Encountering a check engine light in your Nissan Altima can be unsettling, and deciphering the cryptic OBD2 codes that accompany it is often the first step towards resolving the issue. Among the numerous codes that might surface, P0502 and P1122 are relatively common, yet understanding their implications is crucial for effective car maintenance and repair.
This guide, brought to you by the experts at techcarusa.com, will delve deep into these two specific OBD2 codes for Nissan Altima vehicles. We’ll break down what they mean, their potential causes, symptoms to watch out for, and most importantly, how to troubleshoot and fix them. Whether you’re a seasoned DIY enthusiast or just starting to learn about car mechanics, this article will equip you with the knowledge to tackle these codes head-on.
Understanding OBD2 Codes P0502 and P1122
Before we dive into the specifics, let’s briefly understand what OBD2 codes are and how they can help diagnose car problems. OBD2 (On-Board Diagnostics II) is a standardized system that monitors various components and systems in your vehicle. When it detects an issue, it triggers the check engine light and stores a diagnostic trouble code (DTC). These codes are five-digit alphanumeric codes that provide a starting point for diagnosing the problem.
Now, let’s focus on our primary codes:
OBD2 Code P0502: Vehicle Speed Sensor (VSS) Low Input
P0502 indicates a “Vehicle Speed Sensor Low Input”. This means the Engine Control Module (ECM) is receiving a signal from the Vehicle Speed Sensor that is lower than expected, or nonexistent, for a certain period. The VSS is crucial as it measures the speed of your vehicle and sends this information to various systems, including the speedometer, cruise control, transmission control, and anti-lock braking system (ABS).
Potential Causes of P0502:
- Faulty Vehicle Speed Sensor (VSS): The sensor itself might be damaged or malfunctioning.
- Wiring Issues: Damaged, corroded, or loose wiring to the VSS can disrupt the signal.
- Connector Problems: A loose or corroded connector at the VSS or ECM can cause signal loss.
- ECM Malfunction: In rare cases, the ECM itself might be faulty and misinterpreting the VSS signal.
- Instrument Cluster Issues: Although less common, problems within the instrument cluster can sometimes trigger this code.
Symptoms of P0502:
- Check Engine Light is on: This is the most obvious symptom.
- Speedometer Malfunction: The speedometer may be inaccurate or stop working altogether.
- Cruise Control Issues: Cruise control may not engage or may disengage intermittently.
- Transmission Problems: Shifting issues, erratic shifting, or transmission not shifting properly could occur.
- ABS Light might illuminate: As VSS data is used by the ABS system, the ABS light might also come on.
OBD2 Code P1122: Electronic Throttle Control (ETC) Function Circuit Low Input
P1122 signals an “Electronic Throttle Control Function Circuit Low Input”. This code points to a problem within the electronic throttle control system, specifically indicating a low voltage signal in the throttle position sensor circuit. The ETC system replaces the traditional mechanical throttle cable with electronic sensors and actuators to control the throttle valve opening, thus regulating engine power.
Potential Causes of P1122:
- Faulty Throttle Position Sensor (TPS): The TPS might be failing to provide the correct voltage signal.
- Throttle Actuator Issues: The throttle actuator, which physically moves the throttle valve, could be malfunctioning.
- Wiring and Connector Problems: Similar to P0502, wiring issues (shorts, opens, corrosion) and connector problems in the ETC system can cause P1122.
- ECM Issues: Although less frequent, the ECM could be misinterpreting signals or have internal faults related to the ETC system.
- Dirty or Carbon Buildup in Throttle Body: Excessive carbon buildup in the throttle body can impede throttle plate movement and affect TPS readings.
Symptoms of P1122:
- Check Engine Light is on: Again, the primary indicator.
- Reduced Engine Power: The vehicle might enter limp mode, significantly reducing engine power and acceleration.
- Poor Throttle Response: The engine might be slow to respond when you press the accelerator pedal.
- Engine Stalling or Hesitation: The engine might stall at idle or hesitate during acceleration.
- Idle Problems: The engine might idle erratically or at an incorrect RPM.
Diagnosing and Fixing P0502 and P1122 on Your Nissan Altima
Now that we understand what these codes mean and their potential causes, let’s explore how to diagnose and fix them on your Nissan Altima.
Step 1: Reading the OBD2 Codes
While the original forum post mentions a pedal method to read codes, a more straightforward and reliable approach is to use an OBD2 scanner. These scanners are readily available online or at auto parts stores at various price points.
- Locate the OBD2 port: This is typically found under the dashboard on the driver’s side.
- Plug in the scanner: Turn the ignition to the “ON” position (do not start the engine) and plug the scanner into the OBD2 port.
- Read the codes: Follow the scanner’s instructions to read the stored codes. Note down all codes present, including P0502 and P1122.
- Clear the codes (optional but recommended for diagnosis): Most scanners have a “clear codes” function. Clearing the codes and then test driving the vehicle can help determine if the codes reappear, indicating an ongoing issue.
Step 2: Diagnosing P0502 – Vehicle Speed Sensor Low Input
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Visual Inspection:
- Inspect the VSS: Locate the VSS (usually on the transmission or wheel hub). Check for any visible damage to the sensor or its wiring.
- Check Wiring and Connectors: Carefully examine the wiring harness and connectors leading to the VSS for damage, corrosion, or loose connections.
