The P1851 trouble code is a manufacturer-specific diagnostic code related to transmission issues. It typically indicates a problem with the brake band apply solenoid performance in the transmission system. This code is commonly found in various vehicle makes, including Ford, Toyota, and some General Motors models. When this code appears, it suggests that the vehicle’s powertrain control module (PCM) has detected an issue with the brake band apply solenoid circuit or its performance.
| P1851 Code Meaning | P1851 Code Common Causes |
|---|---|
| Brake Band Apply Solenoid Performance | Faulty brake band apply solenoid |
| Transmission Control System Malfunction | Dirty or low transmission fluid |
| Power Steering Pressure Switch Signal Issue | Wiring or connector problems |
| Transmission Transfer Case Contact Plate B Open Circuit | Damaged valve body |
| Left Front Wheel Speed Sensor Circuit Current Out of Range | Failed PCM or TCM |
Brake Band Apply Solenoid Malfunction
The brake band apply solenoid is a crucial component in automatic transmissions that use bands to engage certain gears. When this solenoid malfunctions, it can lead to various transmission-related issues.
Symptoms:
- Delayed or harsh gear shifts
- Slipping transmission
- Erratic transmission behavior
- 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 engage specific gears. When the solenoid fails to operate correctly, it can’t properly control the band’s application, leading to shifting problems.
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 wiring harness and connectors for any signs of damage or corrosion.
- Use a multimeter to test the resistance of the brake band apply solenoid. Compare the readings to the manufacturer’s specifications.
- If possible, use a scan tool to command the solenoid on and off while monitoring 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 faulty, replace it with a new one.
- Wiring Repair: If wiring issues are found, repair or replace the damaged sections.
- Valve Body Replacement: In some cases, a damaged valve body may require replacement.
Cost Estimates:
- Transmission Fluid Service: $100 – $250
- Solenoid Replacement: $200 – $500
- Valve Body Replacement: $400 – $1000
Warning: Transmission work can be complex. If you’re not experienced with automotive repairs, it’s best to consult a professional mechanic.
Low or Contaminated Transmission Fluid
Transmission fluid plays a crucial role in the proper functioning of the transmission system, including the brake band apply solenoid. Low or contaminated fluid can cause various issues, including the P1851 code.
Symptoms:
- Rough or delayed shifting
- Transmission slipping
- Overheating transmission
- Unusual noises from the transmission
Technical Explanation:
Transmission fluid lubricates moving parts, acts as a hydraulic fluid for shifting gears, and helps cool the transmission. When the fluid is low or contaminated, it can’t perform these functions effectively, leading to solenoid and overall transmission problems.
Step-by-step Diagnosis:
- Check the transmission fluid level with the engine running and warmed up.
- Inspect the color and smell of the fluid. Clean fluid should be red or pink and odorless.
- Look for signs of contamination, such as metal particles or a burnt smell.
- If the fluid level is low, check for leaks around the transmission.
Solution Methods:
- Top up the fluid if it’s low, using the correct type specified by the vehicle manufacturer.
- If the fluid is contaminated, perform a complete transmission fluid flush and filter change.
- Repair any leaks found in the transmission system.
Cost Estimates:
- Transmission Fluid Top-up: $50 – $100
- Transmission Fluid Flush and Filter Change: $150 – $300
- Leak Repair: $150 – $500 (depending on the location and severity of the leak)
Recommendation: Regular transmission fluid checks and changes as per the manufacturer’s schedule can prevent many transmission-related issues.
Wiring and Connector Issues
Electrical problems in the transmission system can trigger the P1851 code. Damaged wiring or faulty connectors can disrupt the communication between the PCM and the brake band apply solenoid.
Symptoms:
- Intermittent transmission problems
- Check Engine Light flickering or staying on
- Erratic solenoid operation
Technical Explanation:
The brake band apply solenoid requires a proper electrical connection to function correctly. Damaged wiring, loose connections, or corroded terminals can cause resistance changes or signal interruptions, leading to solenoid malfunction.
Step-by-step Diagnosis:
- Visually inspect the wiring harness and connectors related to the transmission.
- Check for signs of wear, chafing, or exposed wires.
- Disconnect and reconnect all relevant connectors, checking for corrosion or bent pins.
- Use a multimeter to test for continuity in the wiring between the PCM and the solenoid.
- Perform a voltage drop test across suspicious connections.
Solution Methods:
- Clean corroded terminals using electrical contact cleaner.
- Repair or replace damaged wiring sections.
- Replace faulty connectors.
- Apply dielectric grease to connections to prevent future corrosion.
