Diagnosing engine issues on your 2003 Silverado 6.0 often involves using a scan tool to read sensor data. One crucial reading is the Throttle Position Sensor (TPS) voltage. If you’re seeing unusual TPS voltage readings or a P0122 code, this guide is for you. We’ll break down what TPS voltage means, what a P0122 code indicates, and how to diagnose the problem using your scan tool.
Understanding DTC P0122 – Throttle Position Sensor Circuit Low Voltage
The Throttle Position Sensor (TPS) is a vital component in your 2003 Silverado 6.0’s engine management system. It’s essentially a potentiometer mounted on the throttle body that monitors the position of the throttle blade. As the throttle opens and closes, the TPS sends a varying voltage signal to the Powertrain Control Module (PCM). This signal is critical because the PCM uses it to determine engine load, control fuel delivery, ignition timing, and transmission operation.
According to GM’s service information, the TPS should typically read around 0.6 volts at idle and increase to over 4.0 volts at Wide Open Throttle (WOT). A DTC P0122, which stands for “Throttle Position Sensor Circuit Low Voltage,” is triggered when the PCM detects a TPS signal voltage that falls below the expected operating range – specifically, less than 0.2 volts for a continuous period. This low voltage indicates a potential issue within the TPS circuit that needs to be addressed.
When this DTC sets, it’s not just a warning light. The PCM takes action to protect the engine and transmission, which can lead to noticeable drivability problems.
Symptoms of a P0122 Code in a 2003 Silverado 6.0
When your 2003 Silverado 6.0 throws a P0122 code due to low TPS voltage, you might experience several symptoms, including:
- Malfunction Indicator Lamp (MIL) or Check Engine Light: This is the most obvious sign, illuminating on your dashboard to alert you to an issue.
- Transmission Issues: The transmission control module relies on accurate TPS data. A faulty reading can cause:
- Torque Converter Clutch (TCC) not applying.
- High transmission line pressure.
- Fixed or harsh transmission shift points.
- No fourth gear engagement, especially in hot mode.
- PCM Default Values: The PCM may use a default value for the TPS, which can affect overall engine performance and fuel efficiency.
- Potential for Intermittent Issues: Problems like misrouted harnesses, chafed wires, or broken wires inside the insulation can cause intermittent P0122 codes, making diagnosis trickier.
Diagnosing P0122 on a 2003 Silverado 6.0 Using a Scan Tool: A Step-by-Step Guide
To effectively diagnose a P0122 code on your 2003 Silverado 6.0, you’ll need a scan tool capable of reading live data and DTCs, and ideally a multimeter for circuit testing. Here’s a systematic approach based on the factory diagnostic procedure:
Step 1: Initial Scan and OBD System Check
Begin by connecting your scan tool to your Silverado’s OBD-II port and check for Diagnostic Trouble Codes (DTCs). If P0122 is present, note if there are any other codes, particularly P1639 (5 Volt Reference 2 Circuit). A P1639 along with P0122 can indicate a broader issue with the 5V reference circuit. Clear all DTCs and perform a Powertrain On-Board Diagnostic (OBD) system check using your scan tool if available.
Step 2: Monitor TPS Voltage at Idle
Start the engine and let it idle. Using your scan tool, monitor the TPS voltage reading. With the throttle closed (at idle), the TPS voltage should be around 0.6 volts. If the voltage is reading below 0.2 volts, proceed to the next steps.
Step 3: Review Freeze Frame Data and Replicate Conditions
If the TPS voltage is not immediately low, but you’ve had a P0122 code, review the Freeze Frame and/or Failure Records data stored by the PCM when the DTC was set. This data captures the engine parameters at the time of the fault. Note the RPM, engine load, vehicle speed, temperature, and other relevant conditions. Turn off the ignition for about 15 seconds, then restart the engine and attempt to operate the vehicle under the conditions recorded in the Freeze Frame data. Monitor the TPS voltage again with your scan tool to see if the low voltage and P0122 code reappear.
Step 4: Isolate the Accelerator and Servo Control Module (ASCM)
Turn off the ignition. Disconnect the Accelerator and Servo Control Module (ASCM). This module, though mentioned in the original text, might be more relevant to certain models or brake systems – in the context of a TPS issue for a 2003 Silverado 6.0, this step is likely to isolate potential shorts or issues within the throttle body or TPS wiring itself, rather than a separate module. Turn the ignition back ON with the engine OFF and monitor the TPS voltage on your scan tool again.
- If the TPS voltage is still below 0.2V: This suggests the issue is likely within the TPS sensor, its wiring harness leading to the PCM, or potentially the PCM itself. Proceed to Step 5.
- If the TPS voltage is now within the normal range: This could indicate an issue related to the ASCM or its connection. However, for a basic P0122 diagnosis on a 2003 Silverado 6.0, focusing on the TPS and its direct circuit is more pertinent.
Step 5: Test TPS Sensor Circuit with Jumper Wire
Turn OFF the ignition and disconnect the electrical connector at the TPS sensor. Using a jumper wire, carefully connect the 5.0-volt reference circuit terminal at the TPS harness connector to the TPS signal circuit terminal. Turn the ignition ON (engine OFF) and observe the TPS voltage reading on your scan tool.
