P1191

Code

Other P — Powertrain

Key Line Voltage

Brand: Other

Type: P — Powertrain

Views: UK: 36 EN: 51 RU: 54

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Page language: EN

Causes

Weak or discharged battery
Poor or corroded battery terminals / ground connections
Faulty ignition switch or key detection circuit
Open, shorted or damaged wiring in the key/IG supply circuit
Blown or faulty fuse or relay in the ignition or ECU power circuit
Intermittent connector contact (corrosion, loose pins)

Symptoms

Engine cranks but will not start, or intermittent no-start
Random stalling or loss of power while driving
Dash lights dimming or flickering with key on
ECU-related warning lights or limp-home mode
Intermittent operation of ignition-controlled accessories

What to check

Scan for additional fault codes and freeze-frame data
Measure battery voltage at rest, during cranking and with key ON (engine off)
Check voltage on the IG/key supply pin at the ECU with key OFF, key ON, and during cranking
Inspect and clean battery terminals and major ground points; measure voltage drop across ground and positive connections
Visually inspect wiring and connectors for damage, corrosion, loose pins, or aftermarket splices
Check relevant fuses and relays for continuity and proper operation

Signal parameters

Key/OFF voltage: near 0 V (typically
Key/ON (IGN) engine-off: ~11.0–14.5 V (near battery voltage)
During cranking: typically ≥9.0 V (may drop temporarily but should remain above ~8–9 V)
Acceptable fluctuation: stable within ±0.5 V while key is steady; no rapid spikes or dropouts
Intermittent or noisy waveform, repeated dropouts, or sustained over-voltage (>16 V) are out of range

Diagnostic algorithm

Retrieve freeze-frame and live data; note exact conditions when code set (key position, battery voltage).
Visually inspect battery, battery terminals, main grounds and IG circuit connectors for corrosion or looseness. Clean and tighten as needed.
Measure battery resting voltage and voltage at starter battery positive and negative. If battery
With a DVOM, measure voltage at the IG/key supply terminal at the ECU with key OFF, key ON (engine off), and during cranking. Compare to signal_params.
If voltage is low at ECU but good at battery, perform voltage drop tests along positive feed and ground to find high-resistance connection or open. Inspect ignition switch wiring and connectors.
Check related fuses and relays; substitute known-good relay if suspect. Back-probe connector pins while operating switch to detect intermittent contact.
Wiggle harness and ignition switch while monitoring voltage to reproduce intermittent faults. Use a scope if available to see transient drops or spikes.
Verify alternator/regulator output while engine running; ensure charging voltage within spec and no over-voltage conditions.
If all wiring, switches, battery, grounds, fuses and charging system check good, consider ECM power circuit fault — consult manufacturer technical service info and consider ECM bench test/replacement.
Clear codes and perform test drive or repeat conditions to confirm repair; monitor for reoccurrence.

Likely causes

Battery voltage low or voltage drop at battery terminals
Loose or corroded battery or engine ground
Worn or failing ignition switch contacts
Open/short in IG supply wiring or connector near ignition switch or ECU
Blown fuse or sticking relay feeding the IG/key circuit

Fault status

⚠️ Status

The ECM detected the ignition/key supply voltage outside expected range (low, high, intermittent or noisy). This may prevent proper ECU power-up or cause intermittent operation of ignition-controlled systems.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours