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Home / DTC / P1397 — System Voltage Out Of Self Test Range

P1397 — System Voltage Out Of Self Test Range

Detailed page for trouble code P1397.

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Code

P1397

MERCURY P — Powertrain

System Voltage Out Of Self Test Range

Brand: MERCURY
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Weak or discharged battery
  • Corroded or loose battery terminals or cable connections
  • Bad engine/chassis ground or PCM ground
  • Failing alternator or voltage regulator
  • Blown main fuse or fusible link supplying PCM
  • Intermittent ignition switch or wiring fault to PCM power rail

Symptoms

  • MIL (check engine light) illuminated and P1397 stored
  • Difficult starting, slow cranking or no-crank condition
  • Dim or flickering lights, electrical accessories operating erratically
  • Battery warning lamp may be lit
  • Intermittent electrical faults or drivability problems

What to check

  • Read DTCs and capture freeze-frame data and readiness status
  • Measure battery open-circuit voltage (key off), key-on engine-off (KOEO), during cranking, and key-on engine-running (KOER)
  • Visually inspect battery terminals, cable condition, and engine/chassis grounds for corrosion/tightness
  • Check main fuses and fusible links supplying PCM and charging system
  • Check alternator output voltage and ripple (charger load test if available)
  • Verify PCM power and 5V reference (where applicable) at the PCM connector

Signal parameters

  • Battery rest (key off): ~12.4–12.8 V (healthy, fully charged)
  • Key-on, engine-off (KOEO): ~12.2–13.0 V
  • Cranking voltage: typically should not fall below ~9.5 V (depends on vehicle and battery condition)
  • Running/charging voltage (KOER): ~13.5–14.8 V
  • PCM reference signal (if applicable): ~5.0 V ±0.25 V
  • Self-test accept/reject thresholds vary by model year; many systems flag faults when supply voltage is below ~8–10 V or above ~16 V during self-test

Diagnostic algorithm

  1. Record all stored codes and freeze-frame data. Note the conditions when P1397 set (battery voltage at that time if available).
  2. Inspect battery condition, date code, and terminals. Clean and tighten terminals; verify cable integrity.
  3. Measure and record battery voltage: key off, key on engine off, during cranking, and with engine running at idle and at ~2000 rpm. Compare to expected ranges.
  4. If battery voltage is low, perform a battery load test or replace if failing. Retest voltage behavior after repair.
  5. With engine running, measure alternator/regulator output and check for excessive ripple using a multimeter/oscilloscope. Repair/replace alternator or regulator as needed.
  6. Check main power and ground circuits to the PCM: verify continuity, voltage under load, and inspect connector pins for corrosion/looseness. Repair wiring or connectors as needed.
  7. Check main fuses and fusible links supplying the PCM and charging system; replace any blown devices and investigate cause.
  8. If voltage readings and wiring are good, check for parasitic current draw that may have drained the system and caused a false self-test failure.
  9. After repairs, clear codes, perform key cycles, and drive/monitor to confirm the self-test no longer records P1397 and that system voltages remain within range.
  10. If all power/charging and wiring checks are good and the code returns, consider PCM power stage or PCM replacement after consultation with manufacturer repair information.

Likely causes

  • Low battery voltage at key-on or during cranking (battery age/condition)
  • Poor battery terminal/ground connections causing voltage drop
  • Alternator not charging or overcharging causing out-of-range system voltage
  • Open or high-resistance power/ignition feed or fusible link to PCM

Fault status

⚠️ Status
PCM detected system/rail voltage outside allowed range during self-test — check battery, charging system, and PCM power/ground circuits.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0562 P0563 P0622 P0627
296

Browse 296 MERCURY manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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