Do it yourself, replace the ignition coils on your Nissan Altima 2.5L, manufactured in the year 2003. To do this repair you will require new ignition coils from your local hardware store, a 10 mm socket wrench and a ratchet with a 5 mm Allen bit. The repair process is simple and will probably not take you more than 20 minutes.
At the beginning, you have to open the hood of your car and remove the engine cover. It is held by four 5 mm Allen bolts (see the next picture) that you can unscrew using an appropriate tool (a ratchet with a 5 mm Allen bit or an Allen wrench).
After you do it, the cover will just lift off. Then, you will gain access to the four ignition coils, shown in the picture below. Using a 10 mm socket wrench, unscrew the bolts that secure them and disconnect their harnesses. You can do it very simply, by pressing their safety tabs with your fingers, as shown below.
At the end, you just have to install the replacements. Insert the new coils back into their holes and securing them with the 10 mm bolts. Connect the harnesses, place back the engine cover and you are done. These are the steps you have to follow in order to replace the ignition coils on your Nissan Altima manufactured in the year 2003.