Code
P1197
MERCURY
P — Powertrain
Key Off Voltage Low
Views:
UK: 29
EN: 42
RU: 35
AI status
Completed
Completed
100%
Causes
- Weak or discharged battery
- Corroded or loose battery terminals or cable connections
- Poor or corroded engine/chassis ground(s)
- Faulty ignition switch or power distribution (backfeed)
- Open or high-resistance wiring between battery and PCM fuse/power input
- Parasitic current draw when key is off
Symptoms
- Stored P1197 and MIL (Check Engine Light) illuminated
- Low resting battery voltage measured at battery terminals with ignition off
- Difficulty starting or no-start after extended sitting
- Battery drained after vehicle sits overnight (parasitic drain)
- Intermittent electrical issues or erratic module behavior
- Other codes related to low system voltage (modules resetting)
What to check
- Visual inspection of battery terminals, cable ends and ground straps for corrosion, tightness and damage
- Measure resting battery voltage at the battery with ignition off using DMM (typical healthy battery 12.4–12.8 V)
- Perform parasitic current draw test (mA) with all systems off; compare against manufacturer spec
- Inspect and verify main fuses and fusible links feeding PCM and ignition circuits
- Check continuity and resistance of battery negative and engine/body ground connections
- Scan PCM for freeze frame, related codes and live data of system voltage
Signal parameters
- Typical resting battery voltage: ~12.4–12.8 V (fully charged)
- Aging or discharged battery resting voltage:
- Parasitic draw typical threshold:
- PCM may flag low key-off voltage when measured voltage falls below a manufacturer-defined threshold (varies by vehicle)
Diagnostic algorithm
- Read stored DTCs and freeze frame data; note battery voltage at time of code set
- Visually inspect battery, terminals, positive/negative cables and ground straps; clean and tighten as required
- Measure and record battery resting voltage at battery posts with ignition OFF
- If resting voltage is low, fully charge battery and load-test or perform battery capacity test; replace if battery fails
- With charged battery and ignition OFF, perform parasitic draw test at the battery negative cable using an ammeter; isolate circuits if draw exceeds spec to find source
- Verify main fuses/fusible links and PCM power/ignition feed fuses are intact and have good continuity
- Check continuity and resistance of wiring between battery, fuse box and PCM power input; repair any high-resistance or open circuits
- Inspect ignition switch operation and related relays for proper isolation when key is OFF
- If wiring and external components test good, check PCM ground connections and harness connector pins for corrosion or damage
- If all vehicle-side wiring and power/ground are good and problem persists, consider PCM power supply internal fault; consult manufacturer diagnostics for PCM testing or replacement
- Clear codes and road/test after repairs; confirm code does not return and that resting voltage and parasitic draw are within spec
Likely causes
- Battery state-of-charge below normal or aged battery
- High resistance at battery negative or positive connections
- Faulty grounding at engine block or body ground strap
- Parasitic draw from aftermarket accessories or shorted circuit
- Faulty ignition switch not isolating circuits when off
- Damaged wiring to PCM power input or measurement circuit
Fault status
Status
P1197 — Key Off Voltage Low. The PCM detected vehicle electrical system voltage below the expected threshold while ignition is OFF. Investigate battery condition, grounding, wiring, parasitic draw and PCM power input.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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