Transmission Control System MIL Request Circuit High
The transmission control module is trying to turn on the check engine light, but the signal circuit is stuck at too high a voltage, like a light switch that won't make proper contact. This prevents the MIL from illuminating even when there are transmission problems.
The ECM monitors the MIL request circuit voltage from the transmission control module. This circuit should pull to ground (low voltage) when the TCM detects a fault and wants to illuminate the check engine light. A high voltage condition indicates an open circuit, short to power, or faulty wiring preventing proper MIL activation.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| MIL Request Circuit Voltage | 0-1 volt (pulled low when active) | Above 4.5 volts continuously |
| Circuit Continuity | Proper ground path established | Open circuit or short to voltage detected |
Code P0899 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0899 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.