DTC P15A1: Troubleshooting Fuel Economy Mode Switch Performance

The diagnostic trouble code (DTC) P15a1 indicates a “Fuel Economy Mode Switch Performance” issue in your vehicle. This code is triggered when the Engine Control Module (ECM) detects an abnormal signal from the fuel economy mode switch circuit. Understanding and addressing this code promptly is crucial for maintaining your vehicle’s fuel efficiency and overall performance. This article provides a comprehensive guide to diagnosing and resolving DTC P15A1.

Understanding DTC P15A1

DTC P15A1 specifically points to a problem within the fuel economy mode switch circuit. This circuit involves the fuel economy mode switch, the ECM, and the wiring connecting them. The fuel economy mode switch allows the driver to request a fuel-efficient driving mode. When activated, the ECM and Transmission Control Module (TCM) adjust engine and transmission parameters to optimize fuel consumption.

The ECM supplies a 5-volt reference voltage to the fuel economy mode switch. When the switch is pressed, it changes the resistance in the circuit, causing a voltage drop that the ECM interprets as a fuel economy mode request. The ECM then communicates this request to the TCM and Instrument Panel Cluster (IPC). DTC P15A1 sets when the ECM detects that the voltage signal from the fuel economy mode switch is in an indeterminate or unexpected range, suggesting a malfunction in the circuit or switch itself.

Symptoms of a P15A1 Code

While the presence of a DTC P15A1 might not always be accompanied by obvious symptoms, you may observe the following:

Fuel Economy Mode Indicator Lamp Malfunction: The fuel economy indicator light on your dashboard might not illuminate when the switch is pressed, or it may behave erratically.
No Change in Driving Performance: You might not notice any difference in fuel economy or driving characteristics when pressing the fuel economy mode switch.
Check Engine Light (MIL) Illumination: The malfunction indicator lamp (MIL), or check engine light, will typically illuminate to alert you to the problem.
Possible Intermittent Issues: The problem might be intermittent, meaning the symptoms and the DTC might come and go.

Potential Causes of DTC P15A1

Several factors can lead to a P15A1 code. These can be broadly categorized into electrical circuit issues, switch malfunctions, and ECM problems:

Faulty Fuel Economy Mode Switch: The switch itself might be defective, with internal resistance issues or contact problems preventing proper signal transmission.
Wiring Issues: Problems in the wiring harness connecting the fuel economy mode switch to the ECM can cause P15A1. This includes:
- Open Circuit: A break in the wiring, preventing signal flow.
- Short to Ground: The signal wire might be shorting to a ground, causing an incorrect voltage reading.
- High Resistance: Corrosion or loose connections in the wiring can increase resistance, affecting signal integrity.
- Short to Voltage: The signal wire might be shorting to a voltage source, leading to an abnormally high voltage reading.
ECM Issues: In rare cases, a problem within the ECM itself, specifically with the circuits monitoring the fuel economy mode switch, could trigger P15A1.
Ground Circuit Issues: A poor ground connection for the fuel economy mode switch circuit can also lead to this DTC.

Diagnosing DTC P15A1: Step-by-Step Guide

Diagnosing DTC P15A1 requires a systematic approach, starting with basic checks and progressing to more in-depth circuit testing. Here’s a step-by-step diagnostic procedure:

Preliminary Checks:

Diagnostic System Check: Before proceeding, perform a complete Diagnostic System Check of the vehicle using a scan tool to identify any other related DTCs. Address any other codes first, as they might be related or contributing to P15A1.
Review Diagnostic Trouble Code Definitions: Understand the different DTC types (Type B in this case) and their implications for clearing and setting conditions.
Check for Related DTCs: Verify if DTCs P159F and P15A0 are also present. These codes can provide valuable clues. According to the Diagnostic Fault Information table, P159F might indicate shorts to ground or open circuits, while P15A0 might suggest shorts to voltage or ground circuit issues in the Fuel Economy Mode Switch circuit.

