Code
P0512
HUMMER
P — Powertrain
Start Switch Circuit
AI status
Completed
Completed
100%
Causes
- Faulty ignition / start switch
- Open or shorted wiring between start switch and PCM or starter relay/solenoid
- Corroded or loose connectors at switch, relay, starter or PCM
- Failed starter relay or starter solenoid
- Low battery voltage or poor battery connections
- Faulty PCM (less common)
Symptoms
- No-crank or no-start condition when turning the key or pressing START
- Intermittent starting or starter engages only sometimes
- Clicking sound from relay/solenoid but starter does not spin
- Instrument cluster or starter indicator behaves erratically during start
- Malfunction Indicator Lamp (MIL) or message about starter/system starting fault
What to check
- Verify battery state-of-charge and terminal connections are clean and tight
- Check starter relay and associated fuses for continuity and correct operation
- Inspect wiring and connectors from start switch to relay/PCM for damage, corrosion, or loose pins
- Verify park/neutral/clutch interlock switches are in the correct state (vehicle in Park/Neutral) and communicating
- Scan for stored freeze-frame data and related codes that may point to source
- Perform a visual inspection of the ignition/start switch and connector for signs of wear or heat
Signal parameters
- Battery voltage at any start circuit test point: ~12.6 V (engine off, fully charged); ~9–14 V during cranking
- Start switch output when key/START pressed: ~Battery voltage (close to 12 V) to energize relay/solenoid
- Start switch output when off: ~0 V (or open circuit) — no sustained voltage
- Continuity from start switch output to starter relay/solenoid control terminal: low resistance (< 2–5 ohm for short harness, or continuity/infinite for open)
- Starter relay coil resistance: typically a few ohms to a few tens of ohms (refer to service data)
- Voltage drop from battery positive to starter solenoid terminal while cranking: ideally < 0.5–0.8 V
Diagnostic algorithm
- Safety first: Park vehicle securely, chock wheels, and disable any nonspecified systems if required. Use PPE.
- Verify and record battery voltage with a multimeter. Charge or replace battery if below specification.
- Scan OBD for P0512 and any related codes; note freeze-frame and command data during a start attempt.
- Visually inspect ignition/start switch connector, starter relay, fuses, wiring harness, and PCM connector for damage or corrosion.
- With a helper, measure voltage at the start switch output while turning key to START (or pressing START). Confirm the switch produces battery voltage to the relay/solenoid control circuit.
- If start switch provides correct voltage, check for voltage at the starter relay coil terminal and at the starter solenoid control terminal. If voltage is lost between points, trace wiring and repair open/short.
- Test starter relay by swapping with a known good relay of same type or bench-testing. Replace if defective.
- Measure continuity/resistance from start switch output to relay/PCM input. Repair open circuits or high resistance connections.
- Check park/neutral/clutch interlock switch operation and wiring; confirm PCM receives Park/Neutral signal (if interlock prevents start, address that code).
- Inspect grounds and perform voltage drop tests: battery negative to engine/chassis and battery positive to starter solenoid while cranking should be within spec.
- If wiring, switch, relay, battery and interlocks are confirmed good and fault persists, consider PCM input circuit testing or replacement per factory procedures.
- Clear codes and perform multiple start attempts to verify repair. Document findings and repairs.
Likely causes
- Damaged wiring or broken/loose connector at the start switch or starter relay
- Failed starter relay or starter solenoid coil not receiving proper input
- Ignition/start switch contacts worn or stuck
- Battery voltage too low under load causing the start signal to be out of spec
- Park/neutral position switch not reporting Park/Neutral to PCM (start inhibit)
Fault status
Status
Start Switch Circuit — PCM detected abnormal or missing start-request signal. Inspect start switch, wiring, relay/solenoid, battery and interlock switches.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
Manual library for HUMMER
138
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