Understanding the GMC P1475 Diagnostic Trouble Code
When your GMC Sierra, Yukon, or other model’s check engine light illuminates and a scan tool reveals code P1475, it indicates a specific fault within the vehicle’s auxiliary emission control systems. This OBD2 code is defined as “Auxiliary Emission Controls.” Unlike generic codes that point to a single component, P1475 is a manufacturer-specific code for General Motors vehicles, signaling that the Powertrain Control Module (PCM) has detected an electrical or functional problem in a circuit designed to reduce tailpipe emissions beyond the primary systems. This often involves components related to the Evaporative Emission Control (EVAP) system, which prevents fuel vapors from escaping into the atmosphere.
What Does “Auxiliary Emission Controls” Mean?
The term refers to secondary systems that support the main emission control framework. In the context of GMC trucks and SUVs, this P1475 code frequently pertains to circuits controlling devices like the EVAP canister purge solenoid, vacuum solenoids for secondary air injection (if equipped), or other dedicated vacuum-controlled valves. The PCM monitors the electrical current or voltage in these circuits. If the signal falls outside a predetermined range—indicating an open, short, or a component failure—it stores the P1475 code.
Symptoms of a P1475 Code in Your GMC
You may not experience dramatic drivability issues, but several signs can accompany this code:
- Illuminated Check Engine Light (MIL): The primary and most common symptom.
- Failed Emissions Test: The vehicle will likely fail a state or local smog inspection.
- Possible Rough Idle or Stalling: If the fault affects vacuum supply or purge flow, it can disrupt the air/fuel ratio.
- Reduced Fuel Efficiency: A malfunctioning EVAP purge system can impact fuel economy.
- No Obvious Symptoms: Often, the code is present with no noticeable change in vehicle performance.
Common Causes of the P1475 Code in GMC Vehicles
Diagnosing P1475 requires a systematic approach, focusing on the auxiliary emission control circuit. The fault can be electrical, mechanical, or a combination of both. Here are the most prevalent culprits.
1. Faulty EVAP Canister Purge Solenoid Valve
This is the most frequent cause. The purge valve, controlled by the PCM, opens to allow fuel vapors stored in the charcoal canister to be drawn into the engine and burned. A valve that is stuck open, stuck closed, or has an internal electrical failure will trigger P1475. A stuck-open valve can create a large vacuum leak and cause driveability problems.
2. Damaged or Disconnected Vacuum Hoses
The auxiliary emission controls rely on a network of vacuum hoses. A cracked, brittle, melted, or completely disconnected hose—especially the one connecting the purge solenoid to the intake manifold or canister—will break the circuit’s function and set the code. Always inspect hoses for physical damage and secure connections.
3. Electrical Issues in the Control Circuit
Problems in the wiring harness can mimic a failed component. Look for:
- Open or Shorted Wires: Frayed, broken, or pinched wires leading to the solenoid.
- Poor Electrical Connections: Corrosion or looseness at the solenoid connector or PCM terminals.
- Blown Fuse: Check the vehicle’s fuse box for a fuse dedicated to the EVAP or emission controls system.
4. Defective Vacuum Reservoir or Switch
Some GMC models use a vacuum reservoir (a small plastic tank) and switching valves to manage vacuum supply to various emission components. A leak in this reservoir or a failed switch can disrupt the entire auxiliary vacuum circuit.
5. Faulty Powertrain Control Module (PCM)
This is rare but possible. A PCM with an internal driver circuit failure cannot properly control the solenoid. This should only be considered after all other components and wiring have been thoroughly tested and ruled out.
Step-by-Step Diagnostic and Repair Guide for P1475
Proper diagnosis saves time and money. Follow this technical procedure to pinpoint the cause of P1475 on your GMC. You will need a basic multimeter and a scan tool capable of performing actuator tests.
Step 1: Preliminary Inspection and Visual Check
Begin with a thorough visual inspection. Locate the EVAP canister purge solenoid (typically near the engine bay’s firewall or on the intake manifold). Trace all associated vacuum hoses from the solenoid to the intake and to the charcoal canister. Look for obvious cracks, disconnections, or signs of melting. Inspect the electrical connector for corrosion or bent pins.
Step 2: Check the Purge Solenoid Valve Operation
- Mechanical Test: Remove the purge valve. Apply 12 volts directly to its terminals; you should hear a distinct click as it opens. Blow through the ports; air should flow when powered and be blocked when not.
- Electrical Test: With a multimeter, check the solenoid’s resistance. Refer to your vehicle’s service manual for specifications, but typical values range from 20 to 50 ohms. An infinite reading (open) or a reading of zero (short) indicates a bad solenoid.
Step 3: Test the Wiring and Power Supply
With the vehicle’s ignition ON (engine OFF), back-probe the solenoid connector. One wire should have battery voltage (12V). If not, check the related fuse. The other wire is the control wire from the PCM. Using a scan tool’s bidirectional controls to activate the purge solenoid should change the voltage on this control wire. A lack of signal points to a wiring issue or PCM problem.
Step 4: Perform a Vacuum System Integrity Check
Using a hand-held vacuum pump, apply vacuum to the system’s hoses and components. Vacuum should hold steady. A rapid drop indicates a leak. Pay special attention to the vacuum reservoir, check valves, and all plastic T-connectors in the lines.
Step 5: Clear the Code and Verify Repair
After replacing the faulty component (e.g., purge valve, hose) or repairing the wiring, clear the P1475 code with your scan tool. Drive the vehicle through a complete drive cycle to allow the PCM to run its self-tests. If the repair was successful, the check engine light will remain off, and the code will not return.
Conclusion: Addressing P1475 for Reliable Performance
While code P1475 on your GMC may not cause immediate breakdown, it signifies a fault in the vehicle’s critical emission control system. Ignoring it can lead to increased pollution, potential fuel economy loss, and a guaranteed failure during emissions testing. By understanding that this code points to the auxiliary circuits—often centered on the EVAP purge system—you can methodically diagnose the root cause, whether it’s a simple vacuum hose or a faulty solenoid valve. Addressing a P1475 code promptly restores your vehicle’s emission integrity and ensures it continues to run as designed. For complex electrical diagnostics, consulting a professional technician with advanced scan tools is always a recommended step.