What causes your Check Engine Light to come on?

March 5, 2010

Almost everyone has been driving along at some point when their little yellow “Check Engine” or “Service Engine Soon” light pops on with no warning.  Even though most people have experienced it, the check engine light is one of the most misunderstood lights on the dashboard, and knowing what to do to resolve it takes an understanding of what makes it come on.

check engine light

A Typical Check Engine Light

What is a Check Engine Light?

The check engine light is part of your vehicle’s engine management and onboard diagnostics system.  The lamp may read “Check Engine” or “Service Engine Soon”, or may simply show a picture of an engine(shown at right), possibly with the word “Check” or “Service”.  The light is connected directly to the engine control module (ECM), the computer that controls the vehicle’s engine.  The ECM turns the light on, and only when it detects that something is amiss.  The ECM is a computer, so the only way it can tell what is happening in your engine is through electronic signals from an array of sensors mounted throughout the vehicle.  It uses the information it gathers to control the engine’s operation, and to diagnose problems.  Since this information is being transmitted electronically, electrical glitches, loose connections, or faulty sensors can confuse the ECM and cause a Check Engine lamp as well.

The information gathered by the ECM tells the computer about atmospheric conditions, driver inputs, engine parameters, exhaust emissions, and other vehicle systems.  This info  is used to control many aspects of the engine’s performance, including the amount of fuel injected into the engine, ignition timing,  idle speed, and throttle angle on most new vehicle.  When the ECM senses an error and turns on the “Check Engine” light, the computer will often enter a “limp-home” mode, which controls the engine using the available good information, but does not offer the best possible fuel economy and engine performance.  The sophisticated ECM can compensate for many small problems, meaning that while you may not feel a difference in the way the vehicle drives, fuel economy is likely going to be affected, and other components may be damaged by long term driving with the check engine lamp on.

DTC’s and Check Engine Light Diagnostics

So what kinds of errors actually turn on the light, and how does a technician diagnose the problem?  Whenever the ECM detects a problem, it will store a Diagnostic Trouble Code (DTC) in it’s memory, which relates to the problem found.  Computers are strictly math-based, and their logic is very concrete, so for a specific DTC to set, certain parameters must be met.  For instance, an ECM’s parameters may state that the Intake Air Temperature sensor must always read between -40F and 240F.  This makes sense, because it is highly unlikely that anyone would be driving around in weather beyond these extremes.  Thus, if the computer were to read 241F on the Intake Air Temperature sensor, a DTC would set, likely DTC P0111 “Intake Air Temp sensor range/performance”.  A technician diagnosing this vehicle would then know why the light is turned on, because the ECM measured intake air temperature that was out of range.  Now the tech has to determine WHY the intake air temp was out of range.  If the car happens to be driven in the arctic circle, there’s always the possibility that the vehicle actually experienced air temperatures that were out of range on a -42F day.  More likely however, the ECM is receiving the wrong information.  This information comes from the Intake Air Temperature sensor, through some wires and connectors, and into the ECM.  The technician now must verify where the problem lies, likely in a wire, a connector, or a sensor.  Once the technician has tested the wiring and voltages from the sensor to the ECM, he can determine what repairs are needed.

The air temperature sensor is a fairly simple diagnosis, because an air temperature signal is directly related to a measurable value, air temperature.  The technician can simply test outside temperature, compare the the ECM’s value, and see if there is a problem.  However, most DTC’s are set based on more complex derived values.  For instance, a common DTC is P0141 “Bank One Fuel Trim Too Lean”.  This DTC sets when the computer senses that there is not enough fuel entering the engine to match the amount of air that it believes is entering the engine.  The computer assesses the Air to Fuel ratio based on information received from Oxygen Sensors or A/F Ratio Sensors mounted in the exhaust system, which tell the ECM how much oxygen and/or unburnt fuel is leaving the engine through the exhaust pipe.

