What Does U300F Mean?
DTC U300F indicates that a control module has detected a fault with the ignition switch input for the Accessory position. The Accessory position provides power to convenience systems like the radio, power windows, and power outlets without powering the engine or fuel systems. This code is set when the Accessory position signal is missing, erratic, or incorrect when the key is in the Accessory detent.
Common Causes
35%
Worn ignition switch Accessory position contact — the detent makes poor electrical contact, providing an intermittent or absent Accessory signal
25%
Faulty wiring in the Accessory signal circuit — open wire, poor splice, or chafed insulation grounding the circuit
20%
BCM or module connector corrosion on the Accessory input pin preventing clean signal reception
20%
Ignition lock cylinder mechanical issue preventing the switch from fully engaging the Accessory detent
Diagnostic Steps
1
Step 1: Monitor the ignition switch Accessory position on the scan tool — turn the key to the Accessory position and verify the signal is recognized; check that it transitions cleanly between Off and Accessory without bouncing.
2
Step 2: Test Accessory-powered systems — with the key in Accessory, verify the radio, power windows, and power outlets function; if they work but the code is present, the issue is specifically with the signal reporting to the BCM, not the power delivery.
3
Step 3: Backprobe the Accessory signal wire at the BCM connector — measure voltage in the Accessory position versus Off; verify the expected voltage level per the wiring diagram.
4
Step 4: Disconnect the ignition switch connector and test the Accessory contact with an ohmmeter — verify continuity in the Accessory position and open in other positions; replace the switch if the contact is erratic.
5
Step 5: Check the ignition lock cylinder detent spring and actuator — verify the Accessory position clicks firmly into detent and does not slip to Off or Run.
Estimated Repair Cost
$75 - $450
Parts + labor, varies by vehicle and location
DTC U300F indicates that a control module has detected a fault with the ignition switch input for the Accessory position. The Accessory position provides power to convenience systems like the radio, power windows, and power outlets without powering the engine or fuel systems. This code is set when t...
The most common cause of U300F (Ignition Input Accessory) is: Worn ignition switch Accessory position contact — the detent makes poor electrical contact, providing an intermittent or absent Accessory signal
Typical repair costs for U300F range from $75 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Generally safe to drive. The Accessory position fault primarily affects convenience features when the engine is off. However, it may also indicate broader ignition switch wear that could progress to affect the Run or Start positions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U300F to identify the root cause.
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Quick Info
Category
Network
System
Ignition Switch / Accessory Power System
Difficulty
Type
Generic (SAE)
Recommended Tools
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