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DTC codes / P08xx · Transmission / P0802

P0802Transmission Control System MIL Request Circuit/Open

P0802 means the transmission control module (TCM) has detected an open circuit or fault in the dedicated wire it uses to request that the engine computer turn on the malfunction indicator lamp (MIL). The TCM is not reporting a transmission problem directly; it is reporting that its communication path for lighting the check engine light is broken.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The check engine light (MIL) may be illuminated even though the transmission shifts normally.
  • The MIL may fail to illuminate when the ignition is turned on, because the request circuit is open.
  • In some vehicles, the transmission may enter limp mode or default to a fixed gear if the TCM cannot confirm MIL control.
  • A stored P0802 may be the only sign, with no noticeable driveability change at all.
  • Other transmission-related codes may be present if the open circuit affects shared wiring.
  • The instrument cluster may show a transmission fault message or warning on some models.

Common causes, most likely first

  1. Broken, chafed, or disconnected wire in the MIL request circuit between the TCM and the engine control module (ECM) or powertrain control module (PCM).
  2. Corroded or loose terminals at the TCM connector or the ECM/PCM connector.
  3. Failed TCM output driver for the MIL request circuit.
  4. Failed ECM/PCM input circuit for the MIL request signal.
  5. Blown fuse or poor ground affecting the TCM's ability to send the request signal.
  6. Aftermarket wiring or alarm installation that tapped into the circuit and left it open.
  7. Water intrusion or rodent damage to the harness in the transmission tunnel or engine bay.

How to diagnose and fix it

  1. Verify the stored code with a scan tool and check for any related communication or transmission codes before touching wiring.
  2. Inspect the TCM and ECM/PCM connectors for corrosion, loose pins, or pushed-out terminals, and clean or reseat as needed.
  3. Check the MIL request circuit for continuity and short-to-ground using a wiring diagram and a digital multimeter; repair any open or damaged wire.
  4. Test the TCM output signal at the TCM connector with a scan tool or oscilloscope while commanding the MIL on, if the tool supports it.
  5. Check powers and grounds at the TCM, including fuses and ground points, to rule out a supply issue that mimics an open circuit.
  6. If the circuit tests good, substitute a known-good TCM or ECM/PCM only after confirming the fault follows the module, then reprogram or replace as required.
  7. Clear the code and perform a road test to confirm the MIL request circuit operates correctly and the code does not return.

Why the MIL request circuit fails

  • Many technicians replace the TCM when the real fault is a single chafed wire in the harness near the transmission bellhousing, so always ohm the circuit before condemning a module.
  • P0802 can be set by a completely different module than the one you expect; on some networks the MIL request is sent over a data bus, and a bus fault can mimic an open circuit.
  • A missing or blown fuse for the TCM can set P0802 because the TCM cannot energize its output driver, yet the fuse box is rarely checked first.
  • Aftermarket remote starters or alarm systems that splice into the MIL wire are a common cause of an open circuit that looks like a module failure.
  • The code does not mean the transmission is damaged; it only means the lamp request path is broken, so do not overhaul the transmission based on P0802 alone.

Can I keep driving with P0802?

You can usually drive the vehicle with P0802 because the transmission itself may be fine, but do not ignore it; if the transmission enters limp mode or the check engine light stops working, have it diagnosed promptly to avoid missing a more serious fault.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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