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Replacing Oxygen Sensors

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Replacing Oxygen Sensors

What is an oxygen sensor?

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Oxygen sensors work by burning some of the unburned oxygen in the exhaust, producing voltage which varies by the percentage of oxygen in the exhaust, thereby telling the computer how rich or lean the mix is. When there is low voltage, the mix is lean - not enough oxygen is present to create a higher voltage. A high voltage means the mixture is rich - there is a lot of unburned oxygen. Most sensors are designed so that a perfectly balanced mixture results in a reading of .5 volts, with a range of around .1 to .9 volts. Most oxygen sensors also have a heating element to help them to get to 600° F, their operating temperature, quickly - and to keep them hot when the engine idles.

Sensors can last from 30,000 miles to over 100,000 miles, though as they age they tend to become slower, causing the engine to be somewhat less efficient. Dirty gasoline (or lead), contamination of the exhaust with carbon or solvents from sealers, and other sources of pollution can coat the sensor, so that a single tank of bad gas (or the wrong choice of RTV form-a-gasket) or a bad spark plug can destroy the oxygen sensor.

Leaks of air or bad spark plugs can throw off the readings by allowing extra oxygen to end up in the exhaust.

Most oxygen sensors use a zirconia tip, but some use titania, which changes resistance instead of changing voltage; for these sensors the computer feeds in voltage and measures the resistance. These are used in some Jeeps because they are better suited to off-road driving.

Fault codes, 2.2 / 2.5 engines

During a cold engine startup, the logic module will run the engine rich until the coolant sensor indicates a temperature greater than 89.6°F for 1984 models, 80.6°F for Turbo II models, and 95.0°F for all other 2.2 and 2.5 liter models. Then, the logic module will wait for 7.39 seconds before entering closed-loop mode.

After another delay of 59 seconds for 1984 models or 718 seconds (11 minutes, 58 seconds) for other 2.2/2.5 models, the logic module will begin monitoring the oxygen sensor output's neutral region. If the sensor output stays in the neutral region for more than 21 seconds, a fault code 21 is stored. The circuit stays in closed-loop and the logic module will still monitor the oxygen sensor signal and allow the engine to run rich, but prevent it from running lean.
Once in closed loop, if the oxygen sensor output indicates "lean" (below the neutral region) or "rich" (above then neutral region) for more than 121 seconds (2 minutes, 1 second), a code 51 or 52 is stored for 1985 models, respectively, or a code 51 for both conditions for 1984 models. For all other models, if the oxygen sensor output indicates "lean" or "rich" for more than 715 seconds (11 minutes, 55 seconds), a code 51 or 52 is stored, respectively. The circuit stays in closed-loop and the logic module will still monitor the oxygen sensor signal. If the condition persists for four times what it took to set the code, the logic module will allow the engine to run rich, but prevent it from running lean.


Newer cars (1990s and newer) have at least two oxygen sensors, "upstream" and "downstream," one of which provides the engine with its information, while the other provides a reference signal so the computer knows if the first sensor is going bad. Sometimes the computer will say the downstream sensor is bad when in reality the upstream has gone bad; sometimes all it knows is that the two disagree (though usually it can figure out which is the bad one, that's not always the case.)

Generally, you will need a special socket to get oxygen sensors out - a socket with a cutaway area for the wires to poke out. Most replacement sensors come with anti-seize compound already applied, but if yours does not (the greasy silvery stuff on the threads), put a small amount on.

Sometimes it is easier to get to the sensor from above, sometimes from below. On a 1995 V6 minivan, Russell Yee wrote that going underneath is easiest, with no need for extensions, joints, or other special efforts.

Edward R Hennessy wrote about replacing the sensor on an EEK:

Let the car cool about 30-45 minutes. Take the o2 sensor socket, 3/8" to 1/2" inch drive adapter, 1/2" drive ratchet, 1/2" universal joint, and 19" 1/2" drive extension over to the car. Remove the airbox completely. Unplug connector. Install socket on sensor, attach ratchet handle, crank, and out it came (didn't need to spray or anything--this thing was in there for 72,000 miles, though). I didn't need the u-joint and extension, since there was plenty of room for the socket and handle with the airbox removed. Total elapsed time, 20-30 minutes at most.

