How to: Change O2 sensors on a ’94 C4 Corvette

I had some problems with a rough idle on my ’94 Corvette coupe.  Never having this sort of problem on a fuel injected engine before, I did a pretty extensive analysis on the electronic fuel injection looking for anything that would account for the slightly rough idling.  After checking the engine’s real time data with a code scanner, I didn’t see any problems with PCM (Powertrain Control Module) or the sensor inputs.  However, if the readings from the O2 sensors are wrong or biased, then the entire control loop will be off.  Many of the “experts” on the auto forums suggest changing the O2 sensors every 30K to 50K  miles.  The sensors are supposed to be good for 100K miles, but I feel sure they suffer from some degradation before that mileage.  O2 sensors are quite easy to kill, using any compound with Silicone or Silica it that is in or around the engine can kill the O2 sensors. Add to that, this car is 17 years old and has 77K miles on it, just based on the mileage there is good case for changing the O2 sensors.

The ’94 is a ODBI car and it has two O2 Sensors, one just in front of each the two catalytic converters. The ’96 and later cars are ODBII and will have a O2 sensor(s) after the converter(s). This O2 sensor is not used by the PCM to determine fuel mixture, it is used for monitoring emissions by checking the effectiveness of the catalytic converters. There is no need to change the O2 sensors that are behind the cats unless you are having problems passing emissions on a ODBII car.

I purchased two new NTK O2 sensors at Advanced Auto Parts for $112 and decided to change them myself.  (It would just kill me to have to take it to a repair shop for this kind of repair!) The engine compartment of the Corvette is very, very compact, so I knew this was going to be a challenge.   Here is how I went about it and some of the things I learned during the project.

Here are the tools you will need:

3/8 Breaker Bar

Assorted 3/8 extensions

7/8 O2 Sensor Socket or Offset wrench

Floor jack and two Jack Stands

Accessing the sensor from above is just about impossible on the C4 Corvette, so you will have to get at the sensor from the below the car.  I found that the socket was the best fit on the drivers side sensor, and the offset wrench worked better on the passenger side of the car.

The O2 sensor socket is a specialty tool that has a slot cut in the side to allow a exit for the sensors wires.

Even though this is a socket for a 3/8 inch square drive, I recommend you purchase a socket that has a hex on the top like the one shown here.  This allows additional flexibility because you can also use it with a box or open end wrench on the top hex.

I used the socket with a 1 inch box wrench to get the drivers side sensor out and back in.

Because of close clearance with the starter on the passenger side, I needed a offset sensor wrench on that side of the car.  This is intended to be used with breaker bar (in the square) and like the socket has a slot to allow the wrench to go over the wire leads.

Ok, let’s change the sensors.  I had to remove the air dam from my car to get the floor jack far enough under the car to get it jacked up.  You can jack the front of the car up by placing the floor jack under the front cross member.  Be careful of the oil cooler that is mounted right in front of the cross member. (I believe this cooler is for the power steering fluid)

Place the jack stands under the frame rails just behind the front wheels.  Be careful and set the stands so that they are nice and stable.  My stands have small notches in the tops, so I positioned the frame seams in the notches so the stands couldn’t slide around.  Of course, never get under a car that is supported by only a jack(s).

Fish the connector down and disconnect the sensor from the harness connector. The is a small clip on one side, push the tab up and pull the connector halves apart. Put your socket or, in this case, the sensor wrench, over the sensor and work your breaker bar into the drive square. Remember, you are working from the “opposite” side, so it is no longer “Lefty Losey, Righty Tighty” It is backwards from this angle. Break the sensor loose and then you should be able to remove the breaker bar and turn the sensor out with just the wrench.

Before you put the new sensor in, check it to see if it’s threads are already coated with anti-seize compound.  If not, place a small amount of anti-seize compound on the threads, only the threads.  Make sure that your anti-seize is O2 sensor friendly.  You should never use any sealers, cleaners, or other compounds on a fuel injected motor that aren’t O2 sensor friendly.  Put the new sensor in and turn it in finger tight.  For me, getting the sensor’s threads started was the most difficult part of this process. The close quarters combined with the odd angle of the sensor made it hard to do. Then put the socket or wrench on it and tighten it 1/2 additional turn .

Route the wire on the sensor clear of the exhaust and reconnect to the harness connector.

Now, repeat the process on the other side.

While I had the air dam off, I took a few minutes to fish out some of the debris from between the A/C condenser and the radiator.  That red thing turned out to be a Baby Ruth candy bar wrapper!

That does it.  After the change my idle was noticeably better.  It should also help my gas mileage, but that remains to be seen.  I’ll have to put some miles on the car to see how that works out!

4 Comments so far

  1. barryz on May 31st, 2011

    Please change title “How to: Change O2 sensors on a C4 Corvette” to
    How to: Change O2 sensors on a C4 Corvette (1994 ODBII)
    so it is clearer when people visit.
    I have a 1990 and the O2 sensor setup is different.

    BarryZ

  2. admin on May 31st, 2011

    Good input Barry. I changed the title to “Change O2 senors on a ’94 C4 Corvette. I think that is accurate. One thing, the ’94 is an ODBI car, not an ODBII car. ODBII didn’t officially come along until 1996 although I am told (But I have not confirmed) that some of the ’95s were ODBII.

  3. ArmanL on September 10th, 2016

    Thanks for the write up. Especially useful is the mention of specialty tools. I am going out today to purchase the offset wrench and the O2 Sensor socket. I don’t have a fear of doing the job now. I have a similar situation with my 93 in that the idle is mostly ok, but there is a slight miss every few seconds and the car has 115K miles. I will attempt this repair!

  4. admin on September 11th, 2016

    Glad you found the write up useful. Good luck with the O2 Sensor change and let us know if it fixes your problems.

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