U300B

Code

HYUNDAI U — Network/User

Ignition Input Accessory/On/Start

Brand: HYUNDAI

Type: U — Network/User

Views: UK: 13 EN: 15 RU: 11

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Faulty ignition switch (mechanical or electrical failure)
Open, shorted, or corroded wiring/connectors between ignition switch and BCM/SJB
Blown fuse or failed ignition/ACC relay
Poor battery voltage or weak cranking voltage causing incorrect signal levels
Body Control Module (BCM) or Smart Junction Box (SJB) failure or internal input circuit fault
Software/firmware error or calibration mismatch in BCM or other network modules

Symptoms

Accessory circuits not powering in ACC or ON position (radio, windows, HVAC in accessory)
No crank or no start condition when turning to START
Intermittent starting or loss of accessories while driving
Warning lights or multiple network communication DTCs present
Inconsistent ignition status shown in scan tool data list (ACC/ON/START flags not matching key position)

What to check

Read and record all network DTCs and freeze frame with a capable OEM-level scan tool
Verify battery state of charge and cranking voltage (should remain above ~9.5 V during crank)
Visually inspect ignition switch connector, harness routing through column and firewall for damage
Check fuses and ignition/ACC relays related to accessory/IG/START circuits
Use scan tool data list to observe ignition input status bits while operating key/START
Backprobe ignition switch harness and measure ACC/IGN/STAR T voltages as key is turned

Signal parameters

ACC terminal: approximately battery voltage (~12 V) when key in ACC; ~0 V when off
IGN (ON) terminal: ~12 V when key in ON/run; should remain during run and dropping to 0 when off
START (crank) terminal: battery voltage present to START while cranking (may drop slightly under load)
Voltage tolerance: expect ~10–14 V under normal conditions; if below ~9.5 V during crank the signal may be invalid
Logical state reported to BCM: ACC = ON/OFF, IGN = ON/OFF, START = CRANK/NOT CRANK (observe with OEM scanner)

Diagnostic algorithm

Verify customer complaint and conditions when U300B is set. Note whether fault is permanent or intermittent.
Connect OEM-capable scan tool. Read and record all U- and P-codes and freeze-frame data. Clear codes and attempt to reproduce.
Check battery voltage and cranking voltage. If low, charge/repair battery and retest.
With key off, visually inspect ignition switch connector, steering column harness and BCM/SJB connectors for corrosion, loose pins, or damage.
Backprobe ignition switch ACC/IGN/START terminals and monitor voltages while cycling key to ACC, ON, and START. Confirm voltages meet expected signal parameters.
Using scan tool, monitor ignition status flags reported by BCM/SJB while cycling key. Compare physical voltages to module-reported states.
If physical voltages are correct but module reports incorrect state, check continuity from switch terminals to BCM input pins. Repair harness or connector faults as needed.
Inspect/replace related fuses and relays if the circuit has no voltage in expected positions.
If wiring, fuses and relays are good and continuity is good but signal still not recognized, check BCM power/grounds and re-test. Apply wiggle testing while monitoring for intermittent changes.
If BCM shows internal fault or intermittent input despite correct external signals, consider BCM/SJB reprogramming or replacement per factory procedure (replace BCM only after eliminating wiring and switch faults).
After repair, clear codes, verify proper ignition input reporting with scan tool and road/test to confirm the issue is resolved.

Likely causes

Ignition switch connector pins pushed out/corroded causing intermittent contact
Ignition switch internal contact wear resulting in no ACC/IG/START feed
Damaged harness where it passes through steering column or firewall (pinched or chafed)
Faulty ground or power feed to the BCM causing it to read inputs incorrectly
Failed ignition relay or related fuse supplying ACC/IGN circuits
Recent module replacement without proper programming or incorrect software level

Fault status

⚠️ Status

Ignition Input (Accessory/ON/START) signal missing, implausible or inconsistent — ignition position not properly reported to network modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-3.0 hours

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