First off, the ECM may be receiving an improper oxygen sensor reading, meaning the ratio may actually be correct, but the ECM doesn’t think so.  If the Air/Fuel Ratio actually is leaner than expected for the measured airflow into the engine, the measured airflow information entering the ECM may be wrong, caused by a faulty Mass Air Flow (MAF) sensor, or by a physical issue like an intake air leak causing air to enter the engine without passing through the MAF sensor on it’s way in.  If the computer is found to have the correct information regarding Airflow and Air/Fuel Ratio, then the problem could be fuel derived, caused by low fuel pressure, plugged fuel injectors, a restricted fuel filter, or even a bad tank of gasoline or gasoline with an excessive ethanol mixture.  Obviously, a DTC does not simply tell you what part to replace, it tells you which information does not sound correct, and the technician is then required to diagnose the problem to find where this incorrect information comes from.

Loose Fuel Cap?

Another common confusing cause of a Check Engine light is a loose or faulty fuel cap.  Many people are very confused as to why this turns the lamp on, and why simply tightening the cap doesn’t turn the lamp back off.

The ECM on all vehicle from 1996 and newer is required by Federal Law to adhere to a set of parameters know as OBD-II.  The OBD-II parameters require that the ECM keeps the vehicle running in a manner to achieve high fuel economy and low emissions, and performs certain regular test to the vehicle’s system to ensure all emissions systems are operating correctly.  The fuel cap is a part of the Evaporative Emissions or EVAP system, which ensures that no gasoline vapors can leave the vehicle and pollute the atmosphere.  If a fuel system had a leak, or a gas cap was missing or did not seal, the vehicle would constantly give off fuel vapors through evaporation.

EVAP System Chart

A basic EVAP Monitoring system

To ensure that the fuel system is properly sealed, the vehicle routinely performs a test known as the EVAP Monitor, which checks the fuel system for leaks.  The EVAP Monitor test applies a vacuum to the entire fuel system, then seals the system and measures the amount of time required for that vacuum pressure to leak away.  The test is very accurate, and can detect a leak as small as 0.020″ in diameter.  When the fuel cap is loose or missing, this test will fail and light the Check Engine light.

For a very accurate EVAP Monitor test to complete, a certain set of parameters must be met.  On most vehicles, the fuel tank must be between 1/4 and 3/4 full, and the vehicle must be started cold, within a certain external temperature range.  The vehicle must then be driven within a narrow speed range for a set amount of time.  For a Check Engine lamp to turn off after a failed EVAP Monitor, the monitor must run and pass two more times, which may take a few days of driving.  Thus, the lamp won’t shut itself off immediately after retightening or replacing the fuel cap.  And of course, while the fuel cap is a likely suspect for an EVAP system leak, the leak could also come from a piece of tubing, o-ring, seal, or a crack in a component.  This is why even an EVAP system DTC may require quite a bit of diagnostics (leak detection is usually accomplished by using a special smoke machine to pressurize the whole system with a mist of smoke, and searching for the spot where the smoke leaks out).

Finding a Qualified Technician

Now that you have an understanding of the diagnostic process, it has become apparent that a technician must have quite a bit of electronic knowledge, as well as a good understanding of how these complex systems work to thoroughly diagnose your check engine light.  How do you find a good, qualified technician?  The National Institute of Automotive Service Excellence (ASE) certified vehicle technicians in many areas of Automotive repair.  Since 1994, ASE has offered an Advanced Engine Performance Specialist (L1) certification that qualifies a technician’s knowledge of Computer Controlled emissions and drivability systems.  To earn L1 certification, a technician must already possess certification in Engine Repair and Engine Performance, and then pass an Advanced Level test that tests their diagnostic skills on current vehicle technology.

Here at Richardson Auto Care, all of our Technicians hold ASE Certification.  Both Jeff and Rob hold ASE Master Automotive Technician certifications, meaning they have received all 8 major Automotive certifications.  Rob is also certified as an Advanced Engine Performance Specialist (L1) and an ASE certified Medium/Heavy Duty Truck Technician.  Rather than waste your time and money on incorrect diagnostics and repairs, give us a call at 972-243-1204 to schedule an appointment and have you Check Engine lamp properly diagnosed.