What is OBD2 Code P1477 in a Buick?
When your Buick’s check engine light illuminates and a diagnostic scan reveals trouble code P1477, it indicates a specific fault within the vehicle’s Evaporative Emission Control (EVAP) system. Formally defined as “EVAP Purge Flow Sensor Circuit High Input,” this code is common in many GM vehicles, including popular Buick models like the LeSabre, Park Avenue, Century, and Regal from the late 1990s and early 2000s. The EVAP system is crucial for preventing fuel vapors from escaping into the atmosphere, and a P1477 code signifies that the Powertrain Control Module (PCM) has detected an abnormally high voltage signal from the purge flow sensor circuit, suggesting a malfunction that requires attention.
Symptoms and Common Causes of P1477
Unlike some engine codes that cause immediate drivability issues, P1477 often presents subtle symptoms directly related to the EVAP system’s operation. Ignoring it, however, can lead to failed emissions tests and potential fuel system inefficiencies.
Primary Symptoms of Code P1477
- Illuminated Check Engine Light (MIL): This is the most common and often the only initial symptom.
- Failed Emissions Test: The vehicle will likely not pass a state or local emissions inspection due to an active EVAP system fault.
- Possible Fuel Odor: In some cases, a faint smell of gasoline may be detected if the EVAP system is not properly purging vapors.
- Slightly Reduced Fuel Economy: A malfunctioning purge system can disrupt the engine’s air/fuel ratio calculations, potentially leading to a minor drop in MPG.
Root Causes of a P1477 Fault Code
The “High Input” designation points to an electrical issue where the PCM sees a voltage higher than expected on the signal circuit of the purge flow sensor. The culprits typically fall into these categories:
- Faulty EVAP Purge Flow Sensor: The sensor itself is the most common point of failure. Internal electrical faults can send an incorrect high-voltage signal.
- Damaged or Corroded Wiring/Connectors: Exposed wires, chafed insulation, or corroded terminals in the sensor’s circuit can cause a short to power (12V), creating the high input condition.
- Poor Electrical Ground: A bad ground connection for the sensor or the PCM can cause erratic voltage readings.
- Failed EVAP Purge Solenoid/Valve: While the code points to the sensor circuit, a completely stuck-closed purge valve can sometimes be a related contributing factor, though it usually sets its own codes.
- Defective Powertrain Control Module (PCM): This is rare, but a malfunction within the PCM’s internal circuitry for processing this signal cannot be entirely ruled out after all other causes are eliminated.
Step-by-Step Diagnostic Procedure for P1477
A systematic approach is key to correctly diagnosing and fixing P1477. You will need a quality OBD2 scanner, a digital multimeter (DMM), and basic hand tools.
Step 1: Preliminary Checks and Code Verification
Begin by using your OBD2 scanner to confirm the presence of P1477. Clear the code and perform a short test drive. If the code returns immediately or during the drive, the fault is current. Also, check for any other related EVAP codes (like P0440, P0442, P0455) which could indicate a larger system issue. Perform a thorough visual inspection of the EVAP components, wiring harnesses, and connectors around the purge solenoid and sensor (typically located near the engine’s intake manifold or firewall) for obvious damage, disconnections, or corrosion.
Step 2: Electrical Testing of the Purge Flow Sensor
Locate the purge flow sensor (refer to your Buick’s service manual for its exact location). With the connector disconnected and the ignition key in the ON position (engine off), use your multimeter to probe the vehicle-side harness connector.
- Identify the reference voltage wire (usually 5V from the PCM) and the signal return wire. Check for the presence of a clean 5-volt reference and a good ground.
- Next, test the sensor itself. Set the multimeter to resistance (ohms). Measure the resistance across the sensor’s terminals. Compare your reading to the manufacturer’s specification (often found in a repair database like ALLDATA or Mitchell1). A reading of infinite resistance (open circuit) or zero resistance (short circuit) confirms a bad sensor.
Step 3: Circuit Integrity and Voltage Signal Check
This step checks for a short to voltage in the signal wire. Reconnect the sensor. Back-probe the signal wire at the PCM connector or at the sensor connector using a multimeter in DC voltage mode. With the ignition ON and engine off, observe the voltage. A normal reading should typically be around 0.5-1.5 volts at rest. A reading near battery voltage (12V) confirms a “high input” short to power somewhere in that wire between the sensor and the PCM. This requires a meticulous wire-by-wire inspection of the harness.
Repair Solutions and Final Verification
Once the faulty component is identified, the repair path becomes clear. Always disconnect the battery before performing electrical repairs.
Repair 1: Replacing the EVAP Purge Flow Sensor
If electrical testing confirms a faulty sensor, replacement is straightforward.
- Locate and disconnect the electrical connector.
- Remove the sensor from its mounting bracket or the purge valve assembly (it is often integrated into or near the purge solenoid).
- Install the new, OEM or high-quality aftermarket sensor.
- Reconnect the electrical connector.
Repair 2: Fixing Wiring Harness Issues
If the diagnostic points to damaged wiring:
- Carefully trace the entire length of the affected circuit (reference voltage, signal, and ground wires) from the sensor to the PCM.
- Repair any broken, chafed, or corroded wires using solder and heat-shrink tubing for a permanent, reliable fix. Never use twist-on connectors in automotive environments.
- Clean any corroded terminals with electrical contact cleaner and ensure connectors snap together securely.
Step 4: Clearing the Code and Performing a Drive Cycle
After the repair, reconnect the battery. Use your OBD2 scanner to clear the P1477 code from the PCM’s memory. To ensure the fix is complete and to ready the vehicle’s emissions monitors, you must perform a complete drive cycle. This involves driving the vehicle under specific conditions (cold start, city driving, highway driving) to allow the PCM to run its self-tests on the EVAP system. Once the drive cycle is complete, re-scan the vehicle. The code should remain absent, and the EVAP monitor should show “Ready,” indicating a successful repair.