P0899: Transmission Control System MIL Request Circuit High

What it means

Voltage on the transmission control system MIL request circuit is above the expected window. Wire shorted to B+ or driver open.

At a glance

• Usually lights the check-engine light

How it works

The transmission control system uses a MIL request circuit to signal when a transmission fault should illuminate a warning lamp. P0899 sets when that circuit voltage is too high, which can be caused by a short to battery voltage, an open or poor connection, or a module driver fault.

Common causes

Most likely first.

1. Short to voltage in the MIL request circuit
2. Open or poor connection in the circuit
3. Transmission control module circuit fault

Symptoms

• Check-engine or transmission warning light may not respond as expected

How to diagnose it

1. Scan all modules for related communication, lamp-request, and transmission codes, and check whether warning lamps respond as expected.
2. Inspect the MIL request wire and connectors for damage, loose terminals, or contact with a powered wire.
3. Check for an open circuit or poor connection between the TCM and the receiving module using the vehicle wiring diagram.
4. Use a scan tool to compare the TCM lamp-request command with the circuit voltage and the receiving module's reported status.
5. If wiring and connections are sound, test the TCM output and receiving-module input against the manufacturer's specification.

Common fixes

• Repair a MIL request wire shorted to a power supply.
• Restore an open or poor connection in the request circuit.
• Replace a damaged connector or repair corroded terminals.
• Repair or replace a module only if testing confirms its circuit is faulty.

Can I keep driving?

The fault may make the check-engine or transmission warning lamp behave incorrectly, so a missing or unexpected lamp is not a reliable sign that the transmission is healthy. If transmission symptoms appear, reduce driving and stop if control or drive is lost.