What Does U300B Mean?
DTC U300B indicates that a control module has detected a fault with the ignition switch input for the Accessory/On/Start positions. This is a composite signal code meaning the switch is not correctly transitioning through or reporting the Accessory, On (Run), and Start positions. The module expects a specific voltage or signal pattern as the key rotates through these positions, and a discrepancy triggers this code.
Common Causes
35%
Worn ignition switch contacts causing incorrect or missing signals in the Accessory, Run, or Start positions
25%
Open or high-resistance wire in the ignition switch circuit that carries the Accessory/On/Start composite signal
25%
Corroded or damaged connector at the ignition switch or BCM side causing intermittent signal loss
15%
Body Control Module internal input circuit fault for the Accessory/On/Start signal line
Diagnostic Steps
1
Step 1: Use the scan tool to monitor ignition switch position data while slowly rotating the key through Accessory, On/Run, and Start — verify each position is recognized without dropout or skipping.
2
Step 2: Backprobe the specific ignition switch wire for the Accessory/On/Start signal at the BCM connector — verify expected voltage in each key position per the wiring diagram (typically 0V, 5V, or 12V depending on the position and vehicle design).
3
Step 3: Disconnect and inspect the ignition switch connector in the steering column — check for burnt contacts, arc marks, corroded pins, or terminals that have backed out of the housing.
4
Step 4: Measure resistance through the ignition switch in each detent position — compare to manufacturer specifications; an erratic or open reading in any position confirms a failed switch.
5
Step 5: If the switch tests good, trace the wiring from the switch through the steering column harness and any intermediate connectors to the BCM — repair any high-resistance points found.
Estimated Repair Cost
$100 - $500
Parts + labor, varies by vehicle and location
DTC U300B indicates that a control module has detected a fault with the ignition switch input for the Accessory/On/Start positions. This is a composite signal code meaning the switch is not correctly transitioning through or reporting the Accessory, On (Run), and Start positions. The module expects ...
The most common cause of U300B (Ignition Input Accessory/On/Start) is: Worn ignition switch contacts causing incorrect or missing signals in the Accessory, Run, or Start positions
Typical repair costs for U300B range from $100 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
If the switch fails to properly signal the Run or Start position, the vehicle may not start or may stall while driving. An intermittent fault could cause power loss to critical systems while driving. Have this diagnosed promptly.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U300B to identify the root cause.
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Quick Info
Category
Network
System
Ignition Switch / Power Distribution System
Difficulty
Type
Generic (SAE)
Recommended Tools
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