Understanding the check engine light on a modern vehicle may be confusing. Most people are concerned when it comes on. If the light goes off, we assume things are okay. Now it comes on again. Knowing if we should continue driving or stop right now is difficult. To clear the confusion, we need a basic understanding of what the check engine light is trying to tell us.
A check engine indicator is a malfunction-indicator light or MIL for the computer-management system. They use several computers to operate the various systems on modern vehicles. The power control module or PCM controls the engine and constantly tests each system that it monitors. Should any test fail to pass, the PCM turns on the check engine light to inform the driver.
When the MIL illuminates, it shows at least one test has failed to pass. A MIL coming on is to alert the driver of a potential problem, in a monitored system. This is separate from the oil, temperature and battery lights. Each of those systems have lights of their own. However, these other systems may also set a check engine light, at times.
Check engine lights also have multiple states. For instance, a check engine light that is steadily on, means a situation exists that requires service soon. A flashing MIL shows a condition that may damage the engine and needs immediate attention.
Why do check-engine lights come on and go off?
A modern PCM has to test more than 2,000 items on the vehicle. When we start the vehicle, we establish a "drive-cycle." This drive cycle continues until we turn the vehicle off. Not every test can be completed in every drive-cycle. This may add to the confusion of MIL operation.
When a test fails, the check engine light may come on. We start a new drive cycle and if the test that turned the light on does not run the MIL may turn off. This does not suggest we have solved the problem. The light is off because the PCM moves the information about the fault to history, and continues checking the problem. The next time the PCM runs the failed test, the light comes back on.
As confusing as this seems, it is not without some reason. For instance, certain tests require specific conditions to run. An example may be a "cold start." A cold start means the engine coolant temperature is equal to the ambient temperature.
When we start the vehicle, the drive cycle begins. The PCM checks to see if this is a cold start or a restart. If it is a cold start, they run certain tests. On a restart they may attempt other test. Should we turn off the ignition before the failed test is complete, it will abort. When restarting the engine, coolant temperature exceeds ambient temperature and the PCM will not attempt cold start tests. This means a test, which requires a cold start and several minutes to complete, may not run if we only take short trips. This type driving may keep the MIL from coming back on for several days. The vehicle has not resolved the problem. We simply have not met the conditions needed for the tests.
An evaporative emissions example
Another similar situation is with evaporative emissions testing. Most systems require at least 1/4 tank of fuel, but not more than 3/4 tank to run. If the system fails, the MIL will come on. If the fuel level falls to 1/4 tank, the evaporative emissions tests will not run and the light may go off. Fill the tank and the tests will not run until we reach the 3/4 tank level. This could take several days, but when we meet the conditions, the light will again come on.
Multitudes of conditions exist, for the various tests and they vary from one vehicle to another. What seems like a random occurrence of the MIL is logical, according to the way they program the vehicle. Once the MIL has come on, an issue is present, even if it goes back out. When the failed tests run again, the light will come back.
What makes the MIL come on?
The PCM operates components on the vehicle such as the fuel pump, fuel injectors, the ignition, evaporative emissions, and even the air conditioning and heating. These systems may be called outputs, because the PCM controls them. Many components give information to the PCM. Some of these are called inputs, because they supply information needed for outputs. Other components provide test results and confirm operation of the outputs.
For example, when the ignition is switched on, sensors begin feeding information to the PCM. Among other things the computer needs to know the ambient temperature, barometric pressure and engine temperature. It may also turn off the air conditioner compressor, energize the fuel pump and begin watching for crankshaft rotation. If the crankshaft does not rotate, the fuel pump is switched off, to prevent engine flooding.
When the engine is cranked, the fuel pump is again turned on. The position of the pistons in each cylinder is monitored so that fuel and spark can be supplied at the proper time. Fuel is also monitored precisely, based on engine and ambient temperature, barometric pressure, intake vacuum, air flow and several other factors. This allows the engine to run smoothly with minimum fuel use.
The engine’s idle speed is also controlled, based on several factors. This is why an engine may idle faster at times and slower at other times. Once the engine reaches a higher temperature, other sensors begin to operate and provide feedback. For instance, air/fuel sensors confirm oxygen remaining in the exhaust gas.
