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Home / DTC / U300B — Ignition Input Accessory/On/Start

U300B — Ignition Input Accessory/On/Start

Detailed page for trouble code U300B.

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Code

U300B

Generic U — Network/User

Ignition Input Accessory/On/Start

Brand: Generic
Type: U — Network/User
Views: UK: 16 EN: 22 RU: 13
AI status
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Page language: EN

Causes

  • Ignition switch internal fault or worn contacts
  • Open, short to voltage, or short to ground in ignition input wiring
  • Corroded, loose or damaged connector pins at ignition switch, BCM/IPC or PCM/ECM
  • Blown fuse or failed ignition-related relay
  • Poor chassis or module ground
  • Intermittent connection due to vibration or wiring chafe

Symptoms

  • Intermittent or complete loss of accessory power in ACC position
  • Engine fails to crank or starts intermittently
  • Starter cranks when it shouldn’t or won’t crank even though dash lights are present
  • Dimming or flicker of dash/cluster when switching key positions
  • Malfunction indicator or multiple network communication warnings present
  • Stored network/communication DTCs alongside U300B

What to check

  • Read all stored and pending codes and freeze frame data with a scan tool; note related U or P codes
  • Confirm stable battery voltage (12.4–12.8 V at rest) and supply during cranking (no extreme drop)
  • Visually inspect ignition switch connector, wiring harness, and nearby harness support points for damage, chafe, corrosion, or pinched wires
  • Check fuses and ignition-related relays for continuity and proper operation
  • Backprobe ignition switch output pins and the corresponding module input pins to verify voltage in each key position (ACC, ON/RUN, START)
  • Perform wiggle test of ignition switch connector and harness while monitoring signal for intermittent changes

Signal parameters

  • ACC position: ignition switch ACC output ~battery voltage (~11.5–13.0 V) when key in ACC, 0 V when OFF
  • RUN/ON position: ignition switch RUN output ~battery voltage (~11.5–13.0 V) with key in RUN (engine off), present during cranking except where routed through starter cut circuits
  • START position: ignition START terminal ~battery voltage when cranking (short duration) or ground-switched depending on design; expected clear transition to START within milliseconds of key rotation
  • Idle/Off: ignition input should be ~0 V or open when key OFF (no backfeed)
  • Signal quality: transitions should be clean (no rapid bouncing); high series resistance (>1–2 ohms at connector) or voltage drop >1 V under load indicates poor connection

Diagnostic algorithm

  1. Retrieve and record all DTCs, freeze frame and live data; note engine and battery conditions when code set.
  2. Verify battery state and charging system; correct low-voltage conditions before testing signals.
  3. Visually inspect ignition switch, connectors, harness routing to BCM/IPC/ECM and for signs of aftermarket device connections.
  4. With a DVOM or lab scope, backprobe ignition switch output(s) and the module ignition input pin(s). Verify expected voltages in OFF, ACC, RUN, and START positions and check for clean transitions.
  5. If voltage is absent at module input but present at switch output, check continuity of wiring and repair open/shorts. If voltage is present at module but not at switch, inspect ignition switch and related fuses/relays.
  6. Check fuses/relays that feed ignition circuits; replace suspect items and retest.
  7. Perform wiggle test on harness and connectors while observing signal for intermittent behavior; repair chafed wires or corroded terminals.
  8. Isolate aftermarket equipment (alarms, remote starters) and retest if present.
  9. If wiring and switch test good, inspect module grounds and supply pins. If module input line is driven incorrectly or intermittent despite good wiring and switch, consider module replacement or reflashing per manufacturer guidance.
  10. After repairs, clear codes and confirm fault does not return in several key cycles and during road test.

Likely causes

  • Worn ignition switch contacts producing intermittent or incorrect ACC/RUN/START signals
  • Broken or pinched harness conductor between ignition switch and body/engine control module
  • Corroded connector at the ignition switch or module causing high resistance
  • Blown IGN/IGN1 fuse or failed ignition relay causing loss of supply to the input circuit
  • Aftermarket alarm/remote start or improper install creating backfeed or short

Fault status

⚠️ Status
Ignition input (Accessory/On/Start) circuit abnormal — module detected unexpected or missing ignition-switch signal.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

U0100 U0121 U0073 U300A U0401
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