P0122 on Your 2018 Honda Civic: The Evidence Points to a Circuit Fault
A P0122 on your 2018 Honda Civic means the ECM has caught the throttle position sensor voltage dropping below the minimum acceptable level. This is an electrical circuit failure—the TPS signal to the ECM has gone critically low or dead. Our detective has identified that a key communication link in the throttle system has broken down.
The 2018 Civic (10th generation) uses a 2.0L NA four-cylinder or 1.5L VTEC turbo. At 6–8 years old, connector corrosion and component aging are primary concerns. The electronic throttle body's TPS is integrated and not separately serviceable.
Symptoms
- Check engine light on
- Severe or complete throttle loss
- Engine idles but won't respond to gas pedal
- Limp mode with limited RPM
- Possible stalling at intersections
Causes at This Age
- Connector corrosion and pin damage – 6+ years of heat cycling and environmental exposure. This is the #1 suspect at this age. Likelihood: 35%
- Worn throttle body assembly – The internal TPS sensor has degraded and developed a short or open circuit. Likelihood: 30%
- Wiring harness deterioration – Aged insulation cracking, wire chafing against engine components. Likelihood: 20%
- 5V reference circuit fault – Shared reference line pulled low by another failed sensor. Likelihood: 10%
- Poor ground connection – Corroded engine ground strap or body ground. Likelihood: 5%
Repair Costs
- Connector cleaning/repair: $50–$150
- Throttle body replacement: $250–$450 parts and labor
- Wiring repair: $150–$350
- Ground strap replacement: $30–$100
DIY Feasibility: High
Your 2018 Civic is out of basic warranty but may be within the federal emissions warranty (8 years/80,000 miles) through 2026. Check your mileage. For DIY, start with connector inspection: unplug the throttle body connector, clean all pins with contact cleaner, check for bent or recessed pins, apply dielectric grease. Use a multimeter to verify 5V reference at the connector (key ON, engine OFF). If reference voltage is present and connector is clean but the code returns, replace the throttle body. Honda's idle relearn: key ON 10 seconds, start, idle 10 minutes.
How AutoDetective AI Can Help
AutoDetective's AI helps you determine whether a $10 connector cleaning or a $400 throttle body replacement is needed. Upload your scan data for a free diagnosis.