Code
P1521
ALFA ROMEO
P — Powertrain
Supply voltage contact fault
Views:
UK: 21
EN: 28
RU: 21
AI status
Completed
Completed
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Causes
- Corroded, loose or damaged battery terminals
- Blown fuse or failing fusible link on the supply circuit
- Faulty power/ignition relay or poor socket contact
- Damaged wiring harness (open, short to ground or short to battery)
- Poor or corroded ground connection
- Faulty connector (bent pins, contamination, water ingress)
Symptoms
- Malfunction Indicator Lamp (MIL) or warning lamp illuminated
- Intermittent electrical/electronic module failures
- Engine may crank but not start, stall, or run erratically
- Loss of function of systems fed by the affected supply (e.g., throttle, sensors, actuators)
- Poor charging or low system voltage indications
What to check
- Read and record freeze frame and all stored codes with a scanner
- Visual inspection of battery terminals, main grounds and fusible links
- Inspect fuses and relays related to the reported circuit
- Check connector condition at the reported module/sensor (pins, corrosion, retention)
- Measure battery voltage (key off, key on engine off, engine running) at battery and at the component connector
- Perform wiggle test on connectors, wiring harness while monitoring voltage/communication
Signal parameters
- Battery voltage (resting, key ON engine OFF): typically 12.2–12.8 V
- Charging voltage (engine running): typically 13.5–14.8 V
- Voltage at supply contact with key ON (should be close to battery voltage, within ~0.5 V)
- Cranking voltage (while starter engaged): normally >9 V (depends on vehicle)
- Supply circuit should show stable voltage without rapid drops or spikes under load
Diagnostic algorithm
- Retrieve DTC and freeze frame data. Note when the fault occurred (key position, engine state).
- Visually inspect battery, main ground straps, engine/transmission ground, and related fuses/relays. Clean/tighten as needed.
- Identify the specific supply contact/circuit referenced by vehicle manufacturer documentation (wiring diagram).
- With a DVOM, measure voltage at the component/module supply pin with key ON and engine OFF. Compare to battery voltage.
- If low or intermittent, probe the supply upstream (fuse/relay output) and downstream. Check relay operation and fuse continuity.
- Perform wiggle/stress test on connectors/wiring while monitoring voltage or scanner data to reproduce intermittent contact.
- Check continuity and resistance to ground for the module ground circuit. Repair poor grounds.
- Repair or replace damaged wiring, connector terminals, fuse or relay as necessary. Use solder/heat-shrink or proper crimp terminals; replace corroded connectors.
- Clear codes and perform function test / road test. Re-scan to confirm code does not return.
- If wiring, fuse and connector OK and problem persists, consult manufacturer procedures for module bench test or consider module replacement as last resort.
Likely causes
- Loose/corroded connector at the affected module or sensor
- Blown fuse or defective relay in the supply circuit
- Damaged wiring or pin terminal causing intermittent contact
- Poor battery/ground connection reducing supply voltage
Fault status
Status
Supply voltage contact fault detected — open, short, high resistance or intermittent connection on monitored power supply circuit.
Repair difficulty: Medium
Diagnostic time: 0.5 - 2.0 hours
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