P1921

Code

Generic P — Powertrain

Transmission Control Module Power Supply Voltage Low

Brand: Generic

Type: P — Powertrain

Views: UK: 15 EN: 22 RU: 17

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Page language: EN

Causes

Weak or discharged battery
Loose or corroded battery terminals or cable
Blown or open fuse for TCM power feed
Faulty ignition switch or power relay supplying the TCM
Damaged, corroded or shorted wiring in the TCM power feed
Poor or high-resistance engine/chassis ground(s)

Symptoms

Transmission limp/limited function or fixed gear
Harsh, delayed or erratic shifting
Transmission-related MIL (check engine light) or transmission warning lamp
Loss of communication with TCM on scan tool or intermittent comms
Failure to engage drive/gear selection problems
Possible no-start or cranking problems if supply is severely low

What to check

Read freeze frame and all stored codes; note time, engine state and conditions when the DTC set
Check battery state-of-charge and cold cranking amps; perform load test if battery is old or suspect
Visually inspect battery terminals, cables and ground straps for corrosion, tightness and damage
Verify fuses and relays in the power distribution box for the transmission/TCM circuit
Measure voltage at battery positive and at TCM power supply pin(s) with key ON and during cranking
Check continuity and voltage drop between battery positive and the TCM power pin

Signal parameters

Battery voltage (rest, engine OFF): ~12.4–12.8 V (acceptable >12.0 V)
Key ON, engine OFF at TCM power pin: should be within ~11–14.5 V; fault likely if below ~8–9 V
Cranking voltage at battery: typically >9.5 V under load (depends on vehicle); substantial drops below this can trigger faults
Voltage drop between battery positive and TCM power pin: ideally 1.0–2.0 V indicates wiring/connection problem
Ground circuit resistance: ideally

Diagnostic algorithm

Connect a scan tool, record DTCs and freeze frame data. Note if P1921 is accompanied by battery/charging codes (e.g., P0562).
Visually inspect battery, terminals and cables. Clean and tighten terminals. Retest for voltage and DTC reset.
Check vehicle battery voltage at rest. If low, charge or replace battery and retest. Perform battery load/crank test.
With key ON (engine OFF), measure voltage at the TCM power input pin(s). Compare to battery positive. If low, trace feed back to fuse/relay.
Inspect and test the fuse(s) and relay(s) that supply the TCM. Replace suspect fuses/relays and recheck voltage.
Perform voltage drop test on positive feed: measure between battery positive and TCM power pin while under load (key ON and cranking). Repair high drop points (clean/replace cables, connectors).
Check ground(s) for the TCM: measure resistance from TCM ground to battery negative/chassis and inspect ground strap connections. Clean/tighten or repair as needed.
Wiggle wiring harness and connectors while monitoring TCM supply voltage and communication to reproduce intermittent faults. Repair damaged wiring or pins.
Verify alternator output and charging system if system voltage is erratic. Repair charging system faults before further TCM diagnostics.
If wiring, fuses, relays, battery and charging system are confirmed good and low voltage condition persists at the TCM, consider replacing or bench-testing the TCM per manufacturer procedures.
After repairs, clear codes and perform road test under the same conditions to confirm the DTC does not return.

Likely causes

Loose or corroded battery terminals or poor battery condition
Open/blown fuse or relay in the TCM power feed
High-resistance connection or broken wire in the B+ feed or ground
Faulty ignition switch or power distribution problem
Failing TCM (less common)

Fault status

⚠️ Status

Transmission Control Module detected low power supply voltage. Check battery, fuses, power/ground wiring, relays and charging system before replacing the TCM.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours

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