If it was stuck in there, though, it would have taken me a lot longer, but the 1/2 inch drive really helps (and I was ready to swap the ratchet for a 1/2" drive 19" breaker bar if needed, then add the extension and u-joint if I had to, so it might not have been that bad).

Use the right tools and it goes a lot easier. But I was surprised at how easy the access was once I removed the airbox completely.

Not every vehicle is that easy! - an alternative guide by James Dement

I had 120K miles on my 3.3L Grand Caravan and had recently gotten a code 51 (lean condition). This code doesn't mean that your oxygen sensor is bad, but then again it could be so I elected to change my O2 sensor because it had never been changed.

You might be able to see the sensor from looking behind the engine but unless you arms are as long as a monkey's and as tiny as a midget's then you can forget about getting to the sensor from above. You should however disconnect the wiring plug from above before going underneath to try and loosen the sensor.

Then, from underneath the van/car you can with difficulty get to the sensor. I'd let the engine cool as it is almost impossible to not touch the hot exhaust pipe. You might spray some penetrating oil on the area as the sensor generally is very hard to remove. You will need a 7/8" or 22 mm wrench or socket. You can buy a special socket for about $13, but I just made mine from an older deep socket. I cut a slot up one side about 3 inches so the socket could fit over the wires without cutting the wiring off. After much work and effort I got the sensor to break loose then unscrewing it was easy.

The new sensor was a 4 wire unit and matched the original almost perfect. Pep boys sells a similar one from Borg Warner for about $47 that would probably be a good choice also. Others have posted problems with Bosch oxygen sensors so I didn't want to take a chance with them.

Someone said to tie a string on the wiring connector before installation. I found that to be a wonderful idea after trying without luck in getting the new one started. The wiring is in the way and if you pull the wiring up with a string and attach it to the top side of the engine then you can twist the sensor without the wiring messing you up. If you don't understand this - you will after trying to install the new sensor.

Finally got the new sensor started and tightened it up. Then wiped the exposed part of the sensor to remove any junk I'd gotten on it and went top side to attach the wiring connection.

I might also add that a mirror is almost required as you will not be able to see the hole or nut for the sensor from below. I hope this helps others.

Ted Mittelstaedt

I just did my O2 this weekend... on my 1995 Chrysler Town & Country with 3.8L engine. The O2 sensor on this vehicle is easy to get out if you have the right tools. You first need an O2 socket with a 1/2 drive. Get a really high quality one, the cheaper O2 sockets are shorter, and the O2 sensor is quite tall. In fact if you don't have an O2 socket, buy the O2 sensor first then have it with you when you buy the O2 socket. Next you need a 1/2 flex head stubby ratchet, (Harbor Freight sells a set of 3 of these which are perfect for this) and a length of pipe. The pipe slides over the ratchet handle.

The reason you want a flex head ratchet is because the angles are such that a regular ratchet will not fit in the space, nor will an assembly of u joints or suchlike. While Craftsman and SnapOn make really nice flex-head rachets, you won't be wanting to destroy them with a length of pipe.

The O2 sensor should come out with moderate pressure. If it doesen't one trick is to buy a can of Freez-It Freeze Spray from an electronics tools supplier (or other circuit freezing spray) and while the vehicle is up on jackstands, run the engine for 10 minutes to get the exhaust manifold good and hot, then shut it off and soak down the O2 sensor with the freeze spray pretty good, then try breaking free the O2 sensor. Wear heavy leather gloves and a leather jacket to guard against burns.

It also helps greatly to have the vehicle pretty high up on jackstands. Use good jackstands. The best I've come across are the Harbor Freight ones, they go on sale from time to time and have nice wide bases and and go quite high.

Unplug the sensor from the top before you start unscrewing it. Once you get the sensor tightened in, make sure to fasten in the top wiring to the bracket before you get out from under the van.

