Low-octane or poor-quality fuel in the tank during the self-test window; Faulty knock sensor or damaged knock sensor wiring/connector; Carbon buildup or mechanical noise in the engine mimicking knock; PCM calibration or software issue needing a Ford update; Loose or corroded PCM connector pins affecting the knock sensor circuit; Engine coolant temperature or intake air temperature sensor bias skewing the self-test conditions; Rare internal PCM fault in the knock control or self-test circuitry

DTC codes / Ford codes / P1390

P1390 · FordOctane Adjust Out of Self-Test Range

Q: What does code P1390 mean?

Octane Adjust Out of Self-Test Range. P1390 on Ford vehicles means the powertrain control module (PCM) ran its octane adjust self-test and found the octane adjust value outside the expected range. The computer compared the knock sensor signal and ignition timing behaviour against a stored window during a specific self-test routine, and the result did not fall where Ford expects it to.

P1390 on Ford vehicles means the powertrain control module (PCM) ran its octane adjust self-test and found the octane adjust value outside the expected range. The computer compared the knock sensor signal and ignition timing behaviour against a stored window during a specific self-test routine, and the result did not fall where Ford expects it to.

SystemPowertrain

Code typeFord specific

SeverityLow

Fix difficultyIntermediate

AreaIgnition system and misfire

Symptoms you may notice

Illuminated check engine light, often with no other symptoms
Slight loss of power or sluggish acceleration under load
Occasional engine ping or rattle on acceleration with lower-octane fuel
Rough idle or mild hesitation in some cases
Poor fuel economy if the PCM has pulled timing to protect the engine
No drivability complaint at all in many instances

Common causes, most likely first

Low-octane or poor-quality fuel in the tank during the self-test window
Faulty knock sensor or damaged knock sensor wiring/connector
Carbon buildup or mechanical noise in the engine mimicking knock
PCM calibration or software issue needing a Ford update
Loose or corroded PCM connector pins affecting the knock sensor circuit
Engine coolant temperature or intake air temperature sensor bias skewing the self-test conditions
Rare internal PCM fault in the knock control or self-test circuitry

How to diagnose and fix it

Verify fuel octane and quality; if recently filled with low-octane fuel, run the tank down and refill with the recommended grade before retesting
Scan all modules and record freeze-frame data to confirm the exact conditions when P1390 set
Inspect the knock sensor and its wiring for chafing, corrosion, or loose connections, and check sensor mounting torque
Check for mechanical engine noise or excessive carbon buildup that could be interpreted as knock
Verify engine coolant temperature and intake air temperature sensor readings against actual conditions with a scan tool
Check for and install any available Ford PCM calibration updates for the vehicle
If all else passes, perform a pinpoint test of the knock sensor circuit and PCM self-test routine per Ford service information

What Ford's octane self-test really checks

P1390 is a self-test result, not a live knock fault; the PCM runs this test under specific conditions and stores the code when the octane adjust value falls outside the calibrated window.
Many P1390 codes are set after a single tank of low-octane fuel, so correcting the fuel and clearing the code often resolves it without any parts replacement.
A noisy accessory drive, exhaust heat shield, or loose engine mount can create vibration the knock sensor picks up, mimicking knock and skewing the self-test.
Technicians sometimes replace the knock sensor unnecessarily when the real problem is a skewed coolant temperature sensor or a PCM calibration that needs updating.
This code does not always mean the engine is damaged; it means the PCM could not verify the octane adjust value within its expected range during the self-test.

Can I keep driving with P1390?

You can generally keep driving with P1390 because it is a self-test result and the PCM still protects the engine, but avoid heavy load or towing and use the recommended octane fuel until the code is diagnosed and cleared.

Tools that help

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