Faulty ignition coil or ignition module drawing excessive current; Damaged or corroded wiring in the ignition supply circuit; Blown fuse or faulty relay supplying power to the ignition system; Poor ground connection for the ignition circuit; Failed engine control module (ECM) driver for the ignition supply; Aftermarket alarm or remote start system interfering with ignition wiring

DTC codes / Jaguar codes / P1789

P1789 · JaguarIgnition Supply Malfunction

Q: What does code P1789 mean?

Ignition Supply Malfunction. P1789 on a Jaguar indicates that the engine control module (ECM) has detected a fault in the ignition supply circuit, which provides power to the ignition coils or ignition module. The ECM monitors voltage and current on this circuit and sets the code when the signal is outside the expected range, often due to a wiring or component failure.

P1789 on a Jaguar indicates that the engine control module (ECM) has detected a fault in the ignition supply circuit, which provides power to the ignition coils or ignition module. The ECM monitors voltage and current on this circuit and sets the code when the signal is outside the expected range, often due to a wiring or component failure.

SystemPowertrain

Code typeJaguar specific

SeverityMedium

Fix difficultyIntermediate

AreaTransmission

Symptoms you may notice

Engine misfire or rough running, especially under load
Difficulty starting or no-start condition
Illuminated check engine light (malfunction indicator lamp)
Reduced engine performance or power loss
Often no symptoms besides the warning light, particularly if the fault is intermittent

Common causes, most likely first

Faulty ignition coil or ignition module drawing excessive current
Damaged or corroded wiring in the ignition supply circuit
Blown fuse or faulty relay supplying power to the ignition system
Poor ground connection for the ignition circuit
Failed engine control module (ECM) driver for the ignition supply
Aftermarket alarm or remote start system interfering with ignition wiring

How to diagnose and fix it

Check for any stored freeze-frame data and note the conditions when the code set
Inspect the ignition supply fuse and relay, testing for power and continuity
Visually inspect the ignition coil harness and connectors for damage or corrosion
Measure resistance and voltage drop across the ignition supply wiring to locate high resistance
Test the ignition coils or module for primary and secondary resistance according to Jaguar specifications
Check and clean the engine and chassis ground points related to the ignition system
If all external circuits test good, suspect the ECM and perform pin-out and signal tests before replacement

Don't overlook ignition supply circuit grounds

A common oversight is a corroded ground connection on the engine block that causes voltage drops only under load, setting P1789 without obvious misfire at idle.
Aftermarket accessories wired into the ignition circuit, such as alarms or remote starters, often introduce resistance or intermittent shorts that trigger this code.
The code can be set by a failing ignition coil that only breaks down when hot, so testing components cold may not reveal the fault.
P1789 does not always mean the ECM is faulty; the majority of cases trace back to wiring or coil issues, so avoid replacing the ECM prematurely.
On some Jaguar models, a weak battery or charging system fault can cause voltage spikes that the ECM interprets as an ignition supply malfunction.

Can I keep driving with P1789?

If the engine runs smoothly and the light is steady, you can drive cautiously to a repair shop, but avoid heavy acceleration or long trips. If the engine misfires, lacks power, or the light flashes, stop driving and seek immediate professional diagnosis to prevent catalytic converter damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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