Blown fuse or faulty fuse box connection in the ignition supply circuit; Damaged or corroded wiring in the ignition supply harness, including chafed insulation or broken strands; Poor ground connection for the ignition system or engine block; Faulty ignition switch or ignition relay not delivering consistent power; Failed ignition coil or coil pack drawing excessive current or shorting internally; Corrosion or loose pins at the ECM connector affecting the supply voltage; Internal fault in the ECM itself (rare but possible)

DTC codes / Jaguar codes / P1791

P1791 · JaguarIgnition Supply Malfunction

Q: What does code P1791 mean?

Ignition Supply Malfunction. P1791 on a Jaguar means the engine control module (ECM) has detected a problem with the ignition supply circuit, which is the voltage feed that powers the ignition coils and related components. The ECM monitors this supply and sets the code when the voltage is out of range, intermittent, or missing when it should be present.

P1791 on a Jaguar means the engine control module (ECM) has detected a problem with the ignition supply circuit, which is the voltage feed that powers the ignition coils and related components. The ECM monitors this supply and sets the code when the voltage is out of range, intermittent, or missing when it should be present.

SystemPowertrain

Code typeJaguar specific

SeverityMedium

Fix difficultyIntermediate

AreaTransmission

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Engine misfire or rough running, especially under load
Difficulty starting or a crank-no-start condition
Loss of power or reduced engine performance
In rare cases, the engine may stall or cut out while driving

Common causes, most likely first

Blown fuse or faulty fuse box connection in the ignition supply circuit
Damaged or corroded wiring in the ignition supply harness, including chafed insulation or broken strands
Poor ground connection for the ignition system or engine block
Faulty ignition switch or ignition relay not delivering consistent power
Failed ignition coil or coil pack drawing excessive current or shorting internally
Corrosion or loose pins at the ECM connector affecting the supply voltage
Internal fault in the ECM itself (rare but possible)

How to diagnose and fix it

Check all fuses related to the ignition system and replace any that are blown
Inspect the ignition supply wiring and connectors for damage, corrosion, or loose pins
Test the ignition relay and ignition switch for proper voltage output
Measure voltage at the ignition coil connectors with the key on and during cranking
Perform a voltage drop test on the ground circuits for the ignition system
Check for technical service bulletins (TSBs) or known issues for the specific Jaguar model
If all else fails, test the ECM for internal faults or replace it after confirming power and ground integrity

Ignition supply faults often hide in plain sight

A blown fuse is the most common cause, but simply replacing it without finding the underlying short or overload will lead to a repeat failure.
Corroded or loose ground connections can mimic an ignition supply fault and are frequently overlooked during diagnosis.
The ignition supply circuit often shares power with other systems, so a fault may also affect components like the fuel injectors or sensors.
Intermittent faults may not set a code until the vehicle is driven under specific conditions, such as vibration or temperature extremes.
Always verify the battery and charging system are healthy before chasing this code, as low voltage can trigger false ignition supply faults.

Can I keep driving with P1791?

If the engine is running normally, you can drive cautiously to a repair shop, but avoid heavy acceleration or long trips. If the engine misfires, stalls, or fails to start, stop driving and have the vehicle towed to prevent further damage or safety risks.

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.