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DTC codes / Jaguar codes / P1242

P1242 · JaguarSensor Power Supply High Input

P1242 on a Jaguar indicates that the engine control module (ECM) has detected a voltage on a sensor power supply circuit that is higher than the expected regulated value, typically above 5.5 volts on a 5-volt reference bus. The ECM monitors this supply to ensure sensors receive a stable voltage, and a high reading suggests a short to voltage or an internal fault in the supply circuit.

SystemPowertrain
Code typeJaguar specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering (injector circuits)

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Rough idle or poor engine performance if the affected sensor is critical to fueling or timing
  • Intermittent stalling or hesitation under load due to corrupted sensor signals
  • Transmission shift quality issues if the shared reference voltage feeds a transmission sensor
  • Reduced fuel economy or increased emissions as the ECM substitutes default values

Common causes, most likely first

  1. Short to voltage on the sensor reference circuit, often from chafed wiring near hot or moving components
  2. Faulty sensor internally shorted and back-feeding voltage onto the reference line
  3. Corroded or damaged connector pins at the sensor or ECM causing high resistance and voltage spikes
  4. ECM internal fault in the 5-volt reference supply regulator
  5. Water intrusion into a connector or wiring harness, leading to intermittent shorts
  6. Aftermarket accessories or poor previous repairs tapping into the reference circuit

How to diagnose and fix it

  1. Connect a scan tool and record all stored and pending codes, then clear codes and test-drive to see if P1242 returns
  2. Use a digital multimeter to measure the reference voltage at each sensor on the shared 5-volt bus, comparing to the specified value
  3. Visually inspect the harness and connectors for signs of chafing, corrosion, or contamination, especially near the exhaust and engine mounts
  4. Disconnect sensors one at a time to see if the high voltage disappears, isolating the faulty component
  5. Check for aftermarket wiring or accessories spliced into the reference circuit and remove or repair as needed
  6. If the high voltage persists with all sensors disconnected, test the ECM reference output; replace or reprogram the ECM if faulty
  7. Repair or replace the affected harness section or sensor, then clear codes and verify the repair with a road test

Shared Reference Voltage Pitfalls

  • Many Jaguar models share a single 5-volt reference among multiple sensors, so one faulty sensor can pull the entire bus high and set P1242 without the faulty sensor itself setting a code.
  • A common misdiagnosis is replacing the ECM first; always check for a short to voltage on the reference line before condemning the module.
  • Water ingress into the connector at the throttle body or mass airflow sensor is a frequent cause on older Jaguars, especially after steam cleaning or heavy rain.
  • The code can be set by a failing sensor that only shorts when hot, so a cold engine check may not reveal the fault; use a heat gun or road test to reproduce.
  • Aftermarket alarm or remote start installations often tap into the reference circuit, creating intermittent high voltage that sets P1242 without any driveability symptoms.

Can I keep driving with P1242?

You can usually drive the vehicle short distances to a repair shop, but avoid sustained high-speed driving or heavy loads because the ECM may be operating on default values that affect fuel mixture and emissions. If the engine runs rough or stalls, stop driving and have the vehicle towed.

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.

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