Transmission Control System MIL Request Circuit/Open
Diagnostic trouble code P0802 indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an open circuit or malfunction in the Malfunction Indicator Lamp (MIL) request circuit from the Transmission Control Module (TCM). This means the TCM is unable to command the check engine light to turn on when a transmission-related fault occurs.
While this code does not immediately affect drivability, it prevents the driver from being alerted to potentially serious transmission faults. It should be diagnosed and repaired soon to ensure transmission issues can be properly communicated.
Common symptoms
- Check Engine Light (MIL) is illuminated (set by the PCM/ECM)
- Check Engine Light fails to illuminate during the bulb check at startup
- Inability to pass an emissions inspection
- Possible transmission shifting abnormalities (if underlying transmission faults exist but aren't indicated)
- Transmission may enter limp mode (depending on vehicle design)
Likely causes
- Wiring harness damage or open circuit between the TCM and PCM/ECM
- Corroded, loose, or damaged electrical connectors in the MIL request circuit
- Blown fuse or fusible link providing power to the TCM or instrument cluster
- Faulty Transmission Control Module (TCM)
- Faulty Engine Control Module (ECM) or Powertrain Control Module (PCM)
What to check before replacing parts
Verify the Check Engine Light (MIL) operation during the key-on, engine-off (KOEO) bulb check.
Look for: If the MIL does not illuminate during the bulb check, the issue may be a burned-out bulb, instrument cluster fault, or a communication issue.
Scan all modules (PCM, TCM, Instrument Cluster) for additional diagnostic trouble codes.
Look for: Look for communication codes (U-codes) or specific transmission faults that may point to a broader network issue or TCM power failure.
Visually inspect the wiring harness and connectors between the TCM and PCM.
Look for: Check for chafed wires, broken wires, bent pins, or corrosion in the connectors that would cause an open circuit in the MIL request line.
Test the MIL request circuit for continuity between the TCM and PCM using a digital multimeter.
Look for: A reading of infinite resistance (OL) indicates an open circuit in the wiring that needs to be repaired.
Check for proper power and ground at the Transmission Control Module (TCM).
Look for: Voltage should match battery voltage on power pins, and resistance to ground should be less than 5 ohms. Lack of power or ground can prevent the TCM from sending the MIL request signal.
This page explains the generic P0802 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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