Owner's Problem Description
The owner of a 2006 Nissan Altima SL reports that the engine stalls weekly. The vehicle does not accelerate when in Neutral or Park, but restarting the engine temporarily resolves the issue, although the 'Service Engine Soon' light remains illuminated. A Bosch KTS 200 scan revealed a P2138 code related to the accelerator pedal position sensor circuit, which aligns with the symptoms of poor acceleration. The owner is uncertain whether to replace the entire accelerator system or just the throttle position sensor.
Professional Technical Analysis
The 2006 Nissan Altima SL equipped with a 2.5L engine and CVT transmission, currently exhibiting intermittent engine stalling and lack of acceleration, likely has an estimated mileage of over 100,000 miles. The reported symptoms occur primarily during low-speed maneuvers or when idling, with the engine stalling about once a week. The vehicle's history indicates no previous repairs to the accelerator system or throttle body, making the current P2138 code a critical focus. This code signifies an issue in the accelerator pedal position sensor circuit, which is vital for proper throttle operation in this drive-by-wire system. In such systems, the accelerator pedal position sensor sends signals to the Engine Control Module (ECM) which, in turn, adjusts the throttle position accordingly. If this sensor fails, it can lead to a no-acceleration condition or unintended stalling. Additionally, the continuing illumination of the 'Service Engine Soon' light suggests that the fault is persistent and requires immediate attention. An analysis of the vehicle's electrical behavior reveals that the throttle position sensor should operate within a voltage range of approximately 0.5V (fully closed) to 4.5V (fully open), with a consistent signal during operation. It is essential to verify the integrity of the wiring and connectors associated with the accelerator pedal and throttle body to eliminate any potential connection issues. Given the age and mileage of the vehicle, wear and tear on these components should also be considered. Safety concerns arise from the stalling issue, particularly during traffic situations, necessitating prompt diagnosis and repair to ensure reliable vehicle operation.
Possible Causes
Most common causes (ordered by frequency):
- Throttle Position Sensor (TPS) Failure: The P2138 code indicates a fault in the accelerator pedal position sensor circuit, which is integral to the vehicle's throttle control system. This failure can manifest as erratic throttle response or complete lack of acceleration. The TPS should produce a voltage between 0.5V (closed) and 4.5V (wide open). If the voltage readings are inconsistent or outside this range, it could indicate a malfunctioning sensor or wiring issue. Given the vehicle's age and potential wear, replacing the TPS may resolve the stalling and acceleration problems. The OEM part number for the TPS is likely 22660-7S000, typically priced around $100-$150. - Persistent P2138 code, erratic throttle response, voltage readings from TPS inconsistent, engine stalls when idling or at low speeds.
- Wiring Issue in Accelerator Pedal Circuit: The accelerator pedal position sensor relies on a 5V reference, ground, and signal wire. Any damage, corrosion, or disconnection in these wires can disrupt the signal to the ECM, leading to erroneous throttle control and engine stalling. A visual inspection and continuity test of the wiring harness is essential. The repair may involve splicing or replacing damaged wires, which typically costs between $50-$100, depending on the extent of the damage. - Corrosion evident on wiring connectors, intermittent throttle performance, visual inspection shows wear on signal wire.
- Engine Control Module (ECM) Fault: An internal fault within the ECM could cause erroneous readings and control issues with the throttle system. If the accelerator pedal position sensor and wiring are confirmed functional, testing the ECM for faults becomes crucial. This may involve specialized diagnostic tools and could potentially require reprogramming or replacement of the ECM, typically costing $600-$1200 depending on the make and model. - No voltage output from ECM, communication faults detected with diagnostic scanner, potential software issues indicated.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Diagnosis: Begin with a thorough scan using GeekOBD APP or a professional OBD-II scanner to retrieve stored fault codes and freeze frame data. This data provides critical insight into the conditions under which the P2138 code is triggered. Pay attention to any additional codes that may indicate related faults in the throttle control system. Document all findings for further analysis.
- STEP 2 - Live Data Monitoring: Utilize the GeekOBD APP to monitor live data from the accelerator pedal position sensor and throttle position sensor. Check the voltage readings during various driving conditions. The TPS should consistently read between 0.5V and 4.5V. Any irregularities in these readings can pinpoint potential failures. Additionally, monitor engine RPM and throttle response to assess performance under load.
- STEP 3 - Wiring Inspection: Conduct a thorough visual and continuity inspection of the wiring harness associated with the accelerator pedal and throttle position sensor. Look for signs of wear, corrosion, or damage. Use a multimeter to check for continuity between connectors and ensure secure connections. Repair any damaged wires or connectors identified during inspection.
- STEP 4 - Component Testing: If the wiring is intact, proceed to test the throttle position sensor and accelerator pedal position sensor using a multimeter. Measure the voltage output of the TPS while manipulating the throttle plate. Compare the readings against the specified ranges. If the readings are out of range, consider replacing the TPS. Recheck all connections and clear any fault codes before retesting.
