What Does U300D Mean?
DTC U300D indicates that a control module has detected a fault with the ignition switch input specifically for the On/Start combined position signal. This monitors the circuit that provides voltage when the key is in either the On (Run) or Start positions. If this signal is absent when expected, incorrect, or intermittent, the module cannot properly determine whether the engine should be running or cranking, and sets this code.
Common Causes
35%
Ignition switch contact failure on the On/Start circuit — worn or pitted contacts that provide intermittent or weak signal
25%
Open or high-resistance wire in the On/Start signal circuit between the ignition switch and the control module
20%
Blown fuse or relay in the ignition Run/Start power feed that supplies the On/Start signal voltage
20%
Control module or BCM connector corrosion on the On/Start input pin causing signal degradation
Diagnostic Steps
1
Step 1: Monitor the On/Start signal on the scan tool — with the key in the Run position, verify the signal shows On; while cranking, verify it shows Start; check for any dropouts during cranking due to voltage sag.
2
Step 2: Measure voltage at the module's On/Start input pin — should read battery voltage when the key is in Run (~12.5V engine off) and during cranking (may dip to ~10V but should not drop below 9V).
3
Step 3: Check the ignition Run/Start fuse and relay — verify fuse continuity and relay operation; a marginal fuse contact can cause voltage drop under the higher current draw during cranking.
4
Step 4: Inspect the ignition switch connector and the module connector for the On/Start circuit — look for melted terminals, corrosion, or backed-out pins.
5
Step 5: If an intermittent fault is suspected, use a digital storage oscilloscope on the On/Start wire while cranking — look for momentary voltage dropouts that a multimeter would miss.
Estimated Repair Cost
$100 - $500
Parts + labor, varies by vehicle and location
DTC U300D indicates that a control module has detected a fault with the ignition switch input specifically for the On/Start combined position signal. This monitors the circuit that provides voltage when the key is in either the On (Run) or Start positions. If this signal is absent when expected, inc...
The most common cause of U300D (Ignition Input On/Start) is: Ignition switch contact failure on the On/Start circuit — worn or pitted contacts that provide intermittent or weak signal
Typical repair costs for U300D range from $100 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
If the On/Start signal drops out while driving, the module may interpret this as the ignition being turned off, potentially causing a stall. This is a safety concern and should be diagnosed before extended driving.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U300D 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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