Investigating P0121 on Your 2020 Lexus IS
A P0121 code on your 2020 Lexus IS means the ECM has identified an irregular throttle position sensor pattern. The TPS voltage is within expected limits, but the signal trace doesn't match what the computer expects from your pedal inputs—our investigation focuses on the electronic throttle body and its communication with the ECM.
The 2020 IS is available as the IS 300 (2.0L turbo-four with direct injection for RWD, or 3.5L V6 with D-4S for AWD) and IS 350 (3.5L V6 with D-4S dual injection). The 2.0L turbo uses direct injection only, making it more prone to carbon buildup than the 3.5L V6 with its port+direct injection system.
Symptoms
- Check engine light illuminated
- Hesitation during spirited acceleration
- Idle roughness
- Throttle response feels inconsistent
- Slight surging at steady cruise
Causes Ranked
- Carbon buildup on throttle body – At 4–6 years old, carbon deposits from PCV vapors have accumulated. The 2.0L turbo DI model is especially susceptible. Likelihood: 45%
- Throttle body assembly wear – Internal TPS sensor or motor component degradation over time. Likelihood: 25%
- Wiring or connector issue – Age and heat cycling degrade connectors and wiring insulation. Likelihood: 15%
- ECM software update – Toyota/Lexus may have released recalibrations for this generation IS. Likelihood: 10%
- Intake air leak – Cracked intake boot or loose turbo piping (2.0T models). Likelihood: 5%
Repair Costs
- Throttle body cleaning: $100–$180 (shop) / $10–$25 (DIY)
- Throttle body replacement: $350–$600 parts and labor
- Wiring repair: $100–$250
- ECM update: $100–$200 (dealer)
DIY Feasibility: Moderate to High
The 4-year/50,000-mile basic warranty has likely expired, but the federal emissions warranty (8 years/80,000 miles) may cover this repair if your IS has under 80,000 miles. Ask your Lexus dealer before paying out of pocket. For DIY, the throttle body is accessible on both engines, though the 2.0L turbo is slightly tighter due to turbo plumbing. After cleaning, perform a throttle relearn: disconnect the battery for 30 minutes, reconnect, and idle for 10–15 minutes in Park.
How AutoDetective AI Can Help
AutoDetective's AI differentiates between the IS's 2.0L turbo and 3.5L V6 powertrains to rank causes specific to your engine. Upload your scan data for a free tailored diagnosis.