Fault Tree: P0700 (TCM MIL Request)
P0700 usually means the engine ECU has been told to turn the MIL on because the TCM stored its own fault. The root cause is almost always in the transmission module’s codes and data.
Quick triage & tools
- Scan all modules: you need TCM codes (not just engine ECU).
- Voltage first: low system voltage can trigger shift/communication faults.
- Don’t guess: P0700 alone is not a “replace gearbox” code.
Tip: if your scan tool can’t talk to the TCM, jump to U0101 TCM communication →
What you need (minimal)
- Scan tool that can read TCM codes and live data (gear commanded/actual, ATF temp if available).
- Multimeter for power/ground checks if comms or module feeds look suspicious.
- Service info for correct fluid type and checking procedure (many are temperature-specific).
Decision flow
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Confirm what modules report
- Read DTCs in TCM, ECM, and ABS (some gearboxes depend on ABS wheel speed data).
- Capture freeze-frame / event data where available.
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If you have TCM codes, follow the most “root-cause” one
- Prioritise voltage/power, communication, and sensor plausibility codes first.
- Then look at solenoid/pressure control codes and shift performance codes.
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Check ATF temperature & protection modes
- If ATF temp is very high or implausible, the TCM may command limp mode.
- Investigate cooling system issues (cooler flow, fan operation) and driving conditions.
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Basic mechanical sanity
- Correct fluid level & type (follow the proper temp-based procedure).
- Any obvious leaks, burnt smell, or contamination?
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If codes suggest comms/power issues, verify module feeds
- Check TCM fuses/relays and grounds under load (voltage drop test).
- If comms are flaky: check CAN physical layer and connector corrosion/water ingress.
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Confirm the fix
Clear codes, perform the same drive cycle (or relearn if required), and confirm the TCM does not re-log faults.
Related flows
Print / save checklist
Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.
- TCM codes read (not just engine ECU)
- Freeze-frame / event data captured
- Battery voltage checked (resting + cranking)
- TCM power/ground checked under load (voltage drop)
- Fluid level/type verified with correct procedure
- Fix confirmed with repeat drive cycle
What to do next
Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.
Find another symptom flow
Jump to the symptom selector to locate the closest decision tree.
Workshop Guides
Deep-dive how-to tests: voltage drop, wiring diagrams, CAN checks and more.
Browse Diagnostic Codes
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AI Tools
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