Parts or components should not be replaced with reference to only a U0003 DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.
The rule of U0003 code when it comes to emissions-related vehicle repair is that any modification that changes the vehicle from a certified configuration to a non-certified configuration is considered tampering: this applies to both vehicle owners and repair facilities and is, therefore, a Federal offense. Replacing a catalyst with a straight pipe is one traditional example. Likewise, overriding the OBD 2 system through the use of high-tech defeat devices or non-certified computer chips, for example, would also be considered tampering. The OBD system may, however, be repaired back to its original certified configuration with certified performance chips or appropriate aftermarket parts.
Evaporative emissions systems test for a leak in the fuel tank and associated hoses. To perform the test, the computer pulls a vacuum and checks to see if the vacuum holds. Each car maker uses a different techniques to check, but most often it's some type of fuel tank pressure sensor.
Gasoline engines use spark plugs to cause an explosion of fuel within the cylinder. In a properly timed engine, this explosion occurs at the proper moment to send the piston to the bottom of the cylinder and provide power to the drive shaft. If the plug wires are out of sequence, the explosion occurs at the wrong time. The improper timing of the explosion sometimes pushes the cylinder the wrong direction or interferes with the turning of the crank. As a result, the engine stutters or backfires, if it runs at all.