Hold on. You are not reading what I typed. That was the standard reply I use every day when I run into the same comment that "I got a code and I replaced the part". NO ONE else has ever taken offense at that tiny bit of information because it is not common knowledge. Most people assume fixing cars is just that easy and they get angry with mechanics who charge for diagnostic time since "the car tells you what's wrong". It doesn't. You don't get to interpret my comments as you wish to.
Now, if you want to try to figure out the cause of the problem, we can continue, but you didn't even bother to ask a question. You got step one done already. You checked for codes. Dandy, but based on that code, a parts salesman told you to replace a part. That was the first mistake. You got another code, possibly the same one but you didn't say. At that point it should have been obvious the salesman was guessing and as long as your wallet keeps on spewing dollars, he's going to keep on making suggestions. Second mistake.
There are over two dozen fault codes related to the oxygen sensors and they all mean different things and require different diagnostic tests. You didn't list the code number so we can research it further to see what can set that code. Third mistake. You didn't even bother to list the engine size but we would have gotten to that in the course of the conversation. I will never understand why so many people keep those details secret, then get angry when they don't get the expected response. Fourth oversight. You obviously do not know how the oxygen sensor system works but that is to be expected. This forum is mainly for do-it-yourselfers who know little about cars. THAT'S who we're here to help. Had you come here earlier we would have prevented you from buying a catalytic converter unless testing showed it to be ineffective. That's not a common problem.
Your observation that you are not a mechanic and you "do the best you can" will get you into trouble on most of these vehicles. Every day I read about people causing damage to wheel bearings, computers, transmissions, and every other part of their cars. They aren't the same as they were years ago. I applaud anyone who wants to learn more about their car and try to diagnose it them self, but the first step is understanding how the systems work. I can help with that. You'll be better prepared to do as much as possible yourself without fancy diagnostic scanners once you're armed with that information.
First of all, which engine do you have? You said you replaced the upstream and downstream O2 sensors. That implies you have a four cylinder engine. If you have a V-6 or V-8, you have four O2 sensors. Which side of the engine did you replace those sensors on? The front and rear ones do entirely different things and will set different codes. Do you remember the exact code number? Was it the same number both times?
Another point of confusion; one that gets mechanics frustrated and customers angry with mechanics is how some fault codes set. There is always a long list of conditions that must be met to set a code. In almost all cases, one of those conditions is that certain specific other codes can not already be set. That's because the Engine Computer constantly compares sensor readings and reconciles them with each other. When a code is set related to something the computer compares to a second parameter, it can't test that second parameter if there's nothing valid to compare it to so it won't set that second code, ... UNTIL the first problem is fixed. That's when the second code immediately pops up. That happens all the time on some cars with their anti-lock brake systems.
Once you do have the code, it will tell you whether the O2 sensor heater is grounded or shorted to voltage, whether the signal is staying lean or rich too long, whether it's not switching properly, or if it's producing no signal. Not switching properly and staying in one state too long have nothing to do with a bad sensor. Those conditions actually prove it's working. It is simply reporting an improper condition. You have to diagnose the cause of the condition, not replace the sensor. Many people find it easier to understand the coolant temperature sensor. If it reports the engine is overheating, you don't solve the overheating by replacing the sensor.
The next thing that is not terribly common but can drive experienced mechanics crazy is a leak in the exhaust system ahead of the catalytic converter. That is something to look at since you replaced the converter, even if you can't hear it. Between the pulses of exhaust flow, the momentum creates little pules of vacuum that can draw in outside air. The oxygen in that air will be detected by the front O2 sensor which will report a lean condition. The computer will respond by commanding more fuel to all cylinders on that side of the engine. No matter how much more fuel it commands, there will still be that unburned oxygen showing up and it will finally set a code related to the computer losing control over the mixture.
The same thing can happen from a single cylinder misfire. Unburned fuel and air go into the exhaust where the unburned oxygen is detected as a lean condition on that side of the engine.
Your fifth mistake was assuming I'm lecturing you on being dumb. As I pointed out, we work here mostly with people who know little or nothing about cars. I can list dozens of common things I know nothing about. That doesn't make me dumb. The sixth red flag is "I need to know what to try next". The last thing anyone here will do is tell you to "try this" or "try that". That is the most expensive and least effective way to diagnose a problem. If you're going to resort to that kind of shotgun approach, take the truck to a mechanic because it will get fixed faster and cheaper. That is not meant to be as sarcastic as it may sound. The exception is when you're trying to learn about your car or truck. For the average owner, seeing a mechanic is the least expensive route.
One more thing to look at is where the front O2 sensors' wiring runs down behind the engine. On more than one occasion people have found that harness to have a broken attaching clip and the wires fall onto the hot exhaust pipe and melt. The code(s) will depend on which wire melts through and grounds out first.
Please consider a to help us answer more questions.
Wednesday, June 27th, 2012 AT 10:08 PM