The importance of maintenance
If a sensor suggests too much or too little oxygen in the exhaust, the computer may override its calculations and correct the situation. Such could be the case if an intake manifold gasket were starting to leak, or if spark plugs become worn. This feature allows the PCM to adapt to various conditions. It also isolates the driver from feeling the symptoms of the problem.
For instance, spark plugs may be badly worn. The PCM adapts by adding or subtracting fuel and increasing the duty-cycle on the coil packs to increase the spark. This is why proper vehicle maintenance is so critical. Modern vehicles are designed to hide warning signs from the driver. Fuel mileage will not fall off and the vehicle may seem to run fine. Eventually the overloaded coils burn up, the engine begins to misfire and the MIL comes on. Obviously this is a much more expensive problem that proper maintenance could have prevented.
Who’s in control here?
Many components act as both inputs and outputs. An example is the air conditioning. The compressor acts as in input, allowing the PCM to sense the additional load and increase engine speed to compensate. The compressor is also an output, completely controlled by the PCM. If engine temperature exceeds a preset point, the PCM will switch the compressor off, to cut engine load.
Accelerators on most modern vehicles are also an input and an output. When the accelerator is pushed, the throttle position sensor or TPS notifies the PCM. This input is used to increase vehicle speed, set transmission shift points and several other functions. The PCM also controls actual throttle application as an output. For instance when certain malfunctions occur, the throttle is reduced considerably, despite accelerator position.
Diagnostic Trouble Codes or DTC
When a malfunction occurs, the MIL comes on and certain data are stored. These data are used by the repair technician, along with testing, to learn what has happened. A diagnostic trouble code or DTC is a piece of information that helps identify the circuit in which the fault has occurred. The DTC does NOT tell what is wrong and can never be taken literally.
Most amateur and many professional mechanics make this expensive mistake often. The DTC is only a starting point in diagnosis. The MIL is similar to a fire alarm in a skyscraper. It suggests a problem, it does not say what is wrong, where the problem is or what it takes to solve it.
Changing parts in a hope of solving the problem is known as "swaptronics." It is incredibly expensive and very ineffective. Worse, replacing parts can create new problems and further complicate the actual repair.
An example might be an exhaust gas recycle or EGR code. An amateur sleuth might be tempted simply to replace the EGR valve. The professional knows many things can set such a code. When the PCM commands the EGR valve to open, it looks for acknowledgment.
For instance the PCM may expect the vacuum in the intake to fall, because exhaust gas is now flowing in. If the EGR valve is bad, this will not occur. If the sensor reading intake vacuum is bad, it also will not occur. The same goes for a vacuum leak, loose wires or EGR passages plugged with carbon. In fact, several things can all set the same code, many having nothing to do with the EGR valve.
A professional has the knowledge of how the system operates. They perform a diagnosis on the system and tests the inputs plus the outputs. Electronically they eliminate the possibilities and determine the cause. This is far less expensive and more effective than "swaptronics."
Clearing the check engine light and readiness tests
After the problem has been corrected, the codes are cleared from the memory. This will cause the MIL to go off, but will also clear the results of all tests. Once cleared, the PCM will go to work to complete each test and mark them complete. This may take several days before all the conditions are met to allow all tests to run. If a test fails, the MIL will again be turned on.
With State mandated vehicle inspection, the light must be off, no codes can be in history and all readiness tests must be complete, before an inspection sticker is granted. Simply clearing codes will NOT allow the vehicle to pass inspection. There is also no way to shortcut or get around the readiness tests. They must be successfully completed before they will indicate a pass.
This is a very brief overview of a far more complex system, but gives a basic understanding of how things work. On modern vehicles there is not one computer but perhaps 20 to 50. Each is linked to the others, sharing inputs and regulating outputs. Engine diagnostics are one of the most complicated fields in auto service.
Not all shops and dealerships efficiently or properly diagnose engine management problems. True professionals are trained and equipped to solve engine management problems. Paying a true professional to diagnose the vehicle, is always far less expensive.