What is an OBD2 code?

An OBD2 (On-Board Diagnostics) code is a diagnostic trouble code (DTC) generated by your vehicle's computer system when it detects a problem. These codes help identify specific issues with your vehicle's engine, transmission, emissions, or other systems.

How do I read OBD2 codes?

To read OBD2 codes, you'll need an OBD2 scanner. Plug it into your vehicle's OBD2 port (usually located under the dashboard), turn your key to the 'ON' position, and follow your scanner's instructions to retrieve the codes. Each code starts with a letter (P, B, C, or U) followed by four numbers.

What do the different types of OBD2 codes mean?

OBD2 codes are categorized by their first letter: P (Powertrain) for engine and transmission issues, B (Body) for interior systems like airbags, C (Chassis) for mechanical systems like brakes, and U (Network) for computer and module communication problems.

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Home » OBD2 Codes » P0399

Code P0399

Symptoms

• Check Engine Light is on

• Poor engine performance

• Increased fuel consumption

Common Causes

• Faulty cylinder 1 pressure sensor

• Damaged or corroded wiring/connectors

• ECM malfunction

Glossary

ECM: Engine Control Module

OBD-II: On-Board Diagnostics II

Related Codes

• P0398

• P0397

Official description

Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic

Laymans terms

The car's computer is having trouble getting a steady signal from a sensor that checks how hard the engine is working in one of the cylinders.

Explanation

The Engine Control Module (ECM) is detecting an irregular or inconsistent signal from the pressure sensor in cylinder 1.

Technical description

The P0399 code indicates that the ECM has detected an intermittent or erratic signal from the cylinder 1 pressure sensor. This sensor monitors the pressure within the cylinder to ensure optimal engine performance and efficiency. An erratic signal can lead to improper fuel and air mixture, affecting engine performance.

Diagnosis steps

• Use an OBD-II scanner to confirm the code.

• Inspect the wiring and connectors of the cylinder 1 pressure sensor.

• Test the cylinder 1 pressure sensor for proper operation.

Repair procedures

• Replace the faulty cylinder 1 pressure sensor.

• Repair or replace damaged wiring/connectors.

• Reprogram or replace the ECM if necessary.

Tools required

• OBD-II scanner

• Multimeter

• Socket set

Estimated repair cost

$150 - $400

Difficulty level

Moderate

Commonly affected vehicles

• Vehicle 1

• Vehicle 2

• Vehicle 3

Preventive measures

• Regularly inspect engine sensors and wiring.

• Perform routine engine maintenance.

• Ensure proper engine tuning.

Error code type

Sensor Issue

User experience recommendations

• Do not ignore the Check Engine Light.

• Have a professional diagnose the issue if unsure.

• Monitor engine performance for any changes.

Warranty information

Check your vehicle's warranty for coverage on engine sensors.

Historical data

This code is relatively uncommon and often linked to sensor or wiring issues.

Urgency level

Moderate – Should be addressed to avoid further engine performance issues.

Legal and safety notice

Driving with a faulty sensor can lead to increased emissions and potential engine damage.

Testing conditions

• Engine at operating temperature

• Vehicle in a safe, stationary position

Component location

The cylinder 1 pressure sensor is typically located on or near the engine block, close to cylinder 1.

FAQs

Q: Can I drive with a P0399 code?
A: It's not recommended to drive for long periods with this code, as it can lead to engine performance issues.

Q: How can I prevent a P0399 code?
A: Regular maintenance and inspections of engine sensors and wiring can help prevent this issue.

Last updated: Mar 2, 2026 • Source: internal database

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