P1851 Code: Meaning, Symptoms, Causes & Fix

Overview

Symptoms

• transmission warning message
• gear hunting
• reduced acceleration

Common causes

• Broken terminal inside harness affecting transmission transfer case contact plate 'b' open circuit
• Open circuit in wiring
• Connector pin damage/corrosion or harness stress near related components

How to diagnose P1851

1. Confirm P1851 and capture freeze-frame data with a professional scan tool.
2. Inspect wiring, connectors, and grounds related to transmission transfer case contact plate 'b' open circuit for obvious defects.
3. Use a continuity test from module pin to component to identify opens/high resistance.
4. Check for related DTCs to identify the root-cause chain before replacing parts.
5. After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P1851

Moderate: This code is moderate severity: the vehicle may still operate, but fault progression can increase repair cost over time.

Can you drive with P1851?

Most vehicles can be driven short-term with P1851, but postpone heavy load/highway driving until repaired.

If the warning lamp is flashing or drivability/safety is affected, avoid driving and diagnose immediately.

How to fix it (step-by-step)

1. Repair obvious wiring/connector faults first to prevent repeat parts replacement.
2. Address the root fault mode (open) in the transmission transfer case contact plate 'b' open circuit circuit/system.
3. Apply the required relearn/adaptation procedure if specified by service information.
4. Road-test and verify P1851 remains cleared under the conditions that originally set the code.

Fix options

• Clear corrosion and protect terminals.
• Restore continuity and verify pin tension.
• Clear DTCs and verify with live data and completed drive cycle.

Typical repair cost

$300-$1500

Common mistakes when diagnosing P1851

• Replacing parts without verifying voltage, ground, and signal integrity first.
• Ignoring related stored/pending codes that indicate upstream faults.
• Skipping post-repair verification drive cycle and readiness checks.

P1851 in common vehicles

P1851 appears across makes such as BMW, Mercedes-Benz, Audi, Volvo. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P1851?

P1851 is commonly set by open faults affecting transmission transfer case contact plate 'b' open circuit.

How much does it cost to fix P1851?

For P1851, repair costs are typically $300-$1500 depending on exact root cause, labor rates, and part quality.

Can I clear P1851 without repair?

You can clear P1851 temporarily, but the code usually returns until the underlying fault is corrected.

Helpful tools

• Estimate repair cost
• Decode your VIN
• Calculate fuel cost