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P0121 Throttle Position Sensor Issue on 2004 Hyundai Sonata 2.4L A/T - Idle Surges in Park/Neutral

Model: 2004 Hyundai Sonata Fault Code: P0121 Posted: 2012-09-20 14:43

I recently encountered a persistent issue on my friend's 2004 Hyundai Sonata 2.4L automatic transmission vehicle (198k miles). The car exhibits an idle surge between 600-1500 RPM when parked in PARK or NEUTRAL, and the check engine light is illuminated with fault code P0121. After replacing the throttle position sensor (TPS) with a new unit, the problem persists—idle remains erratic in PARK/NEUTRAL, while it runs normally at 800-900 RPM when in DRIVE. I followed all recommended troubleshooting steps for P0121 as outlined in the manufacturer's documentation. All sensor readings were within specified ranges. I also tested continuity between the TPS connector and ECU connector—each of the three wires showed nearly zero ohms, indicating a good electrical connection. Additionally, I verified coolant levels and inspected the idle air control (IAC) valve, both of which are within normal specifications. Given that the TPS replacement did not resolve the issue, I am now considering other potential causes. While a faulty ECU is a possibility, it remains a costly option to replace without exhausting all other diagnostics first. The persistent high idle in PARK/NEUTRAL suggests an underlying intake system or sensor-related problem—especially given known issues with intake gaskets on certain Hyundai models. I'm seeking advice from the community: could vacuum leaks be contributing to this behavior? Are there known TPS compatibility issues with 2004 Sonata models? Or should I consider a complete used throttle body assembly as a next step?

Related fault codes
P0121
Comments (2)
Anonymous 2012-09-20 18:21

Could there be a vacuum leak causing idle surges? Vacuum leaks are known to trigger P0121 codes and result in unstable idling, especially in PARK or NEUTRAL. Some 2004 Hyundai Sonatas have reported intake gasket failures—worth checking for signs of leakage or cracks. Where did you source your TPS from? A used throttle body assembly might be a more reliable solution than just replacing the TPS alone.

Anonymous 2012-09-21 13:33

Yesterday, I sprayed brake cleaner over the throttle body area while the engine was running—no improvement. The idle surge persisted both before and after. UPDATE: After that test, the car now idles consistently at around 1500 RPM in PARK/NEUTRAL. This development has led me to suspect a possible ECU malfunction, though I’m still cautious about replacing it without further diagnostics.