Transmission Control System MIL Request Circuit High
The P0899 code indicates that the transmission control module (TCM) has detected a high voltage condition in the malfunction indicator lamp (MIL) request circuit. This circuit is used by the TCM to request the powertrain control module (PCM) to illuminate the check engine light. A high voltage reading suggests a short to power, a faulty TCM, or wiring issues. This can lead to incorrect MIL operation and potential transmission performance problems.
01. Most Common Causes
- Short to power in the MIL request circuit wiring
- Faulty transmission control module (TCM)
- Damaged connector or corrosion in the circuit
- PCM internal failure
02. Symptoms You Will Notice
- Check engine light illuminated
- Transmission may not shift properly
- Reduced fuel economy
- Possible limp mode activation
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to retrieve all stored codes and freeze frame data.
Step 2: Visually inspect the wiring and connectors for the MIL request circuit for damage, corrosion, or shorts.
Step 3: Test the circuit voltage with a multimeter to confirm high voltage condition.
Step 4: If wiring is okay, check for TCM or PCM faults by performing manufacturer-specific tests.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the transmission may enter limp mode, causing poor performance and potential damage.
How much does it cost to fix?▼
Repair costs typically range from $50 to $150 for diagnostic and minor wiring repairs, but could be higher if the TCM or PCM needs replacement.