P2639: Torque Management Feedback Signal A Low

P2639 means the torque management feedback signal A stays below its valid range, so the transmission controller cannot read a usable torque confirmation from the engine. No manufacturer explains this exact code publicly. The closest documented relatives are Allison's P2637, where the TCM checks over the SAE J1939 data link whether the engine honours shift torque reductions, and Honda's P2638 on its nine-speed gearbox. If the feedback travels as a network message rather than a wire, a low value usually stems from a calibration mismatch between engine and transmission software or a missing message; on a hard-wired signal, check for a ground short. CARB requires such inputs to be tested against range limits. Shift quality may suffer and torque reduction may be disabled.

Symptoms

• Transmission warning or stored code
• Firm or flared shifts

Most likely causes

• Engine and transmission software versions not matched
• Torque feedback message missing or corrupted on the data bus
• Hard-wired feedback line shorted to ground

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the fluid level and condition
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Read the input and output speed in live data
4. Check the connector and wiring
5. Check for other related engine codes
6. Consult the manufacturer's procedure for the transmission

Good to know

• It can trigger limp mode.

What each character of P2639 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 6: Control modules and outputs
• 39: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2639?

Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.

What causes code P2639?

The most common causes are: engine and transmission software versions not matched; torque feedback message missing or corrupted on the data bus; hard-wired feedback line shorted to ground.

Related codes

• P2637Torque Management Feedback Signal A
• P2638Torque Management Feedback Signal A Range/Performance
• P2643Torque Management Feedback Signal B Low

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10189170 9999
• static.nhtsa.gov: MC 10100176 9340

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.