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DTC codes / Jaguar codes / P1797

P1797 · JaguarCAN TCM ECM Circuit Malfunction

P1797 on a Jaguar indicates that the Transmission Control Module (TCM) and the Engine Control Module (ECM) have detected a fault in the Controller Area Network (CAN) communication circuit that links them. The code is stored when either module fails to receive expected CAN messages from the other within a specified time window, or when the data received is implausible.

SystemPowertrain
Code typeJaguar specific
SeverityMedium
Fix difficultyProfessional
AreaTransmission

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the instrument cluster.
  • Transmission may enter limp mode, defaulting to a single gear or restricted shift pattern.
  • Harsh or delayed gear shifts, or the transmission may refuse to shift out of a particular gear.
  • Engine may idle poorly, stall, or exhibit reduced power due to missing torque reduction requests from the TCM.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • Speedometer or tachometer may behave erratically if CAN data used by the instrument cluster is affected.

Common causes, most likely first

  1. Damaged or chafed wiring in the CAN bus between the ECM and TCM, often near connectors or where the harness passes through the bulkhead.
  2. Corroded, loose, or water-contaminated connectors at the ECM or TCM, leading to high resistance or intermittent open circuits.
  3. Failed CAN transceiver inside the TCM or ECM, preventing proper differential signaling on the bus.
  4. Internal fault in the TCM causing it to stop transmitting or receiving CAN messages.
  5. Internal fault in the ECM causing it to stop transmitting or receiving CAN messages.
  6. Electrical interference from a faulty alternator or ignition system corrupting CAN bus signals.
  7. Low battery voltage or poor grounds causing modules to reset and drop off the CAN bus intermittently.

How to diagnose and fix it

  1. Perform a thorough visual inspection of the CAN wiring between the ECM and TCM, looking for chafing, pinching, or corrosion at connectors.
  2. Measure battery voltage and check both engine and transmission grounds for cleanliness and tightness; low voltage can cause spurious CAN faults.
  3. Use a scan tool to read live CAN bus data and check for other communication codes (U-codes) that may indicate a broader network issue.
  4. Disconnect the TCM and ECM connectors and inspect terminals for corrosion, fretting, or backed-out pins; clean and reseat as needed.
  5. With a digital multimeter, check CAN High and CAN Low resistance and voltage levels at the TCM and ECM connectors to verify bus integrity.
  6. If wiring and connectors are good, substitute a known-good TCM or ECM (whichever is more accessible) to isolate the faulty module.
  7. After repairs, clear codes and perform a road test with a scan tool to confirm CAN communication is restored and no codes return.

CAN Bus Faults Often Misdiagnosed as TCM Failure

  • A significant number of P1797 cases are caused by a chafed CAN wire in the harness near the transmission bell housing or behind the dashboard, not by a failed TCM or ECM.
  • Water ingress into the TCM connector, often from a clogged drain or failed seal, can cause intermittent CAN faults that only appear in wet weather.
  • A weak battery or failing alternator can drop system voltage enough to make modules reset and set P1797 without any permanent wiring fault.
  • Some technicians replace the TCM only to find the code returns because the ECM connector has fretting corrosion on the CAN pins, which is easily overlooked.
  • P1797 can be set by a faulty ignition switch or relay that momentarily cuts power to the TCM, mimicking a CAN circuit failure.

Can I keep driving with P1797?

If the transmission has entered limp mode or shifts erratically, avoid highway driving and seek service promptly; if only the warning light is on and the car drives normally, you may drive cautiously to a workshop but avoid heavy loads or long trips.

Tools that help

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Guidance is general and written for Jaguar vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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