P0728

What it means

P0728 is the generic code for an intermittent fault in the engine speed input circuit as seen by the transmission control module. The TCM takes engine speed from a crankshaft signal, usually shared over the vehicle network or a direct wire, and compares it with other inputs such as turbine speed. The code sets when that signal drops out, becomes erratic or disagrees briefly with what is expected, then returns to normal.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Harsh, delayed or erratic gear changes
• Gearbox going into limp mode or staying in one gear
• Rev counter flickering or dropping to zero momentarily
• Torque converter lock-up not engaging or shuddering
• Intermittent engine misfire, stalling or hesitation if the crank signal is faulty

Possible causes

• Loose, corroded or damaged wiring or connector in the engine speed signal circuit between the crankshaft sensor, engine control unit and transmission control unitlikely
• Failing crankshaft position sensor giving a dropout signal when hot or vibratinglikely
• Damaged or contaminated reluctor wheel or incorrect sensor air gappossible
• Network communication fault or poor connection on the data bus carrying the engine speed signal (depends on model)possible
• Low battery voltage, poor earth or power supply problem affecting the control unitspossible
• Faulty transmission control unit or engine control unit, or an outdated calibration known for this fault on some modelsrare

How to diagnose

1. Read all stored codes and freeze frame data from both engine and transmission control units, noting speed, temperature and load when the fault set.
2. Check for related codes such as crankshaft sensor, network communication or misfire faults, and deal with those first.
3. Watch live engine speed on the scan data for both control units while running and compare them; look for dropouts or differences.
4. Inspect the crankshaft sensor, its connector and wiring for oil contamination, chafing, heat damage and corrosion, and wiggle-test the harness with the engine running.
5. Test battery condition, charging voltage and main earths, as low or unstable voltage can cause signal dropouts.
6. Check the sensor mounting, air gap and reluctor wheel for damage or debris, following the model-specific procedure.
7. Use an oscilloscope to check the crankshaft signal pattern for missing or distorted pulses, and test the signal wire for continuity and shorts.
8. Check for manufacturer service information about calibration updates or known network faults for the specific model and gearbox.

How to fix

• Repair or replace damaged wiring and clean or replace corroded connectors in the engine speed signal circuit.
• Replace the crankshaft position sensor if its signal is intermittent or out of specification.
• Repair or replace a damaged reluctor wheel and set the sensor air gap correctly where adjustable.
• Correct battery, charging or earth faults, then clear the codes and retest.
• Rectify data network faults found in the wiring or modules, and update the control unit calibration only if service information lists it as a fix for this fault.

Can I keep driving?

Nearby codes

• P0727GenericEngine Speed Input Circuit No Signal
• P0729GenericGear 6 Incorrect Ratio