- If the TPS voltage is now above 4.7V: This indicates that the wiring between the TPS connector and the PCM, as well as the PCM itself, are likely functioning correctly in terms of voltage supply and signal reception. The problem is likely the TPS sensor itself. Proceed to Step 12 (TPS sensor replacement).
- If the TPS voltage is still below 4.7V: This points to a potential issue in the 5-volt reference circuit, the TPS signal circuit wiring, or the PCM. Proceed to Step 6.
Step 6: Test TPS Signal Circuit with Test Lamp
Disconnect the jumper wire. Connect a test lamp (like J 34142-B or a standard test light) between a known good battery positive (B+) source and the TPS signal circuit terminal at the TPS harness connector. Turn the ignition ON (engine OFF) and observe the TPS voltage on the scan tool.
- If the TPS voltage is now above 4.7V: This suggests that the issue is likely in the TPS sensor itself or potentially in the ground circuit of the TPS. Proceed to Step 8 to check the 5V reference circuit.
- If the TPS voltage is still below 4.7V: This indicates a potential open or short to ground in the TPS signal circuit. Proceed to Step 10 to test the TPS signal circuit wiring.
Step 7: Test 5-Volt Reference Circuit for Open or Short to Ground
Turn OFF the ignition. Disconnect the PCM connector C2 (refer to a wiring diagram for your 2003 Silverado 6.0 to identify the correct connector and pinout). Using a multimeter in resistance mode, test the 5-volt reference circuit wire (at the PCM connector and TPS connector ends if accessible) for:
- Open Circuit: Check for continuity along the wire. An open circuit would mean no continuity (infinite resistance).
- Short to Ground: Check for resistance between the 5-volt reference circuit wire and a known good ground. A short to ground would show very low resistance (close to 0 ohms).
Repair any identified open circuits or shorts to ground in the 5-volt reference circuit as needed, using proper wiring repair techniques.
Step 8: Check 5-Volt Reference Circuit Connection at PCM
If no wiring issues were found in Step 7, suspect a poor connection at the PCM connector for the 5-volt reference circuit. Inspect the PCM connector pins and terminals for corrosion, damage, or looseness. Clean and repair any poor connections as necessary.
Step 9: Test TPS Signal Circuit for Open, Short to Ground, or Short to Sensor Ground
Turn OFF the ignition and ensure the PCM C2 connector is still disconnected. Using a multimeter, test the TPS signal circuit wire (again, at both PCM and TPS connector ends if possible) for:
- Open Circuit: Check for continuity along the wire.
- Short to Ground: Check for resistance between the TPS signal circuit wire and a known good ground.
- Short to Sensor Ground: Check for resistance between the TPS signal circuit wire and the sensor ground circuit wire.
Repair any identified open circuits or shorts in the TPS signal circuit.
Step 10: Check TPS Signal Circuit Connection at PCM
Similar to Step 8, if no wiring issues were found in Step 9, inspect the PCM connector for the TPS signal circuit for poor connections. Clean and repair as needed.
Step 11: Replace the TPS Sensor
If all wiring and PCM connection tests have passed, and the tests in Step 5 and 6 indicated a likely sensor issue, replace the Throttle Position Sensor. Refer to a repair manual for the specific replacement procedure for your 2003 Silverado 6.0.
Step 12: Replace the PCM (as a last resort)
PCM replacement is rare and should only be considered if all other diagnostic steps have been exhausted and you strongly suspect a faulty PCM based on your testing and elimination of other possibilities. Important: If you replace the PCM, it will likely need to be programmed to your vehicle.
Step 13: Verify Repair with Scan Tool and Clear DTCs
After performing any repairs (wiring fixes, sensor replacement, or PCM replacement), reconnect all components, and turn the ignition ON. Use your scan tool to clear all DTCs, including P0122. Start the engine and let it reach normal operating temperature. Use your scan tool to monitor the TPS voltage reading again to ensure it is now within the normal range at idle and responds correctly when the throttle is opened.
Step 14: Run DTC Test and Verify Pass
Using your scan tool, select the specific DTC test for P0122 (if available) and run the test. Operate the vehicle under the conditions specified in the “Conditions for Running the DTC” section of the original service information (if you have access to it), or under typical driving conditions where the P0122 code previously appeared.
- If the scan tool indicates the test ran and passed: The repair is successful. Proceed to Step 15.
- If the scan tool indicates the test failed or the P0122 code returns: Go back to Step 2 and re-evaluate your diagnosis or re-perform the diagnostic steps.
Step 15: Final System Check
Finally, use your scan tool to review for any remaining DTCs that you haven’t addressed. If there are any new or unresolved codes, diagnose those accordingly. If no DTCs are present and the P0122 is resolved, the system is OK.
Conclusion
Diagnosing a P0122 code on your 2003 Silverado 6.0 requires a systematic approach using a scan tool and potentially a multimeter. By following these steps, you can effectively pinpoint the cause of the low TPS voltage, whether it’s a faulty sensor, wiring issue, or, in rare cases, a PCM problem. Remember to always consult a repair manual specific to your vehicle for detailed wiring diagrams and component locations, and if you are not comfortable with electrical testing or automotive repair, it’s best to seek assistance from a qualified mechanic.
