In modern cars, dozens of computers control everything from wiper blades and power windows to the internal combustion engine and transmission. The two main computers we usually worry about are the engine or powertrain control module (ECM or PCM) and the transmission control module (TCM). They are commonly referred to as "brain box".
Physically, the ECM and TCM may be located anywhere in the vehicle, such as in the trunk, under the dash, or under the hood. Using dozens of sensors, such as those that measure engine coolant temperature or transmission output shaft speed, the ECM monitors engine and transmission function. Using this data, it can fine-tune actuators to deliver more power when needed and reduced emissions whenever possible.
If the ECM detects a problem, such as sensor data out of sync or air flow readings that don’t “make sense,” it will turn on the check engine light, also known as the malfunction indicator lamp or service engine soon light (CEL, MIL, or SES). At the same time, the ECM stores a diagnostic trouble code (DTC) in memory.
A check engine light graphic
If the check engine light comes on, one or more of some 10,000 DTCs might be stored in ECM memory. While the DTC does not tell an auto repair technician what to replace, it can lead them in the right direction to make a repair. Once repairs are completed, the technician clears or “resets” the DTCs, turning off the CEL. If you’re a do-it-yourself guy or you just don’t want to see the light, you have a couple of options to reset the check engine light, aside from pulling the bulb (like some car dealers do) or covering it with electrical tape, not recommended. The are;
1. Fix the Problem
The best way to reset the check engine light is to fix the problem that the ECM is reporting. Once the ECM sees that the problem is no longer occurring, such as a cylinder misfire or loose gas cap (tank cover), it will clear the DTC and turn off the check engine light on its own.
The only problem with this method is that it’s a waiting game. Each vehicle has its own criteria for self-clearing DTCs and turning off the CEL, so it may take days or weeks for the ECM to do it on its own. If you can’t wait that long, there are two more methods to reset the check engine light.
2. OBD2 Scan Tool
The easiest way to reset the check engine light and clear any codes is to use a scan tool, which plugs into the ODB2 DLC (On-Board Diagnostics Generation Two Data Link Connector) port, usually somewhere on the driver’s side. Check your owner’s manual for the location. There are different types of scan tools, each varying in price, capability, and usage.
To operate the scan tool correctly, please read the documentation/manual that came with your particular tool or app for specific instructions.
An OBD Scan Tool
3. ECM Hard Reset
One final option is called a “Hard Reset,” which requires you to disconnect the battery. With the vehicle turned “OFF,” disconnect the battery negative (–) terminal clamp. With the battery disconnected, depress the brake for a about a minute. This will deplete any energy in vehicle capacitors. Additionally, you can turn the key in the ignition to the "start" position two or three times. You can then release the brake and reconnect the battery.
At all times when the battery is being disconnected or reconnected, please take the key out of the ignition. Failure to this may wipe the ECU. In cars like Peugeot and Mercedes, the ECM may need replacements before the vehicle can start. Please, be cautious.
Depending on the vehicle, this may or may not work, because ECM memory may not be voltage dependent.
If the hard reset is successful, DTCs and the CEL will be cleared. Still, your vehicle may not “feel right” for a couple of days until the ECM and TCM relearn their fine-tuning. After hard resets on my car, I drive it for 10 minutes at varying speeds to help with the calibration. Your car may be different, but it's nothing to worry about.
Some car radios and aftermarket alarm systems may go into anti-theft mode, as well, and you may be prevented from starting the car or using the radio without a certain code or procedure.
A positive battery terminal
The easiest and recommended way is the use of a OBD scan tool to find the fault and fix it. It's the safest too.
Why Do We Need This?
The main reason for the check engine light is to let you know that your vehicle isn’t running as well as it was designed, and is probably generating higher emissions than it should. At the same time, you might also notice a decrease in performance or fuel economy. The best thing to do is to fix the problem that the ECM is detecting. This will keep your emissions down (we don't care abut this in Nigeria) and reduce refuelling costs by keeping the engine running optimally.
Check engine, oil pressure and battery warning lights
Types of Scan Tools
Bluetooth OBD2 dongles are generally the least expensive and smallest, but they require a smartphone app to operate the dongle and communicate with the vehicle. Paid apps give you more control than free apps, such as being able to view live data or perform live testing, but some free apps should be able to read and clear DTCs.
A bluetooth OBD2 device.
USB OBD2 cables are the next tier up, and are comparable to what many a big independent auto repair technician use to diagnose the check engine light. A USB OBD2 cable connects your laptop to your car, and requires software to operate. Again, paid OBD2 software gives you more control than free software packages.
An OBD2 cable. This is used with a laptop.
A laptop running a vehicle scanning software
Standalone OBD2 scan tools can range in price from lower than USB cables to several thousand dollars. For DIYers, you don’t need to invest in a top-of-the-line scan tool for basic diagnostic functions, such as reading and clearing DTCs.
An OBD2 Scan tool
An engine control module (ECM) aka. brain box
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