Code
P1421
HUMMER
P — Powertrain
Intake Air Low Pressure Switch Circuit High Voltage
AI status
Completed
Completed
100%
Causes
- Short to battery voltage (wiring pin shorted to 12V)
- Failed/shorted intake air low pressure switch (sensor)
- Corroded or loose connector pins at switch or harness
- Damaged/chafed wiring harness (insulation worn, pinched, or contacting chassis power)
- Poor or open ground or reference supply to the sensor
- Faulty PCM/ECM input circuit (less common)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Reduced engine performance or entry into limp/reduced-power mode (depending on system)
- Erratic boost control or turbo-related behavior
- Poor fuel economy or hesitation
- Intermittent or permanent fault code stored
What to check
- Read and record stored DTCs and freeze frame data with a scan tool
- Perform a visual inspection of the intake low pressure switch, connector, and wiring for damage, corrosion, or loose pins
- Check fuse(s) and harness routing for recent repairs or modifications
- Backprobe the switch connector and measure signal voltage with key ON, engine OFF
- Verify reference voltage (typically 5 V) and ground at the switch connector
- Perform continuity and resistance checks from the switch connector to the PCM
Signal parameters
- Reference supply (typical): 5.0 V ± 0.5 V at sensor reference pin
- Switch signal when open: ≈ 4.0–5.0 V (pulled high to reference)
- Switch signal when closed: ≈ 0.0–1.0 V (grounded or pulled low)
- Expected circuit resistance closed: low (few ohms); open: high/OL
- PCM input threshold for 'high' may be near reference voltage; measured values vary by model
Diagnostic algorithm
- Verify the code with a scan tool and record freeze frame and live data for the intake low pressure switch signal.
- Clear the code and perform a road or functional test to see if the code returns; note any conditions when it sets.
- Visually inspect the sensor, connector, and wiring (look for melted insulation, pin corrosion, rodent damage, or recent repairs).
- With ignition ON (engine OFF), backprobe the sensor connector: verify reference voltage (≈5 V), signal voltage, and ground continuity to chassis and PCM.
- If signal is high (> expected) with key ON and sensor unplugged, suspect short to voltage in harness or PCM pull-up — inspect wiring for contact with 12 V circuits.
- Perform continuity/resistance checks from the sensor signal pin to the PCM input pin; check for short to battery (+) or other circuits.
- If wiring and connectors check good, bench-test or replace the intake air low pressure switch and re-check signal behavior.
- If replacement sensor does not fix and wiring is confirmed good, consider PCM input fault — verify with manufacturer wiring diagrams and consider PCM testing or replacement as final step.
- After repairs, clear codes and confirm via test drive that the DTC does not return and system operates normally.
Likely causes
- Short to 12 V on the switch signal circuit
- Failed intake air low pressure switch
- Corroded/loose connector causing intermittent high reading
- Wiring harness damage near intake or turbocharger area
Fault status
Status
Intake Air Low Pressure Switch Circuit High Voltage — signal above expected threshold (electrical fault).
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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