Just coded my 1999 Kia Sephia, P0303, and P0336 came up not sure what that means. needs help.

Hello ayltaz: My name is Roger and I Will help you. The first code p0303 relates to a cylinder misfire in #3 cylinder. This could be a bad plug or plug wire. The second code relates to the crankshaft position sensor. You can just replace the sensor or have this problem tested as to replace a sensor because of a code can get real expensive and may not fix the problem. Should you need further help please just ask. Please rate the answer. Thank You for using Fix Ya. Roger

P0303 means a misfire has been detected in engine cylinder #3
DIAGNOSTIC TROUBLE CODE (DTC) – P0336 58X CRANKSHAFT POSITION
SENSOR NO PLAUSIBLE SIGNAL
Circuit Description
The 58X reference signal is produced by the crankshaft
position (CKP) sensor. During one crankshaft revolution,
58 crankshaft pulses will be produced. The engine
control module(ECM) uses the 58X reference signal to
calculate engine rpm and CKP. The ECM constantly
monitors the number of pulses on the 58X reference circuit
and compares them to the number of camshaft position
(CKP) signal pulses being received. If the ECM
receive and incorrect number of pulses on the 58X reference
circuit, DTC P0336 will set.
Conditions for Setting the DTC

This DTC can be stored in “key-on” status.

Detected number of teeth is differs by 3 or higher.
Action Taken When the DTC Sets

The Malfunction Indicator Lamp (MIL) will illuminate.

The ECM will record operating conditions at the time
the diagnostic fails. This information will be stored in
the Freeze Frame and Failure Records buffers.

A history DTC is stored.
Conditions for Clearing the MIL/DTC

The MIL will turn off after four consecutive ignition
cycles in which the diagnostic runs without a fault.

A history DTC will clear after 40 consecutive warm-up
cycles without a fault.

DTC(s) can be cleared by using the scan tool.

Disconnecting the ECM battery feed for more than 10
seconds.
DIAGNOSTIC AIDS
An intermittent may be caused by a poor connection,
rubbed-through wire insulation or a wire broken inside
the insulation. Check for :
Poor connection – inspect the ECM harness and connectors
for improper mating, broken locks, improperly
formed or damaged terminals, and poor terminal-to-wire
connection.
Damaged harness – inspect the wiring harness for damage.
If the harness appears to be OK, disconnect the
ECM, turn the ignition ON and observe a voltmeter connected
to the 58X reference circuit at the ECM harness
connector while moving the connectors and the wiring
harness related to the ECM. A change in voltage will indicate
the location of the fault.
Review the failure records vehicle mileage since the
diagnostic test failed may help determine how often the
condition that caused the DTC to be set occurs. This
may assist in diagnosing the condition

If you need KIA Fault Codes list Kia Fault Codes

#3 cylinder missfire
and crankshaft position sensor circuit
try a new crank sensor but check wires first
spark plugs and wires might help with missfire code