A: Not many years ago, every car manufacturer had different code assignments and different DLCs (Diagnostic Link Connector) in all sorts of places on the car and a plethora of protocols that, by design, made it impossible for independent auto shops to access trouble codes for diagnosis or easily correct emission problems. However, in 1996, in an attempt to standardize the mess, the United States enacted a Federal Law requiring all cars sold in this country to be OBDII (the On Board Diagnostics system version 2) compliant. This meant that DTC(Diagnostic Trouble Codes), were finally standardized. Now, whether you own a Ford, a Jeep or a SUBARU, if you had a problem, say a “knock sensor circuit malfunctions,” the DTC would be the same > PO325. Or, lets say you had a rough running engine and the DTC came up PO301. This will always mean “cylinder 1 misfire detected.” A DTC would be the same no mater what kind of car it came from. This Law meant that cars 1996 and newer could now be diagnosed with generic OBDII scanners readily available to independent repair shops. This much-needed Law loosened the strangle hold manufacturers had on auto repair. It gave independent auto repair facilities access to this powerful diagnostic tool along with the ability to address emission problems hitherto greedily held by the manufacturers in an attempt to monopolize the auto repair industry. This law also enforced other standards like the shape and location of the DLC (Data Link Connection) -the place the scanner plugs into your car.
Q: My Check Engine light is on, what’s that all about?
A: All modern vehicles have a computer or the ECM (Electronic Control Module) that controls the operation of the vehicle powertrain (the engine and transmission). The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With constantly growing demands for better fuel economy and new stricter emission regulations it’s not very easy to achieve. The engine parameters need to be constantly and precisely monitored and adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc. That’s why today’s cars have much more electronics than in early days – there is a large number of various sensors and other electronic devices that help the vehicle computer or ECM to precisely control the engine and transmission operation and monitor emissions.
The vehicle computer system has self-testing capability. When the computer senses that there is a problem with some of the components it stores the correspondent (Diagnostic Trouble Code) DTC in its memory and lights up the “Check Engine” light to tell you that there is a problem and your car needs to be looked at by a qualified technician. The technician will hook up a scanner to the car’s computer and retrieve the stored trouble code(s). The technician will look it up in the service manual provided by a car manufacturer. The SUBARU service manual contains a list of codes (a few hundred) and describes what each code means and what needs to be tested.
A: DTCs are retrieved from the ECM by a scan tool or scanner. Some scanners are generic, with limited diagnostic capability while others are “Dedicated”, or manufacturer specific. The dedicated scan tools tend to be more sophisticated, complex and superior in their ability. However, they are very expensive ($3700.00 SUBARU Select Monitor plus $350.00 / cartridge) and they are limited to a single manufacturer such as SUBARU.
Q: How does the ECM System works?
A: There are a number of sensors that provide the ECM with all necessary inputs such as the engine temperature, ambient temperature, vehicle speed, load, etc. According to these inputs, the ECM makes initial adjustments adding or subtracting fuel, advancing or retarding the ignition timing, increasing or decreasing idle speed, etc.
There is a primary (upstream) Oxygen Sensor installed in the exhaust before the catalytic converter that monitors the quality of combustion in the cylinders. Based on the feedback from this oxygen sensor the ECM makes further adjustments to the air-fuel mixture to reduce emissions.
There is another, secondary (downstream) oxygen sensor installed after the catalytic converter in the exhaust that monitors the catalytic converter’s efficiency.
There are a few additional vehicle systems related to the emission control. For example, there is an Evaporative system (EVAP) that prevents gasoline vapors inside the gas tank from escaping into the atmosphere. The EVAP system also contains a number of sensors and actuators controlled by the ECM.
The computer or ECM constantly tests operation of all sensors and components. When any of the sensor signals is missing or out of normal range, the ECM sets a fault and illuminates the “Check Engine” light also called MIL (Malfunction Indication Light) and stores the corresponding Diagnostic Trouble Code (DTC) in the ECM memory.
The same happens if a mechanical component of a controlled system fails. For example, a mechanical problem inside the transmission also can turn the “check engine” light on. Even something as simple as loose gas cap will cause the “check engine” light to come on. The SUBARU fuel system is a sealed system of which the cap is an integral part. The ECM constantly checks if the gas tank is sealed properly and will set code PO442 (evaporative emission control system malfunction) if the gas cap is not properly tightened after refueling.
To sum up, when the “Check Engine” light comes on and stays on, there is a problem with your vehicle. This could be a problem with the engine, transmission, or some emission-related component or system.
The stored trouble code can be retrieved with the special scan tool by the technician. The code itself does not tell exactly which part to replace, it only gives a direction where to look for – the technician has to perform certain tests specific for each code to find the exact cause of the problem. I, personally have a big problem with the MIL. Although the ECM will illuminate the “Check Engine” light for emissions related problems, it stays conspicuously OFF with two very serious problems, low oil pressure and engine overheating. It is my belief these two parameters should not only be monitored by the ECM on SUBARUs, but should also illuminate the “Check Engine” light
Q: What do I do if my SUBARU’s “Check Engine” light comes on?
