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 » P1871

Code P1871

Symptoms

• Harsh or delayed shifting

• Transmission stuck in gear

• Check Engine Light illuminated

Common Causes

• Faulty 2-1 shift solenoid

• Short circuit in the wiring

• Faulty transmission control module (TCM)

Glossary

Solenoid: An electromechanical device used to control fluid flow in the transmission.

TCM: Transmission Control Module, the computer that controls transmission operations.

Related Codes

• P1870

• P1872

Official description

2-1 Shift Circuit High

Laymans terms

This code means that the car's computer has detected a problem with the electrical circuit that controls shifting from second gear to first gear. It's like when a light switch is stuck on and won't turn off.

Explanation

The P1871 code is triggered when the transmission control module (TCM) detects a high voltage condition in the 2-1 shift solenoid circuit.

Technical description

The P1871 code indicates a fault in the transmission's 2-1 shift solenoid circuit, where the voltage is higher than expected. This can be due to a short circuit, faulty solenoid, or wiring issues. The solenoid is responsible for controlling the hydraulic pressure that enables the transmission to shift from second gear to first gear.

Diagnosis steps

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

• Inspect the wiring and connectors for damage.

• Test the 2-1 shift solenoid for proper operation.

Repair procedures

• Replace the faulty 2-1 shift solenoid.

• Repair or replace damaged wiring or connectors.

• Replace the transmission control module if necessary.

Tools required

• OBD-II scanner

• Multimeter

• Socket set

Estimated repair cost

$200 - $500

Difficulty level

Moderate

Commonly affected vehicles

• Chevrolet Silverado

• GMC Sierra

• Cadillac Escalade

Preventive measures

• Regular transmission fluid changes

• Inspect wiring harnesses periodically

• Avoid harsh driving conditions

Error code type

Transmission Issue

User experience recommendations

• Avoid driving the vehicle until repaired.

• Consult a professional mechanic for diagnosis.

Warranty information

Transmission components may be covered under powertrain warranty.

Historical data

This code has been commonly reported in vehicles with high mileage or those that have experienced transmission issues.

Urgency level

High – This issue can lead to transmission failure if not addressed promptly.

Legal and safety notice

Driving with a malfunctioning transmission can be dangerous and may lead to further damage.

Testing conditions

• Vehicle at operating temperature

• Transmission in drive mode

Component location

The 2-1 shift solenoid is located inside the transmission valve body.

FAQs

Q: Can I drive with a P1871 code?
A: It's not recommended to drive with this code as it can cause further transmission damage.

Q: How do I reset the P1871 code?
A: After repairs, use an OBD-II scanner to clear the code from the vehicle's computer.

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

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