Circuit/System Verification:

Intermittent Condition Test: If the condition is not consistently present, refer to procedures for “Testing for Intermittent Conditions and Poor Connections” to identify potential wiring or connector issues that may be causing the fault intermittently.
Verify Absence of Related DTCs: Ensure that DTCs P159F and P015A0 are not set. If they are, diagnose and repair those codes first, as they might be the root cause or related to P15A1.
Scan Tool Observation: With the engine running, use a scan tool to monitor DTC information. DTC P15A1 should not be setting if the system is functioning correctly.
Operate Vehicle Under DTC Conditions: Drive the vehicle under the “Conditions for Running the DTC” (ignition ON) to see if the code resets. You can also try to replicate the conditions recorded in the Freeze Frame/Failure Records data, if available.

Circuit/System Testing:

Note: Ensure you have completed the Circuit/System Verification before proceeding with these tests.

Ground Circuit Test:
- Turn ignition OFF and disconnect the harness connector X1 at the Fuel Economy Mode switch.
- Using a Digital Multimeter (DMM), test for resistance between the ground circuit terminal 3 of the harness connector and a known good ground.
- The resistance should be less than 5 ohms.
- If the resistance is higher, investigate the ground circuit for high resistance and repair as needed. If the ground circuit is normal, the ECM (K20) might be faulty.
Switch Resistance Tests:
- Measure the resistance between the 5V reference voltage circuit terminal 1 and the signal circuit terminal 2 of the switch. The expected range is 266-294 ohms. If outside this range, replace the Fuel Economy Mode switch (S112).
- Measure the resistance between the 5V reference voltage circuit terminal 1 and the ground circuit terminal 3 of the switch. The expected range is 1,414-1,562 ohms. If outside this range, replace the Fuel Economy Mode switch (S112).
- Depress the fuel economy mode switch button and measure the resistance between the 5V reference voltage circuit terminal 1 and the ground circuit terminal 3 of the switch again. The expected range is 486-538 ohms. If outside this range, replace the Fuel Economy Mode switch (S112).
Circuit Continuity Tests:
- Disconnect the harness connector X2 at the ECM.
- Test the 5V reference voltage circuit for continuity between terminal 29 of the ECM harness connector X2 and terminal 1 of the Fuel Economy Mode switch harness connector X1. Resistance should be less than 5 ohms. If higher, repair or replace the 5V reference voltage circuit.
- Test the signal circuit for continuity between terminal 18 of the ECM harness connector X2 and terminal 2 of the Fuel Economy Mode switch harness connector X1. Resistance should be less than 5 ohms. If higher, repair or replace the signal circuit.
ECM Replacement (If Necessary): If all circuit tests are normal and the switch tests indicate a problem, or if the ground circuit test indicated a potential ECM issue, replacing the ECM (K20) might be necessary.

Repairing DTC P15A1

After identifying the faulty component or circuit, perform the necessary repairs. This may include:

Fuel Economy Mode Switch Replacement: If the switch resistance tests fail, replace the S112 fuel economy mode switch.
Wiring Repair: Repair any open circuits, shorts to ground, shorts to voltage, or high resistance issues in the wiring harness. This might involve repairing damaged wires, cleaning corroded connectors, or replacing wiring sections. Refer to “Wiring Repairs” and “Connector Repairs” documentation for guidance.
ECM Replacement and Programming: If the ECM is diagnosed as faulty, replace it and perform the necessary setup and programming procedures as outlined in “Control Module References”.
Ground Circuit Repair: If a poor ground connection is identified, clean and tighten the ground connection point or repair the ground wire as needed.

Post-Repair Verification:

After completing any repairs, it’s crucial to verify the fix:

Diagnostic Repair Verification: Perform a “Diagnostic Repair Verification” procedure to ensure the issue is resolved.
Clear DTCs: Clear the DTC P15A1 using a scan tool.
System Verification: Repeat the “Circuit/System Verification” steps to confirm that the DTC does not reset and the fuel economy mode switch is functioning correctly.
Road Test: Road test the vehicle to ensure the symptoms are gone and the vehicle is performing as expected.

By following these diagnostic and repair steps, you can effectively address DTC P15A1 and restore the proper functionality of the fuel economy mode switch system, ensuring optimal fuel efficiency and vehicle performance. Remember to always consult the vehicle-specific service manual for detailed procedures and specifications.