2001 Vauxhall Corsa Exhaust Emission light

The Engine Computer detected a problem, set a diagnostic fault code, then turned the light on to tell you. The last thing you want to do is erase that code until it is recorded, otherwise you will lose that valuable information. What you need to do now is get the problem to act up again and set that code so it can be read. Erasing the codes never makes the problem go away.

It's important to understand that fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis. About half the time when a part is referenced in a code, that part is actually the cause. The other half of the time it's a wiring problem related to hat part.

Engines vibrate because of a misfire condition. The fault code can tell you which cylinder is misfiring but not why. The codes may also indicate the results of that misfire, such as "running too lean", from the excessive unburned oxygen in the exhaust. That is what your mechanic is supposed to diagnose. Any fault codes should be erased after the repairs are completed.

"Hi, Just wanted to ask a few follow up questions on the exhaust emission light. Now since the mechanic has erased the code. How long would it take for the problem to come again and it would it just help if I changed the oxygen sensor or the crankshaft sensor? Because if it comes when I am on the motorway heading home, I would not be able to go to a garage and my motorway journey tkaes 6 hours. I was wondering if for the symptoms I had mentioned earlier would prove for either a sensor to be faulty? Please help me. Many Thanks"

Please don't start a new question each time. Follow the link in your automated e-mail to come right back to this thread. I might not see your new question and the other people aren't going to want to jump into the middle of a conversation they know nothing about. That does you a disservice.

As far as how long it will take for the problem to act up again, there is know way to know that. If I knew the code, I could guess at some generalizations, but with intermittent problems it can be minutes, weeks, months, or never. That's why knowing the code is so important.

Changing random parts is not the answer, and it will add new variables to the problem. No two sensors are ever exactly the same. The Engine Computer has to learn the characteristics of the sensors by comparing them to each other and various operating conditions. When a problem is detected, it may not be bad enough or have lasted long enough yet to set a fault code, but it can prevent the computer from relearning the new sensor's readings. That can cause the engine to run worse due to the new sensor reading differently than the old one, or it can run better because the computer thinks there's a problem with that new sensor so it disregards it and "injects" pre-programmed backup values to run on.

Your car is going to have multiple oxygen sensors. If one is defective, the computer will set the appropriate fault code. When they're working properly they simply report the amount of oxygen in the exhaust. If that amount is wrong, you don't replace the sensor; you find and fix the cause. Don't replace the messenger because you don't like the message. The oxygen sensors after the catalytic converter only report on its efficiency. They have no bearing at all on how the engine runs.

If the crankshaft position sensor fails, the engine will stop running. On some car brands those sensors sit high up on the engine where heat migrates to when the hot engine is stopped. Very often they fail by becoming heat-sensitive. The symptom commonly is the engine runs fine as long as you keep on driving and air is flowing over the engine, but when the engine is stopped for a few minutes, the heat makes the sensor fail resulting in a no-start condition until it cools down for about an hour, then it will work again.

Most commonly an engine misfire like you described originally is caused by old spark plugs and wires but there are many other potential causes. The fault code could be the result of that misfire, or it could be indicating the cause of the misfire. At this point you really have to wait for the problem to occur again so we can see what code is set.

You'd have to ask him what he meant. If the Check Engine light turned on, there had to be a reason, and without fault codes you could spend weeks trying to diagnose every circuit. The code can indicate an electrical problem with a sensor or output circuit or it can indicate an improper operating condition such as "running too rich too long" or something like that.

Also, on some car brands, they use "pending" codes which refer to things the Engine Computer is keeping an eye on before it actually sets the fault code. The problem has to act up a certain number of times or for a certain period of time before the code is set. On those cars, you have to read the "hard" codes and the pending codes. Hard codes may also be called "stored" codes.

If there were no codes to read, it could be because it was a pending code and it got erased when you turned the engine off. There isn't much use for those kinds of codes but the engineers seemed to think they had some value.

It's also possible your mechanic meant he read the code but couldn't find the reason for it to set. A good example is "running too lean". That could be due to a vacuum leak, an engine misfire, or a sensor problem. The fault code just indicates the starting point in a long list of diagnostics.

You can ask all the questions you want to, as long as I know the answer. First let me ask a few. Which country are you in? Can you explain what is involved with this "MOT"? I've seen a lot of references to it, but I don't know everything it involves. It seems like some cars fail this test for very minor problems.

Since your car did not fail due to that exhaust leak, it likely is not a serious one. In fact, a lot of cars come with mufflers that have a tiny hole on the bottom, at the rear, for condensed water vapor to drip from. That hole is bigger than a lot of unintentional leaks.

Depending where that leak is, there is little chance it is related to any running problem. In some rare cases, a leak before the catalytic converter can allow fresh air to get sucked in and be detected by the oxygen sensor. That can result in setting a "running too lean too long" fault code, but there's little chance it will make a noticeable difference in the way the engine runs.

What is needed now is for the running problem to occur again so you can have the diagnostic fault code read. Without that code, the list of possible causes is huge. Old spark plugs and wires are always good suspects but it's not common for them to act up so intermittently. If it was raining or snowing, water on a deteriorated spark plug wire can cause a misfire and rough running engine.

I can offer come generalizations too about the Check Engine light. There are well over a thousand potential fault codes. Only about half of them relate to things that could adversely affect emissions. Those are the ones that turn the light on. When the problem is relatively minor, if it goes away, the light will turn off while you're driving. When an intermittent problem is a little more serious and goes away, the light will stay on until you stop and restart the engine, like happened to you. If it's still more serious, even though the problem goes away, the light will stay on whenever the ignition switch is on. When it's really bad, the light will be flashing. That means stop the engine as soon as possible because too much raw fuel is going into the exhaust system and is going to overheat the catalytic converter. That can turn into an expensive repair.

Thank you. Here in the U.S. The Federal government sets the safety and emissions standards all new cars have to meet, but emissions testing and safety inspections are left up to each of the 50 states. I have no testing by me of any kind. Unfortunately it is well-known that our politicians are idiots, and they are in an adversarial role with the auto manufacturers. The manufacturers have given us a lot of nice innovations, but most of what has been mandated by the politicians has been of questionable value lately. If the politicians would keep their noses out of what they don't understand, competition and the free market would give us less expensive and more reliable cars.

Oh well. Keep the questions coming. I'll answer them if I can.

Nope. What you described is well within normal driving conditions the car was designed for. The running problem you had is intermittent, but without knowing the fault code or other details, it's hard to know if it can be expected to show up again. You have to think of all the possible variables. Tank of bad gas that's used up now. Speck of carbon that broke off the throttle position sensor resulting in a wrong signal voltage, and it fell out of the way now. Loose hose clamp or some other leak in the fresh air tube between the mass air flow sensor and throttle body, that was overlooked but inadvertently tightened properly after searching for the cause of the problem. Chip of carbon stuck in the EGR valve holding it open, and it has since blown out.

Those are things that would likely not act up again. Things that ARE waiting to act up again include corroded splices and connector terminals, a sensor starting to fail, and a computer problem. Wear in a valve or other mechanical part often also start out as intermittent problems.