The P1851 trouble code is a manufacturer-specific diagnostic code related to the vehicle’s transmission system. It typically indicates an issue with the brake band apply solenoid performance or a problem with the transmission transfer case contact plate B circuit. This code can affect various vehicle makes and models, causing transmission-related symptoms that require attention.
| P1851 Code Meaning | P1851 Code Common Causes |
|---|---|
| Brake Band Apply Solenoid Performance Issue | Faulty brake band apply solenoid |
| Transmission Transfer Case Contact Plate B Open Circuit | Damaged or corroded wiring |
| Transmission Control System Malfunction | Low or dirty transmission fluid |
| Powertrain Control Module (PCM) Detection of Circuit Problem | Faulty transmission control module (TCM) |
| Hydraulic System Pressure Control Issue | Clogged transmission fluid passages |
Brake Band Apply Solenoid Performance Issue
The brake band apply solenoid is a crucial component in automatic transmissions that use bands to engage different gears. When this solenoid malfunctions, it can lead to various transmission problems and trigger the P1851 code.
Symptoms
- Delayed or harsh gear shifts
- Slipping between gears
- Transmission overheating
- Check engine light illumination
Technical Explanation
The brake band apply solenoid controls the application of the transmission band, which wraps around a drum to stop its rotation and facilitate gear changes. When the solenoid fails to perform correctly, it can’t properly modulate the hydraulic pressure needed to engage or disengage the band, leading to shifting issues.
Step-by-Step Diagnosis
- Connect an OBD-II scanner to retrieve the P1851 code and any additional codes.
- Check the transmission fluid level and condition. Low or dirty fluid can cause solenoid issues.
- Inspect the transmission wiring harness for any visible damage or corrosion.
- Use a multimeter to test the resistance of the brake band apply solenoid. Compare the readings to the manufacturer’s specifications.
- Perform a pressure test on the transmission to check for proper hydraulic pressure at the solenoid.
- If possible, use a scan tool to command the solenoid on and off while monitoring transmission data to verify its operation.
Solution Methods
- Transmission Fluid Service: Drain and replace the transmission fluid, ensuring the correct type and amount are used.
- Solenoid Replacement: If the solenoid is determined to be faulty, replace it with a new one.
- Wiring Repair: Repair or replace any damaged wiring or connectors associated with the solenoid circuit.
- Transmission Flush: In some cases, a complete transmission flush may be necessary to remove contaminants affecting solenoid performance.
Cost Estimates
- Transmission Fluid Service: $100 – $250
- Solenoid Replacement: $200 – $500 (parts and labor)
- Wiring Repair: $100 – $300
- Transmission Flush: $150 – $400
Warning: Transmission work can be complex. If you’re not confident in your ability to perform these repairs, it’s best to consult a professional mechanic to avoid causing further damage.
Transmission Transfer Case Contact Plate B Open Circuit
An open circuit in the transfer case contact plate B can disrupt the transmission’s ability to engage four-wheel drive or switch between different drive modes, triggering the P1851 code.
Symptoms
- Inability to engage four-wheel drive
- Erratic transfer case behavior
- Transmission warning light
- Reduced vehicle performance
Technical Explanation
The transfer case contact plate B is part of the electrical system that controls the engagement of different drive modes in vehicles with four-wheel drive capabilities. An open circuit means there’s a break in the electrical path, preventing proper communication between the transfer case and the vehicle’s computer.
Step-by-Step Diagnosis
- Verify the P1851 code using an OBD-II scanner and check for any related codes.
- Inspect the transfer case wiring harness for visible damage, loose connections, or corrosion.
- Use a multimeter to check for continuity in the transfer case contact plate B circuit.
- Examine the transfer case control module for signs of damage or water intrusion.
- Test the transfer case motor and position sensors for proper operation.
- Perform a voltage drop test across the circuit to identify any high-resistance areas.
Solution Methods
- Wiring Repair: Repair or replace any damaged wiring in the transfer case circuit.
