1995 BMW 740 02 sensors

First you have to understand you're buying a car made by one of the world's least customer-friendly manufacturers. Parts are going to be very expensive, and almost all repairs will have to go back to the dealer. BMW does not release paint codes or service information. When someone is selling a car without fixing known problems first, you can be sure it's going to cost you more than you expect, and more than they're letting on. If solving a problem was as easy as screwing in a couple of parts, why didn't they do it?

Second, you have to wonder how it was determined two oxygen sensors failed at the same time. If someone read the diagnostic fault codes and found one related to a sensor, those codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. There's about a dozen fault codes related to O2 sensors, and over half of them are the result of a defective condition being reported by a properly-working sensor. Changing the sensor for one of those codes doesn't fix the cause of the problem. A sensor referenced in a code is only the cause of that code about half of the time. You also have to test the wiring to be sure a broken wire or corroded connector terminal isn't the cause of the problem.

As far as ignoring a problem, two things must be considered. '95 and older cars only have an oxygen sensor in front of the catalytic converter. Its readings are used by the Engine Computer to fine tune the fuel / air mixture going into the engine. Without good readings to go by, you're going to get too much or too little fuel. Too little fuel will cause a stumble or hesitation. To avoid that, the factory-programmed default values for fuel metering typically err on the rich side. The engine will run fine but a lot of wasted raw fuel will be going out the tail pipe. If that gets bad enough, the catalyst in the converters can overheat and melt causing a lot more trouble, and very expensive repairs. At the very least, you'll be spending more money of fuel than necessary.

Finally, any fault code related to something that could adversely affect emissions must turn on the Check Engine light. To set a fault code there is always a long list of conditions that must be met, and one of those is that certain other codes can't already be set. The computer compares various sensor readings and operating conditions to each other to determine when something is wrong. As an example, it knows that when the engine has been off at least six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature. When a fault code is set for one of them, the computer knows it can't rely on its reading for comparing to other sensors, so it suspends some of those tests. If a failure occurs with that second sensor, it can seriously affect engine performance, but you'll never know the cause if an additional code doesn't set to indicate which circuit has the problem. If the Check Engine light is already on when you're driving, you'll never know if a second code sets. That second problem can be a very minor one that will turn into an expensive one if it is ignored.

This also leads to a common source of frustration BY and ABOUT mechanics. When the first problem is ignored for a long time, like you're asking about doing here, that gives a second, totally unrelated problem plenty of time to develop. If the computer has suspended some self-tests because of the first problem, when your mechanic reads the fault codes so he can give you a repair estimate, he will have no way of knowing about the second problem. His goal is to fix the problem right the first time, but in this case, with no way of knowing about the second problem, he can only tell you about the first one that has the fault code. It isn't until after that first problem is fixed that the computer will resume all of the tests it runs, and that's when it detects the second problem and turns the Check Engine light on again. You incorrectly assume the mechanic didn't diagnose or repair the problem properly, and the mechanic is frustrated because he has to start the diagnosis all over and tell you more parts or services are needed. They hate having to do that.

A lot of those self-tests that resume after repairing the first problem don't run right away. That means the Check Engine light is likely to turn on again hours or days after leaving the repair shop. This happens most commonly to people who tend to ignore problems for a long time. That's why the same people keep coming back angry and they assume the mechanic is incompetent. If they run to a different shop, and the new problem gets fixed, they blame the first mechanic for not fixing their car right the first time. Most of that can be avoided when you take care of a problem right away.

Nope. I didn't say that. I just get discouraged when people get tricked into buying a car that isn't what they think they're getting, then they are shocked when they find out how some manufacturers sucker them into spending a lot of money on upkeep and repairs. I simply want you to be informed before you buy the car, and I want you to realize there's a good chance new sensors aren't needed, aren't going to solve the problem, whatever it is, and if it was that simple, the owner would have done it already to increase the value of the car.

A Jaguar is another uncommon car that is expensive to maintain and repair, but doors and oil leaks can be fixed. If you like the car, keep it and fix what is needed. Those things could break on the BMW too, then you'll be right back where you are now along with any other needed repairs that aren't known yet. You're familiar with your car and you know if nothing else is wrong with it.

There is no car that is immune from oil leaks, failed computer modules, air and water leaks, and all the other breakdowns you can think of. The only difference is expensive cars are built and repaired with expensive parts.

You never said what the problem is, just that two sensors need to be replaced. If one fails, or if one reports an unacceptable condition, like "running too lean too long", the Check Engine light will be on while you're driving. If the light is not on, most likely the sensors are working.

Also, if the light is on and the fault code refers to a condition, like running too lean, it's the cause of the condition that must be diagnosed and repaired. You don't replace the messenger if you don't like the message. That's where, if there IS a fault code in memory, we need to know the exact code number, not a generic description. The problem here is those codes aren't very specific on '95 and older cars. Starting with the "on-board diagnostics, version 2", (OBD2) emissions system on all '96 models sold in the U.S. There's well over a thousand potential fault codes and they get real specific.

The most common failure with an oxygen sensor is a problem with their internal heaters. It's rare for one to burn out, and really rare for two to do that. What is much more common is to have a break in that wire or a corroded splice or connector terminal. A break in one wire can affect both sensors. Replacing the sensors won't fix a broken wire. That goes back to my fear that new sensors wont solve the problem, because the current owner would have done that already.

When the Check Engine light is on, there is going to be a diagnostic fault code, but remember what I said earlier, those codes never ever say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. It's entirely possible new sensors are needed, but why would both fail at the same time if there wasn't some other underlying cause, and why didn't the owner replace them? Better yet, ask the owner to replace them, then you'll buy the car when the Check Engine light stays off while you're driving.

That's fine, but I can guarantee you they do not say to replace them. The sensors could be defective, but what are you going to do if you replace them and the Check Engine light is still on? Take a look at this page starting with code 129:

https://www.2carpros.com/trouble_codes/obd2/P0100

Not one of those codes says a sensor is defective. If you look through all 18 pages of potential codes, you'll see that not a single one says to replace a part. They only say which circuit to diagnose, THEN you replace a part if that's what the diagnosis leads to.

Now, to make life a little easier, check out this page"

https://www.2carpros.com/articles/bmw-obd1-1995-and-earlier-code-definitions-and-retrieval-procedure

If you have a true '95 model car, this is the page that applies. There's only two oxygen sensor codes but they don't get specific enough. You still have to do the diagnosis, then replace a part when everything else has been eliminated. The first step is to connect a scanner to view live data to see if the sensor is switching properly. It should switch between "rich" and "lean" about two times per second. If it is, it's working. If it stays too rich or too lean too long, you have to introduce extra air through a vacuum leak, or extra fuel like starting fluid or carburetor cleaner to see if the oxygen sensor responds appropriately. If it does, it's working, and it is simply reporting the too-lean or too-rich condition as a result of some other defect, and that is what has to be diagnosed.

If this car is a late '95 production, it could have the '96 emissions system on it in which case you'd be able to use the first link with the huge number of O2 sensor fault codes. To know for sure, look at the emissions sticker under the hood. It will state, "this vehicle complies with 1996 emissions standards" if it has the newer system.

Once you're certain of the year of the emissions system, you can look up the O2 sensors on the Rock Auto web site. I found them from $32.00 to $112.00 for a '95 model. For the '96 model there will be two additional O2 sensors after the catalytic converters. Prices range from $56.00 to $64.00 for a '96 model but I had to select a different engine size to find something to look at. They don't state which locations they're for so those prices are just for reference. This is a discount parts supplier so expect to pay more locally, especially if you have to buy them from the dealer.