Reset ECU (Clear Check Engine Light) in Nissan Altima

To clear all Check Engine light codes you need to reset the ECU (Engine Control Unit). The ECU is located on the driver's side on the right of gas pedal below the steering wheel. It is covered by a plastic piece. You will need to remove the plastic cover to get access to the ECU. You can pry open the the plastic cover. It does not have any screws. The ECU looks as in the picture below and has a Flathead screw.



Follow the steps below.
Do not start the engine ever during the entire procedure.

Procedure Start:
1. Turn ignition key to the "ON" position but do not start the engine. (The ignition key will remain in this position throughout the entire procedure)
2. Turn the ECU switch fully clockwise, wait two seconds.
3. Turn the ECU switch fully counter clockwise.

Now you are in the trouble diagnostic mode with the check engine light blinking. To determine the MIL (Malfunction Indicator Lamp) code number see below.

Continue on to resetting ECU:

4. Turn the ECU switch fully clockwise again, wait two seconds.
5. Turn the ECU switch fully counter clockwise again.
6. Turn ignition key to the "OFF" position.
Procedure End:

Note: Never start the engine in the entire procedure.

You can note down the ECU codes by reading the Long & Short flashes in the Check Engine Light. To determine the MIL (Malfunction Indicator Lamp) code number use the following rules to decode the flashes:
1. Long flashes (0.6 second) are the first digit.
2. Pause (0.9 second).
3. Short flashes (0.3 second) are the last digit.
4. Pause (2.1 seconds).
Next code (if present)
The long flashes will occur before the short ones. If there are multiple error codes, then there will be a noticeable pause (2.1 sec) before the next code starts. The ECM code repeats itself until you turn the ignition key to the OFF position, at which point the ECM resets itself to standard get-in-and-drive-mode. Additionally, if you get four blinks of ten (0000), the ECM is indicating no malfunction.
Use the following table to correlate the MIL code with ECU code
Ref: http://www.troublecodes.net/Nissan/