I checked my ECU just now because my service engine lights came on again, and it tested out for 0903 and 0304, I'm not that knowledgeble with cars so I wanted to know if this is something that I could fix myself, or should I bring it to a shop, and if I do bring it to a shop, how much would it cost?

 

Check the stickies. Post back what you find the codes to mean and then we'll help you out.

 

Get a haynes or chilton manual from your local auto parts store.

 

0304
Diagnostic Trouble Code 0304 points to a problem with the Knock Sensor. The KS is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is senses as vibrational pressure. This pressure is converted into a voltage signal and sent to the Engine Control Module (The computer).

This malfunction is detected when an excessively high or low voltage from the KS is entered to the ECM. Possible causes include...
- the harness or connector (The KS circuit is open or shorted).
- the KS

The KS may be checked with an ohmmeter.

1) Raise the hood. View the engine from the driver's fender. Look into
the deep valley between the cylinder banks and below the intake manifold.
Identify the KS as a black item fastened to the block by a single vertical
bolt. A wire harness wrapped in black leads toward you, out of the valley.
That is the KS sub-harness.

2) Follow the KS sub-harness to it's nearest connector. This is connector F121.
It is located near the upper right-hand corner of the valve cover of the forward cylinder bank, as viewed from the front of the car.

3) Disconnect F121. You have to do a "press the latch and wiggle and
pull" to disconnect it. F121 has only two pins; if you see more than two
pins, you have the wrong connector. Use a digital ohmmeter capable of
measuring more than 10 Megohms. You want to measure the pins of F121, not
the sockets of the matching connector. Measure the resistance between a
good ground (such as the battery negative terminal) and pin #2 of connector
F121. On my car this is the highest of the two pins, the one closest to
the front of the car. The factory spec is 500 - 620 Kohms.

The manual says you have to remove the intake manifold to replace the KS. However, I think that a person with good dexterity and a 12mm ratcheting box wrench could replace the KS without disturbing the manifold.

Important note: the dealer can do a more thorough test with his CONSULT electronic tester. The resistance test described here is a useful but limited test. A Knock Sensor which fails this test is definitely bad. However, a KS which passes this test is not guaranteed to be good.

0903
Diagnostic Trouble Code 0903 indicates a problem with the Evaporative Emission (EVAP) Canister Vent Control Valve or its associated electric circuit.

This part is used by the Engine Control Module to perform the fuel system leak test. Since it is used only for self-diagnosis your engine will run perfectly well despite the dashboard Malfunction Indicator Lamp.

The EVAP canister and this valve are located under the car, near the rear left tire. You might check to make sure all the electrical connectors in that area are secure.

There's no telling how expensive the repair will be, because no definite diagnosis has been made. Maybe you just have to re-plug a loose connector. Since the EVAP system is part of the emissions control system you might have warranty coverage.

 

Ok now that you did your homework...

Ignore the 0304 until you fix the 0903 code. Most likely that valve is stuck open or closed. Get yourself a manual to help work through the diagnosis/repair. If neither the description in the text you pasted or the manual make any sense to you, its time to find a mechanic or someone local to help you out.