First lets clear up some misinformation. There are well over 2000 potential problems that can be detected by the Engine Computer, and it will set a diagnostic fault code to indicate that. About half of those codes refer to something that could adversely affect emissions. THOSE are the codes that cause the Check Engine light to turn on. You can get an idea of the severity of the problem by when the light turns off when the problem is intermittent. For the least severe, it will turn off while you're driving. For the more severe, you must turn the ignition switch off, then restart the engine. By far the most serious is when the Check Engine light is flashing. That means you're supposed to stop the engine as quickly as safely possible. Too much unburned fuel is going into the exhaust system where it will overheat and damage the catalytic converter. That is an expensive repair in addition to the original cause of the problem.
When you know a fault code is already set, you may think the cause is not serious, but how will you ever know if a really serious problem develops? The light is already on. For this reason, ignoring a minor problem can turn it into a major one without you ever knowing it until it's too late.
Also, to be sure you're aware, diagnostic fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. If you list the exact code number, we can try to determine if it is a clue to this problem.
Also be aware there is always a long list of conditions that must be met for a fault code to set, and one of those conditions is certain other codes can't already be set. Having one of those "minor" codes stored can cause some other self-tests to be suspended, so problems in some circuits will go undetected. For example, the Engine Computer watches to be sure engine coolant temperature comes up to normal within six minutes. If a fault code is set for an electrical problem related to the coolant temperature sensor's wiring, the computer knows it can't rely on readings from that circuit, so it won't set any codes that are detected through that sensor. Depending on that minor code you have now, the cause of that might have to be corrected before the computer can resume all of its tests and detect something related to the current problem.
When no fault code sets related to the idle speed problem, you need a scanner with recording capability. The "Record" button is pressed while the problem is occurring on a test-drive, then that is played back slowly, later, and you can watch to see what changed. A glitch in one sensor's readings can cause the idle to change. The readings from other sensors can show the results of what caused the change. When the problem occurs so seldom, you either need a diagnostic fault code to start the diagnosis, or you need a recording of the sensor data to analyze.
Monday, December 5th, 2016 AT 4:01 PM