2001 Hyundai Elantra Idle Surge

Suggested Repair

After replacing the throttle position sensor, disconnect the negative battery cable for a while so the computer relearns with the new sensor. For the P0303 cylinder 3 misfire, check the spark plug and wire first, then the coil pack, oxygen sensors, and fuel injector in that order, and note mileage and whether plugs and wires have been replaced.

Conversation Summary

A 2001 Hyundai Elantra developed erratic shifting, high RPM spikes, and a check engine light with code P0121. After the throttle position sensor was replaced and the codes cleared, it ran briefly, then idle became unstable and RPMs bounced between 1500 and 2500 in park, with code P0303 for a cylinder 3 misfire. The expert advised disconnecting the battery so the computer resets with the new sensor, then checking the spark plug and wire, coil, oxygen sensors, fuel injector, and other possible causes in order, and asked about mileage and prior plug and wire service.