A: The simplest thing to do is to come into Nice Car for a proper diagnostics. We have the skills, knowledge, a dedicated (SUBARU ONLY) OBDII scanner, proper tools and technical manuals needed to quickly diagnose and correct the problem.
Q: Is it safe to drive with the “Check Engine” light is on?
A: It really depends on the code and what caused it. In some cases, driving with the “Check engine” light ON will cause more damage to the vehicle. A car may even stall while driving. If your check engine light comes on, we certainly recommend visiting Nice Car as soon as possible, just to be on a safe side. A rule-of-thumb; if the “Check engine” light is on and the car runs fine, shifts fine, and in every respect seems to be normal, then it is probably something that needs to be looked at soon, but not something needing immediate attention. However, if the Check Engine light is flashing, or the car is running or performing poorly, it means the ECM has detected a serious problem that needs immediate attention before damage is done to the engine, transmission or catalytic system. Have the your vehicle inspected as soon as possible.
Q: Will disconnecting the battery reset the “Check Engine” light?
A: NO – Disconnecting the battery might reset the check engine light on some cars, but NOT on SUBARUs. I recommend bringing your car in for or proper diagnosis, and here is why
Often, the problem may be actually covered under warranty repaired free by the dealer. For example, if you have the code P0420 – Catalyst System Efficiency Below Threshold it’s very possible that your catalytic converter is still covered by the original emission warranty and might be replaced free of charge (this would cost you close to $2000 otherwise).
Many problems, if not repaired in a timely manner, can cause more serious damage to the system it is part of costing much more than the original repair.
Disconnecting the battery will cause basic settings of the vehicle’s computer to be erased (e.g. idle settings, fuel trim settings, transmission shift points, etc.)
The Readiness code can be erased, which may prevent your car from completing an emissions test. (Readiness code is an indication that certain emission control components of your car have been self-tested)
The radio, if code-protected, may be locked after disconnecting the battery.
Q: What if I take my car into one of those parts places that offer FREE computer scans and will turn the light OFF for me?
A: If you just turn the “check engine” light OFF, it will come back on because the problem(s) still exists. It has not been fixed. Meanwhile, you could be doing damage to your car. You have to understand why parts places offer to scan your car for free in the first place. They want to sell you parts – lots of parts.
Q: Doesn’t the code tell you what part to replace?
A: NO – Diagnostic Trouble Code (DTC)s are NOT a magic bullet. The code itself doesn’t tell the technician exactly what component is defective – it only indicates where to look, what engine parameters are out of normal range. Competent automotive establishments are good problem solvers. They treat problems like fun mysteries that need solving. And like any good mystery, evidence must be collected, tests performed, and results analyzed before a diagnosis is made. Trouble Codes are simply another tool (although a very powerful one) in the technician’s toolbox. In the hand of a skilled diagnostician, codes can save vast amounts of money, parts and time.For instance, if the technician retrieves the code PO303 from your SUBARU’s computer and reads the description offered in the service manual, they find that #3-cylinder has had, or currently has a misfire in that cylinder. That is all the information that can be gleaned from that code. A good technician will realize that a misfire in #3-cylinder can be caused by many things: a faulty sparkplug or sparkplug wire, a bad coil pack, bent or burnt valve(s), faulty fuel injector, leaking or blown head gaskets just to mention the obvious ones. He or she could replace all of these parts until they happen to hit the right one – costing the customer a pot load of money and taking scores of hours, or they could do further testing using the codes as a guide to pinpoint the part(s) that are truly defective.Sad to say however, many garages’ definition of diagnostics is “throw parts at it until something sticks.”
Q: Can I pull the “Check Engine” code myself?
A: YES – Having a newer car (1996 or newer), the appropriate scan tool with the proper software, some technical knowledge and the requisite service manuals – it is not difficult. There are a number of scan tools and software available that can scan your SUBARU’s computer for trouble codes. However, you need to have some technical knowledge to diagnose the problem; the trouble code itself does not tell exactly which part to replace. See answer to the previous question.
Q: Where can I find specific trouble codes and test procedure for my SUBARU?
A:There are a number of websites out there (including this one) for the do-it-yourselfer.
Q: Will overfilling the gas tank cause the Check Engine light to come on?
A: Yes, overfilling the gas tank can trigger the “check engine” light to come on. Modern cars are equipped with an Evaporative System that prevents gasoline vapors from escaping into the atmosphere. When we overfill the gas tank, the excess gasoline can enter the part of the Evaporative system called Charcoal Canister and this can cause some problems. Don’t fill the tank past the first click of the pump.
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