The vehicle in question exhibits a hard RPM limitation at approximately 3000 RPM under all driving conditions (idle, neutral revving, and under load). No abnormal noises or vibrations are present, but the engine appears to “hit a wall” at this threshold.

Vehicle Details: 2012 Sedan, 2.4L 4-cylinder, 120,000 miles, no recent modifications

2. Diagnostic Steps Taken

Initial Assessment & Safety Check

• Verified proper oil level and condition
• Confirmed adequate coolant level
• Battery voltage at 12.6V (engine off), 14.2V (running)
• No visible vacuum leaks or disconnected sensors

OBD-II Scan Results

• Stored code: P0121 (Throttle/Pedal Position Sensor “A” Circuit Range/Performance)
• Pending code: P0172 (System Too Rich Bank 1)

Live Data Analysis

• Throttle Position Sensor (TPS) shows erratic voltage jumps between 12% and 78% at steady pedal position
• MAF sensor reading 3.2 g/s at idle (within spec) but fails to increase linearly with RPM
• Short-Term Fuel Trim (STFT): +18% to +25% at 2500 RPM
• Long-Term Fuel Trim (LTFT): +12%
• Oxygen sensors show slow response time (Bank 1 Sensor 1: 0.1-0.8V oscillation at 1Hz)

Fuel System Tests

• Fuel pressure: 48 psi (spec: 50-55 psi)
• Injector pulse width is normal but slightly elevated

Ignition System Check

• Spark plugs show normal wear (gap 0.042″, light tan deposits)
• All coils producing a strong spark (tested with ignition tester)

3. Identified Root Cause(s)

The primary issue stems from a failing throttle position sensor (TPS), causing:

• Erratic throttle position signals to the ECU
• Forced rich condition (compensating for perceived throttle inconsistencies)
• ECU-initiated RPM limitation as a fail-safe measure

Secondary contributing factors:

• Marginally low fuel pressure (possible early fuel pump wear)
• Sluggish oxygen sensor response (not primary cause, but affecting fuel trims)

4. Recommended Repairs

Priority 1: Immediate Repairs

1. Replace throttle position sensor (OEM recommended) – Most likely to resolve RPM limitation
2. Clean throttle body (carbon buildup observed)
3. Perform ECU reset and throttle relearn procedure

Priority 2: Recommended Follow-up

1. Monitor fuel pressure over the next 1,000 miles (potential fuel pump replacement)
2. Replace the upstream oxygen sensor (Bank 1 Sensor 1) if fuel trims remain elevated after TPS replacement

5. Preventive Maintenance Tips

• Throttle Body Maintenance: Clean every 30,000 miles to prevent carbon buildup
• Fuel System Care: Replace fuel filter every 50,000 miles; use top-tier gasoline
• Sensor Longevity: Apply dielectric grease to electrical connectors during sensor replacement
• Diagnostic Habits: Scan for codes monthly, even without warning lights, to catch issues early

Verification & Test Drive Results

After TPS replacement and throttle body cleaning:

• RPM now reaches 6500 (redline) without hesitation
• STFT normalized to ±5% across RPM range
• TPS voltage now shows smooth progression from 0.5V (closed) to 4.5V (WOT)
• Code P0121 has not returned after 3 drive cycles