- Contact Plate Replacement: If the contact plate is faulty, replace it with a new one.
- Connector Cleaning: Clean and secure all connectors in the transfer case electrical system.
- Control Module Replacement: In rare cases, a faulty transfer case control module may need replacement.
Cost Estimates
- Wiring Repair: $150 – $400
- Contact Plate Replacement: $300 – $800 (depending on vehicle make and model)
- Connector Cleaning: $50 – $150
- Control Module Replacement: $500 – $1,500
Recommendation: Transfer case repairs often require specialized tools and knowledge. Consider having a professional diagnose and repair transfer case issues to ensure proper function and prevent potential drivetrain damage.
Low or Dirty Transmission Fluid
Insufficient or contaminated transmission fluid can cause various transmission problems, including solenoid malfunctions that may trigger the P1851 code.
Symptoms
- Rough or delayed shifting
- Transmission slipping
- Overheating transmission
- Unusual noises from the transmission
Technical Explanation
Transmission fluid serves multiple purposes, including lubricating moving parts, cooling the transmission, and providing hydraulic pressure for gear changes. When the fluid is low or dirty, it can’t properly actuate solenoids or provide the necessary pressure for smooth operation, leading to performance issues and potential damage.
Step-by-Step Diagnosis
- Check the transmission fluid level with the engine running and warmed up.
- Inspect the color and smell of the transmission fluid. It should be red or pink and not have a burnt odor.
- Look for signs of fluid leaks under the vehicle.
- If equipped, check the transmission fluid temperature using an OBD-II scanner.
- Perform a transmission fluid pressure test to ensure proper hydraulic function.
Solution Methods
- Transmission Fluid Top-Up: If the fluid is low but in good condition, simply add the correct type of fluid to the proper level.
- Transmission Fluid and Filter Change: Replace the transmission fluid and filter if the fluid is dirty or old.
- Transmission Flush: For severely contaminated systems, a complete flush may be necessary.
- Leak Repair: Identify and fix any transmission fluid leaks to prevent future issues.
Cost Estimates
- Transmission Fluid Top-Up: $50 – $100
- Transmission Fluid and Filter Change: $150 – $300
- Transmission Flush: $150 – $400
- Leak Repair: $200 – $500 (varies widely depending on the source of the leak)
Warning: Always use the transmission fluid specified by your vehicle manufacturer. Using the wrong type of fluid can cause severe transmission damage.
Faulty Transmission Control Module (TCM)
A malfunctioning Transmission Control Module (TCM) can lead to various transmission issues and may trigger the P1851 code.
Symptoms
- Erratic shifting patterns
- Transmission stuck in a single gear
- Check engine light or transmission warning light
- Complete transmission failure
Technical Explanation
The TCM is the brain of the transmission system, controlling shift timing, pressure, and other critical functions. When it fails, it can misinterpret sensor data or fail to properly control transmission components, leading to performance issues and diagnostic trouble codes.
Step-by-Step Diagnosis
- Scan for additional transmission-related codes that might indicate a TCM issue.
- Check the TCM for signs of physical damage or water intrusion.
- Inspect the TCM’s power and ground connections for corrosion or loose terminals.
- Use a scan tool to monitor TCM data and look for inconsistencies or out-of-range values.
- Perform a TCM reset and relearn procedure if supported by the vehicle.
- In some cases, bench testing the TCM may be necessary to confirm a failure.
Solution Methods
- TCM Reset: Sometimes, resetting the TCM and clearing codes can resolve minor issues.
- Software Update: Check for available TCM software updates from the manufacturer.
- Wiring Repair: Fix any damaged wiring or connectors associated with the TCM.
- TCM Replacement: If the module is confirmed faulty, replace it with a new or remanufactured unit.
Cost Estimates
- TCM Reset: $50 – $100
- Software Update: $100 – $200
- Wiring Repair: $100 – $300
- TCM Replacement: $500 – $1,500 (plus programming costs)
Recommendation: TCM replacement often requires specialized programming equipment. It’s typically best to have this procedure performed by a dealership or specialized transmission shop to ensure proper calibration.
