Transmission Codes
Transmission fault codes indicate a problem detected by the Transmission Control Module (TCM) — the computer that manages gear shifts, torque converter lock-up, and other automatic transmission functions. These codes deserve prompt attention.
Last reviewed May 2026 · Find This Code Editorial Team
Understanding transmission codes
Most transmission fault codes (P07xx) are set by the Transmission Control Module (TCM) and reported to the Engine Control Module (ECM). This is why you'll often see a generic indicator code like P0700 in addition to a more specific TCM code that identifies the exact fault.
When reading transmission codes, always scan for all stored codes — the companion code alongside P0700 contains the real diagnostic information.
Always start with the fluid. Low or degraded transmission fluid is the most common underlying cause of transmission solenoid and shift quality codes. Check the fluid level and condition before any electrical diagnosis.
Electrical vs. mechanical causes
Transmission codes can have two very different root causes — and the correct diagnosis determines whether you need a $50 solenoid or a $3,000+ transmission rebuild.
Electrical causes
- → Failed shift solenoid (most common)
- → Solenoid wiring damage or corrosion
- → Faulty TCM or TCM power supply
- → Transmission range sensor fault
Mechanical/hydraulic causes
- → Clogged solenoid (dirty fluid)
- → Worn valve body
- → Worn clutch packs or bands
- → Torque converter wear
Transmission codes covered on Find This Code
Transmission code diagnosis tips
- Read all codes. P0700 is always accompanied by a more specific TCM code. Don't stop at P0700 — the companion code tells you what the transmission actually detected.
- Check the fluid first. Inspect transmission fluid level and color on the dipstick. Dark brown or black fluid with a burnt smell indicates degraded fluid that should be changed before further diagnosis.
- Test the solenoid. Most shift solenoids can be tested for resistance with a multimeter. Compare resistance to the specification in your service manual before assuming the solenoid is faulty.
- Get a professional diagnosis. Transmission repair is complex and expensive if done incorrectly. A transmission specialist can distinguish electrical faults from internal mechanical wear before any parts are purchased.