Code
P1800
MITSUBISHI
P — Powertrain
Automated-M/T fail(relay OFF)
Views:
UK: 7
EN: 18
RU: 10
AI status
Completed
Completed
100%
Causes
- Failed automated‑MT relay (coil open or contacts welded)
- Blown fuse supplying the relay
- Open, short or high‑resistance wiring between ECU and relay
- Poor relay or ground connector connection (corrosion, loose pins)
- Faulty ECU/transmission control output driver
- CAN or communication error preventing proper relay command or status feedback
Symptoms
- Transmission warning lamp or MIL illuminated
- Automated shift function disabled or vehicle stuck in limp/manual mode
- Inability to select or engage gears under automated control
- Intermittent loss of transmission control or gear selection
- No audible relay click at expected times
What to check
- Read all stored codes and freeze frame data; note related transmission or communication codes
- Visually inspect relay, relay socket, wiring harness and connector for damage or corrosion
- Check fuses related to transmission/relay power
- Listen for relay energizing when ignition/engine conditions call for it
- Check battery voltage and grounds for adequate supply while performing tests
Signal parameters
- Relay control command from ECU: switched voltage (typically 0 V/12 V) when relay is commanded ON
- Relay coil resistance (ohms) at ambient temperature
- Supply voltage at relay power terminal (battery voltage present with ignition ON)
- Ground continuity from relay ground terminal to chassis/ECU ground
- CAN/serial messages between transmission controller and ECU (presence and error counts)
Diagnostic algorithm
- Confirm fault: read code(s) and clear then attempt to reproduce; note when code returns
- Check related fuses for continuity and correct rating; replace if blown and re‑test
- Visually inspect relay and socket for corrosion, overheating or damaged pins; reseat connector
- Swap the automated‑MT relay with a known good identical relay (if available) and see if code clears or behavior changes
- With a voltmeter, check for battery supply at the relay power terminal and for ECU control voltage at the relay coil terminal while commanding the function (observe switching between ~0 V and battery voltage)
- Measure coil resistance of relay off vehicle or compare to specification; replace relay if out of range
- Check continuity and resistance of control and supply wires between ECU and relay; repair any opens or shorts
- Verify solid ground at relay ground terminal; clean or repair as needed
- Scan for CAN/communication faults and address any bus issues (loose connectors, shorts)
- If wiring, fuses and relay check good and ECU control output never switches, suspect ECU/transmission control unit driver fault — consider professional ECU testing or replacement
Likely causes
- Relay failure or stuck/open contacts
- Blown/loose fuse or poor power supply to relay
- Corroded/loose relay socket or connector causing intermittent contact
- Wiring break or short in relay control or supply circuit
- ECU output driver fault (less likely than external wiring/relay)
Fault status
Status
Automated-M/T relay OFF detected — relay did not energize when commanded. Check relay, power/fuse, wiring, grounds and ECU output.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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