Cost Estimates:
- DIY Wiring Repair: $20 – $100 for materials
- Professional Wiring Repair: $100 – $500 (depending on the extent of the damage)
Warning: Incorrect wiring repairs can lead to more severe electrical issues. If you’re unsure, consult a professional automotive electrician.
Valve Body Problems
The valve body is a critical component in the transmission that houses various solenoids and directs the flow of transmission fluid. Issues with the valve body can affect the brake band apply solenoid’s performance and trigger the P1851 code.
Symptoms:
- Harsh or delayed shifts
- Transmission slipping
- Incorrect gear engagement
- Transmission fluid leaks
Technical Explanation:
The valve body contains a complex network of passages and valves that control fluid flow to different parts of the transmission, including the brake band apply solenoid. Wear, debris accumulation, or manufacturing defects can disrupt this flow, leading to solenoid performance issues.
Step-by-step Diagnosis:
- Perform a transmission pressure test to check for proper fluid pressure.
- Use a scan tool to monitor transmission data and solenoid operation.
- If possible, remove the transmission pan to inspect for metal debris, which could indicate internal damage.
- Check the valve body for visible damage or wear.
Solution Methods:
- Clean the valve body if debris is the issue.
- Rebuild the valve body if wear is evident.
- Replace the entire valve body if damage is severe or a rebuild isn’t possible.
Cost Estimates:
- Valve Body Cleaning: $200 – $400
- Valve Body Rebuild: $400 – $800
- Valve Body Replacement: $500 – $1000
Recommendation: Valve body work often requires transmission removal. Unless you’re an experienced mechanic, this job is best left to professionals.
PCM or TCM Failure
In rare cases, the P1851 code may be caused by a failure in the Powertrain Control Module (PCM) or Transmission Control Module (TCM).
Symptoms:
- Multiple transmission-related codes
- Erratic transmission behavior
- Other electrical system issues
Technical Explanation:
The PCM or TCM controls various aspects of the transmission’s operation, including the brake band apply solenoid. If these modules fail or have software issues, they may not properly control the solenoid, leading to the P1851 code.
Step-by-step Diagnosis:
- Scan for additional trouble codes that might indicate a broader PCM/TCM issue.
- Check for any available software updates for the PCM/TCM.
- Perform a PCM/TCM reset and see if the issue persists.
- Use advanced diagnostic equipment to test PCM/TCM functionality.
Solution Methods:
- Update PCM/TCM software if updates are available.
- Reflash or reprogram the PCM/TCM.
- Replace the PCM/TCM if determined to be faulty.
Cost Estimates:
- PCM/TCM Software Update: $100 – $200
- PCM/TCM Replacement: $500 – $2000 (varies widely depending on the vehicle make and model)
Warning: PCM/TCM replacement often requires specialized programming. This job should be performed by a qualified technician with the proper diagnostic equipment.
In conclusion, the P1851 code primarily indicates issues with the brake band apply solenoid performance in the transmission system. While it can be caused by simple problems like low transmission fluid, it can also point to more complex issues involving the valve body or control modules. Always start with the simplest and most common causes, gradually working towards more complex solutions. If you’re unsure about any step in the diagnostic or repair process, it’s best to consult a professional mechanic to avoid potentially costly mistakes.
Frequently Asked Questions About P1851
- Can I drive my car with the P1851 code?
While you may be able to drive short distances, it’s not recommended. The code indicates transmission issues that could worsen and lead to more expensive repairs if ignored. - How urgent is it to address the P1851 code?
It’s relatively urgent. Transmission problems can escalate quickly, potentially leading to complete transmission failure. Address the issue as soon as possible to avoid more extensive damage. - Will the P1851 code clear itself?
In most cases, no. The code will typically remain until the underlying issue is resolved and the code is cleared using an OBD-II scanner. - How much does it typically cost to fix a P1851 code?
Costs can vary widely, from about $100 for a simple fluid change to over $2000 for major transmission work. The exact cost depends on the root cause and the vehicle make and model. - Can a P1851 code be caused by a recent battery disconnection?
It’s unlikely. While some codes can be triggered by battery disconnection, P1851 is typically related to actual transmission issues rather than electrical resets. - Is the P1851 code covered under warranty?
It depends on your vehicle’s warranty terms and the cause of the code. Transmission issues are often covered under powertrain warranties, but always check your specific warranty details. - Can I clear the P1851 code without fixing the problem?
While you can clear the code, it will likely return if the underlying issue isn’t resolved. It’s best to diagnose and fix the problem rather than just clearing the code. - How long does it take to fix a P1851 code?
Repair time can range from an hour for simple fixes like fluid changes to several days for complex transmission work. The exact time depends on the cause and the required repairs.