Clogged Transmission Fluid Passages
Debris or sludge buildup in transmission fluid passages can restrict fluid flow, affecting solenoid operation and potentially triggering the P1851 code.
Symptoms
- Delayed or harsh shifts
- Transmission overheating
- Erratic transmission behavior
- Loss of certain gears
Technical Explanation
Transmission fluid passages are designed to deliver pressurized fluid to various components, including solenoids and clutch packs. When these passages become clogged, it can lead to inadequate pressure or fluid flow, causing components like the brake band apply solenoid to malfunction.
Step-by-Step Diagnosis
- Check the transmission fluid condition and level.
- Perform a transmission pressure test to identify any low-pressure areas.
- Inspect the transmission pan for metal debris when draining the fluid.
- If possible, remove and inspect the valve body for signs of debris or damage.
- Use a borescope to inspect internal transmission passages if accessible.
Solution Methods
- Transmission Fluid and Filter Change: Replace the fluid and filter to remove contaminants.
- Transmission Flush: Perform a complete flush to clear debris from passages.
- Valve Body Cleaning: Remove and clean the valve body if necessary.
- Ultrasonic Cleaning: In severe cases, ultrasonic cleaning of components may be required.
Cost Estimates
- Transmission Fluid and Filter Change: $150 – $300
- Transmission Flush: $150 – $400
- Valve Body Cleaning: $300 – $800
- Ultrasonic Cleaning: $200 – $500 (additional to other services)
Warning: If significant metal debris is found in the transmission fluid or pan, it may indicate internal transmission damage requiring a rebuild or replacement.
Addressing the P1851 code promptly is crucial to prevent further transmission damage and ensure your vehicle’s proper operation. While some diagnostic steps can be performed by experienced DIY mechanics, many transmission issues require specialized tools and expertise. If you’re unsure about any aspect of the diagnosis or repair process, it’s best to consult with a certified transmission specialist or your vehicle’s dealership service department.
Regular maintenance, including timely transmission fluid changes and addressing any unusual symptoms promptly, can help prevent issues that lead to the P1851 code and other transmission problems. Always refer to your vehicle’s owner’s manual for specific maintenance schedules and recommended fluids to keep your transmission in optimal condition.
Frequently Asked Questions About P1851
- Can I drive my vehicle with the P1851 code active?
While the vehicle may still operate, it’s not recommended to drive with an active P1851 code. Continued operation could lead to further transmission damage and potentially unsafe driving conditions. - How urgent is it to address the P1851 code?
The P1851 code should be addressed as soon as possible. Transmission-related codes can quickly lead to more severe and costly issues if left unresolved. - Will the P1851 code clear itself?
It’s unlikely that the P1851 code will clear itself. The underlying issue needs to be resolved, and the code must be cleared using an OBD-II scanner. - Is the P1851 code covered under warranty?
Coverage depends on your vehicle’s warranty terms and the specific cause of the code. Check your warranty documentation or consult with your dealership for information. - Can a transmission flush fix the P1851 code?
A transmission flush may resolve the P1851 code if it’s caused by dirty fluid or clogged passages. However, if the issue is related to electrical or mechanical failures, a flush alone won’t fix the problem. - How much does it typically cost to repair a P1851 code issue?
Repair costs can vary widely, ranging from $100 for a simple fluid service to over $2,000 for major transmission repairs. The exact cost depends on the root cause and the vehicle make and model. - Can I replace the brake band apply solenoid myself?
While it’s possible for experienced DIYers to replace the solenoid, it’s generally recommended to have a professional perform this task due to the complexity of transmission work and the risk of causing additional damage. - How can I prevent the P1851 code from occurring in the future?
Regular transmission maintenance, including fluid changes at recommended intervals, addressing any transmission-related symptoms promptly, and avoiding harsh driving conditions can help prevent issues that lead to the P1851 code.