Will an O2 Sensor Cause Surging?

An O2 sensor is essentially a chemical generator. One side of the O2 sensor's sensing element is exposed to the exhaust stream and the other side to the outside air. When the sensor gets hot, electrons will flow from the high-oxygen side to the low-oxygen side of the element, creating a flow that we call electricity. Most cars use what we refer to as narrow-band sensors; they only produce a readable voltage at a certain temperature and when there's a precise amount of oxygen in the exhaust stream. Sensor voltage will drop off if there's either too much air or too much fuel in the exhaust stream.

The oxygen sensor is your computer's primary eye on what's going on in the engine. Without a way to monitor how much air and fuel is coming out of the engine, the computer is only making an educated guess about what the engine should be doing according to the other sensors' input. If the oxygen sensor tells the computer that there's not enough oxygen in the exhaust -- a "rich" condition that indicates excessive fuel -- then the computer will reduce the amount of fuel injected relative to air. The computer uses the O2 sensor to maintain its careful balance of about 14 parts air to one part fuel at all times.

An O2 sensor rarely fails outright. In general, O2 sensors will gradually lose efficiency as they wear out or the sensor probe gets clogged with contaminants. Under normal circumstances, the O2 sensor will respond within milliseconds to tiny changes in oxygen content in the exhaust. But that loss of efficiency in the O2 sensor slows the sensor's response time, so that it responds to changes in oxygen content in tenths of a second instead of thousandths of a second.

Your computer very rarely settles on a set amount of air and fuel; it's always making minor adjustments to the ratio using information from the O2 sensor. Under normal circumstances, these adjustments happen so quickly that you don't notice them; they have a stabilizing effect on engine rpm at idle and under load. But the longer the response time, the more the computer has to alter air/fuel ratio to keep sensor input at a safe average. Think of this "hunting" technique as trying to drive down a straight road if you could only turn your steering wheel hard left or hard right. Because you can't make tiny course corrections, the car ends up wandering all over the road just to sort of stay on it.

While you're cruising down the road, O2 sensor hunting will manifest as a constant surging. This surging can be extremely disconcerting if you happen to be prone to motion sickness, since the constant acceleration and deceleration feels something like riding a boat through heavy seas.

Normally, your computer will throw a check-engine light and an O2 sensor failure code when the sensor gets bad enough to surge. But this isn't always the case; sometimes, the O2 sensor can trick the computer into thinking that everything's fine. One quick way to diagnose O2 sensor surge is to unplug the O2 sensor's wiring harness. When you do that, the computer will default to a standby program called "open-loop mode," where it completely ignores O2 sensor input and uses its best guess from other sensors. You can expect a check-engine light and a loss of power, but the primary thing you're looking for here is a reduction or elimination of surging. If it goes away when you unplug the O2 sensor, then odds are good that you've found your problem.