One other thing there are a number of sensors that my auto parts place claimed were matches to this. Some had plugs and others didn't. Before buying the new sensor, unplug the old one from the top and look carefully at the connector to see what it looks like.


We have a full section on this in the Neon repairs area; essentially, it's easy once the airbox is out.


From Mark McMackin, Chrysler Group Senior Specialist: "Don't spray silicone or WD40 in the connector. Those two fluids will kill an O2 quicker than lighting. The sensor needs to breath oxygen (O2) and it does it through the wiring. Silicone will send a sensor's output south so fast you won't know what hit you."

Technical discussion (2.2 - 2.5 engines)

by Bob O'Neill and Bob Lincoln

The Oxygen sensor (commonly called the O2 sensor) measures the amount of oxygen in the exhaust stream.

The O2 sensor is mounted near the flange where the exhaust down pipe mounts to either the swing valve for turbo engines or the exhaust manifold for non-turbo or TBI engines. As the exhaust passes the O2 sensor, oxygen molecules react with the zirconium sensor, which produces a small voltage when exposed to oxygen based on the amount of oxygen it 'senses'. This happens when the O2 has reached 662° F, being heated by the exhaust gases or an electric heater built into the sensor.

An air/fuel mixture that is too rich means there is too little oxygen sensed in the exhaust stream and the O2 sensor produces a higher output voltage. An air/fuel mixture that is too lean means that there is too little oxygen sensed in the exhaust stream resulting in too much fuel and the O2 sensor produces a lower output voltage. So when the O2 sensor sends the signal to the computer the fuel delivery is adjusted to insure proper air/fuel mixture.

Before the 1987 model year the O2 sensor was a single wire unit and it relied on the hot exhaust gases to heat the O2 sensor. In 1988 and 1989 the O2 sensor was a three wire unit; three wire O2 sensors used a PTC or Positive Temperature Coefficient heating element that keeps the sensor temperature between 923° F and 1292° F. This not only extends the life of the sensor by baking away contaminants but it brought the sensor up to 'normal' operating temperature faster.

In 1989 models and later the O2 sensor is a four wire unit which has an additional ground. The three wire unit used the case of the sensor as the sensor's ground. The four wire unit used the additional wire to ground the sensor element. This improves performance and to some degree the accuracy of the sensor. This extra ground also helps to maintain good ground of the element as the system ages. Over time the rust that forms on the cast iron manifold and swing valve tends to affect the sensor's ability to maintain a good ground.

As the ECU receives the signal from the O2 sensor it decides if the engine is running too lean or too rich. If the ECU receives a voltage less than 0.352v for model years after 1984 and 0.391v for model year 1984 it considers the engine running too lean. If the ECU receives voltages greater than 0.547v it considers the engine to be running too rich. The voltages between too lean and too rich are ignored by the ECU since they are in the 'normal' range. As the signal from the O2 sensor is received by the ECU the ECUs programming uses this information to add or remove fuel from the delivery. If the voltage from the sensor is too low the ECU adds fuel and if it's too high it reduces the fuel delivery. By reacting to the signal from the O2 sensor the ECU adjusts the fuel delivery once per second as long as the O2 sensor is in good operating condition.

The term used to describe the ECUs control of the engine's air/fuel delivery is called 'Closed Loop', which refers to intelligent engine control based on constant feedback from sensors. Open loop operation (cold start, or when there is a fault) refers to 'dumb' engine control based on pre-programmed tables that ignore sensor input). There is an EPROM in the ECU which holds the air/fuel tables which are referenced by the ECU as part of its decision making process for air/fuel delivery. Under WOT or Wide Open Throttle the ECU gives up the programming of the closed circuit and causes the engine to run rich. This protects the engine from detonation as the RPM rises. Also the engine will run rich at cold start because the engine needs more fuel when cold. It also gives the O2 sensor time to heat up to normal operating temperature.

Also see replacing Neon oxygen sensors.

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