AutoFixDB - Free OBD-II diagnostic trouble code lookup. Find causes, symptoms, diagnostic steps and repair solutions.

Overview

DTC P0399 indicates an intermittent or erratic signal from the cylinder 1 pressure sensor circuit, often due to wiring/connectors, sensor failure, or ECM issues, leading to performance symptoms and requiring systematic electrical diagnosis.

Common Symptoms

Check engine light illuminated
Rough idle/vibration at idle
Loss of power / reduced acceleration
Cylinder 1 misfire or knock
Intermittent engine stalling

Most Common Causes

Faulty cylinder 1 pressure sensor high

Sensor can wear or fail internally causing erratic pressure readings.

Damaged or corroded wiring/connectors in sensor circuit high

Intermittent contact or shorts induces erratic signal to ECM.

Faulty engine control module (ECM) input circuit medium

Less common; internal ECM fault could misinterpret valid signals.

Indirect engine/mechanical issues affecting pressure readings low

Severe internal engine wear or abnormal combustion could skew sensor data.

Diagnostic Steps

Connect professional OBD-II scanner and record freeze frame and live data for P0399 and related codes

Expected: P0399 present, no conflicting codes immediately obvious

Interpretation: Confirm the fault and gather initial context
Visually inspect wiring and connectors to cylinder 1 pressure sensor

Expected: No corrosion, broken wires, loose pins

Interpretation: Damaged wiring/connectors often cause intermittent signals
Perform continuity, resistance, and voltage checks on sensor circuit with multimeter

Expected: Circuit within manufacturer spec and stable signal

Interpretation: Out-of-spec or unstable indicates wiring or sensor issue
Replace cylinder 1 pressure sensor if signal remains erratic after wiring verified

Expected: Stable pressure signal and no recurrence of code

Interpretation: Sensor was likely root cause
If code persists, test ECM input and replace/reprogram ECM as required

Expected: No code after ECM service

Interpretation: ECM internal fault was contributing

Repair Solutions

Repair or replace damaged wiring/connectors in sensor circuit medium

Estimated Cost: $50 - $150

Depends on extent of harness damage

Replace cylinder 1 pressure sensor medium

Estimated Cost: $150 - $600

Part cost varies widely by vehicle

ECM reprogram or replacement if internal fault detected hard

Estimated Cost: $400 - $1200

High cost; last resort

Common Misdiagnoses

Avoid these common diagnostic mistakes:

Misinterpreting P0399 as general misfire code (e.g., P0301) without circuit testing

Frequently Asked Questions

Technical definition of P0399 OBD-II Code

Technically, the P0399 code represents Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic. It usually points to a fault within the Sensors.

Should I stop driving if I have P0399?

With a severity level of medium, may cause rough idle, reduced power, misfire in cylinder 1 and intermittent stalling; engine may run poorly or inefficiently.

Symptoms associated with DTC P0399

Drivers often report: Check engine light illuminated, Rough idle/vibration at idle, Loss of power / reduced acceleration when this code is present.

What causes the P0399 code?

This code is frequently triggered by: Faulty cylinder 1 pressure sensor, Damaged or corroded wiring/connectors in sensor circuit, Faulty engine control module (ECM) input circuit.

How much does it cost to fix P0399?

The cost varies significantly based on the root cause. For example, replacing a Repair or replace damaged wiring/connectors in sensor circuit typically costs between $50 and $150, while fixing a ECM reprogram or replacement if internal fault detected can range from $400 to $1200.

What cars are affected by the P0399 code?

This is a universal code. You might see P0399 on Ford, Chevrolet, Toyota, Honda, Nissan, or any other OBD-II vehicle.

P0399: Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic