search feature. Some discussion has centered on the theory that an O2 sensor won't even be able to measure oxygen without some HC present in the exhaust. I'll have to do some more testing to double-check that. If you can't wait for my report, then hang a heated O2 sensor in the open air and see if it reads low or high. Be careful, though. It can get mighty hot!

In its normal environment, the O2 sensor is very accurate at measuring exhaust oxygen levels, thereby making an accurate assumption of fuel mixture. Outside of the normal range, however, the O2 sensor's response can be somewhat unpredictable, rendering its assumption of oxygen content and fuel mixture unreliable. Remember, anything outside of a nice sweep from the O2 sensor is cause for further investigation.