P0739

What it means

P0739 is a generic powertrain code meaning the transmission control module has detected a high signal in the engine speed output circuit. The TCM expects a valid engine speed signal, normally supplied by or shared with the engine control unit, and it has seen a voltage or signal level above the expected range. The exact signal path and how the fault is tested vary by make, model and transmission, so check the manufacturer's data for the vehicle.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Gearbox in limp-home mode, often stuck in one gear
• Harsh, delayed or erratic gear changes
• Rev counter erratic or not working on some vehicles
• Torque converter lock-up not engaging, with higher fuel consumption

Possible causes

• Damaged wiring, such as a short to voltage or chafing, in the engine speed signal circuit between the engine control unit and TCMlikely
• Corroded, loose or contaminated connector terminals at the TCM or engine control unitlikely
• Faulty crankshaft position sensor or related signal feeding the engine speed outputpossible
• Electrical interference or poor earth connections affecting signal levelspossible
• Internal fault in the TCM, or a gearbox with the TCM integrated into the valve body, such as water or fluid ingresspossible
• Fault in the engine control unit output stage, or outdated software where the manufacturer has identified it as a known causerare

How to diagnose

1. Read all stored and pending codes from the engine and transmission modules, and record the freeze frame data
2. Check for related engine speed, crankshaft sensor or communication codes, as these may point to the real source
3. Inspect the wiring and connectors between the engine control unit and TCM for chafing, corrosion, bent pins and water ingress
4. Check battery condition, charging voltage and the main earth and supply connections to the TCM
5. Watch live engine speed data on both modules and compare it with the rev counter while the engine runs
6. Using the manufacturer's wiring diagram, test the signal circuit for short to voltage, open circuit and correct signal level
7. Check the crankshaft position sensor signal with a scope if the engine speed reading is wrong
8. Check for manufacturer software updates or technical bulletins for the vehicle before condemning the TCM

How to fix

• Repair or replace damaged wiring and fix any short to voltage in the signal circuit
• Clean, repair or replace corroded or damaged connectors and terminals
• Replace a faulty crankshaft position sensor if its signal is confirmed faulty
• Restore poor earth or supply connections and correct any charging problems
• Repair or replace the TCM or valve body unit if testing confirms an internal fault, then carry out any required programming or adaptation
• Apply a manufacturer calibration update only where a known software fault is documented for the vehicle

Can I keep driving?

Nearby codes

• P0738GenericTCM Engine Speed Output Circuit Low
• P073AGenericStuck in Gear 5