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VSS Testing (Multimeter required):
- Check for power and ground: Use a multimeter to verify that the VSS is receiving power and ground. Refer to your Altima’s repair manual for specific pinouts and voltage specifications.
- Signal Output Test: With the vehicle safely raised and the wheels able to rotate (ensure safety precautions are taken), use the multimeter to check the VSS signal output while slowly rotating the wheel/transmission input shaft. Compare the readings to the specifications in your repair manual.
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Wiring Continuity Test (Multimeter required):
- If wiring damage is suspected, use a multimeter to perform a continuity test on each wire in the VSS circuit, from the sensor connector to the ECM connector.
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ECM Check (Advanced):
- If all wiring and sensor tests pass, the ECM might be the issue, although this is less likely. ECM testing often requires specialized equipment and expertise, so consider professional diagnosis at this stage.
Step 3: Diagnosing P1122 – Electronic Throttle Control Function Circuit Low Input
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Visual Inspection:
- Throttle Body Inspection: Check the throttle body for carbon buildup or obstructions. Clean the throttle body if necessary using throttle body cleaner and a soft cloth.
- Inspect TPS and Actuator Wiring and Connectors: Similar to the VSS, inspect the wiring and connectors for the TPS and throttle actuator for any damage, corrosion, or loose connections.
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TPS Testing (Multimeter required):
- Voltage Check: With the ignition ON, use a multimeter to check the TPS voltage at idle and as the throttle is opened. Compare readings to the specifications in your repair manual. A smooth and consistent voltage change should be observed as the throttle is moved.
- Resistance Test: Some TPS sensors can also be tested for resistance. Refer to your repair manual for specific procedures.
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Throttle Actuator Test (Scanner with Actuator Control Recommended):
- Some advanced OBD2 scanners have actuator control functions. These can be used to command the throttle actuator to open and close the throttle body, allowing you to check its functionality.
- Manual Actuator Check: Carefully try to manually move the throttle linkage (engine off). It should move smoothly without binding.
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Wiring Continuity Test (Multimeter required):
- Perform continuity tests on the wiring between the TPS, throttle actuator, and ECM, similar to the VSS wiring test.
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ECM Check (Advanced):
- Again, if all other components test okay, the ECM could be the issue, but professional diagnosis is recommended for ECM problems.
Step 4: Repair and Resolution
Based on your diagnosis, the repair steps will vary:
- Replacing Faulty Sensors: If the VSS or TPS is found to be faulty, replace it with a new, quality sensor. Ensure proper installation and connection.
- Wiring Repair: Repair any damaged wiring (splice, replace sections) and clean corroded connectors. Ensure secure connections.
- Throttle Body Cleaning: Clean the throttle body thoroughly to remove carbon buildup.
- ECM Replacement (Last Resort): If the ECM is diagnosed as faulty, it will need to be replaced and potentially reprogrammed. This is best left to a qualified mechanic.
Step 5: Verify the Repair
After performing any repairs:
- Clear the OBD2 codes: Use your scanner to clear all DTCs.
- Test Drive: Take your Altima for a test drive under various driving conditions to see if the check engine light returns and if the symptoms are resolved.
- Rescan for Codes: After the test drive, rescan for OBD2 codes to ensure no codes have reappeared.
DIY Pedal Method for ECM Diagnostics/Reset (From Original Text)
While OBD2 scanners are recommended, the original forum post describes a pedal method for accessing diagnostic mode and resetting the ECM. Use this method with caution and at your own risk, as incorrect procedures could potentially cause issues.
ECM Diagnostics/ECM Reset Procedure (As described in original text):
- Sit in the driver’s seat.
- Turn the ignition key to the ON position and wait three seconds. (Do not start the car.)
- Fully depress and release the accelerator pedal five times in less than five seconds.
- Wait exactly seven seconds.
- Fully depress the accelerator pedal for ten seconds until the MIL (SES) light flashes.
- Release the accelerator pedal and start counting flashes to obtain the four-digit trouble code.
- Long flashes (0.6 seconds) indicate the first digit.
- Faster blinks (0.3 second) for the next three digits with 1.0-second pause between digits.
- Ten blinks indicate a zero.
- Four blinks of ten (0000) indicate no malfunction.
- To clear the code: Hold down the accelerator pedal for more than 10 seconds while in Diagnostic Test Mode II. Release the pedal to erase codes.
- Turn the ignition key to the OFF position to exit diagnostic mode and reset the ECM.
Important Note: This pedal method is described as being from an Altima forum and “working just fine”. However, its reliability and applicability across all Altima models and years may vary. Always consult your vehicle’s repair manual for the most accurate and recommended diagnostic procedures.
Conclusion
Dealing with OBD2 codes P0502 and P1122 in your Nissan Altima can seem daunting, but with a systematic approach to diagnosis and repair, you can effectively resolve these issues. Remember to prioritize safety, use the right tools, and consult your vehicle’s repair manual for specific procedures and specifications.
If you’re uncomfortable performing these diagnostics or repairs yourself, or if the problem persists, it’s always best to seek assistance from a qualified mechanic at a trusted repair shop. Keeping your vehicle properly maintained ensures its longevity and your safety on the road. Stay tuned to techcarusa.com for more expert car repair guides and tips!
