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Home / DTC / P1791 — Transmission Control Module Loop

P1791 — Transmission Control Module Loop

Detailed page for trouble code P1791.

34,405codes
59brands
11,914generic
22,491specific
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P1791

CADILLAC P — Powertrain

Transmission Control Module Loop

Brand: CADILLAC
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Open or short in TCM wiring harness or connector
  • Poor or intermittent power supply to the TCM (blown fuse, battery/charging low)
  • Faulty or high‑resistance ground to the TCM
  • Corroded/damaged connector pins at TCM or mating connector
  • CAN bus or serial data communication fault between TCM and other modules
  • Internal TCM software error or internal TCM failure

Symptoms

  • Malfunction Indicator Lamp (MIL) or Transmission Warning light illuminated
  • Transmission stuck in limp mode or limited gears
  • Erratic, harsh, delayed, or no shifting
  • DTCs related to communication or transmission control stored (may include U‑codes)
  • No communication with TCM when scanned or intermittent comms
  • Reduced vehicle performance or inability to move in drive/gear

What to check

  • Use a capable scan tool to read freeze frame data, trouble codes and live TCM communication status
  • Check for related and history codes (U‑codes, P0700, vehicle speed sensor codes)
  • Verify battery voltage and charging system — voltage should be ~12.6 V at rest and 13.5–14.8 V when running
  • Inspect TCM connectors and wiring for corrosion, damage, pin push‑out, water intrusion, crushed wires, or rodent damage
  • Check transmission/TCM fuses and relays — replace or test as required
  • Confirm scan tool can communicate with the TCM and note any intermittent loss of communication

Signal parameters

  • Battery supply to TCM: ~12 V with engine off, ~13.5–14.8 V with engine running (minimal drop under cranking)
  • Ignition/switched power present at TCM when key ON
  • Ground: solid low resistance to chassis/negative battery (typically
  • CAN bus idle voltages: CAN‑H ~2.5 V, CAN‑L ~2.5 V (differential ~0 V idle); during data CAN‑H rises and CAN‑L falls — verify bus activity with scope or scan tool
  • TCM communication rate: depends on vehicle network (commonly 500 kb/s or 250 kb/s) — verify with appropriate tool
  • Transmission solenoid coil resistance: consult service manual — abnormal open/short can load TCM outputs

Diagnostic algorithm

  1. Connect a diagnostic scan tool and record all stored and pending codes; note freeze frame and live data related to transmission
  2. Attempt communication with the TCM; if no response, verify vehicle grounds and battery/fuse power to TCM
  3. Visually inspect TCM connector(s) and wiring harness for damage, corrosion, or water intrusion; repair any damaged wiring or connectors
  4. Check and verify fuses/relays for TCM and transmission control circuits; replace if blown or faulty
  5. Measure supply voltage and ground at the TCM connector with ignition ON and engine cranking; repair if out of specification
  6. Check CAN‑bus: measure CAN‑H and CAN‑L at the TCM and at another module to confirm continuity and correct idle voltages; repair shorts/opens
  7. Perform wiggle tests on harness and connectors while monitoring communication and live data — isolate intermittent faults to a harness section or connector
  8. If solenoids or sensors show out‑of‑spec resistances or short/opens, repair or replace affected components and retest
  9. Clear codes and perform a road test while monitoring live data to reproduce the fault; if code returns, note conditions
  10. If wiring, power, grounds, and communication lines check good, see if a software update/reflash is available for TCM/ECM; perform as per service bulletin
  11. If all external checks pass and a proven circuit fault is not found, consider replacement of the TCM (follow OEM programming and relearn procedures)

Likely causes

  • Loose or corroded TCM connector pin(s)
  • Blown transmission/TCM fuse or poor battery/charger voltage
  • Damaged CAN bus wiring (short to ground, short to Vb, or open)
  • Intermittent ground at transmission/TCM mounting point
  • Failed TCM (after other causes ruled out)

Fault status

⚠️ Status
Transmission Control Module Loop — communication or circuit problem detected between the transmission control module and its control/communication loop.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-4.0 hours

Similar codes

P0700 P0705 P0720 U0100 U0101
206

Browse 206 CADILLAC manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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