P0845 – Transmission Fluid Pressure Sensor/Switch “B” Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Designator B

Definition source: SAE J2012/J2012DA (industry standard)

P0845 is a powertrain diagnostic trouble code indicating a fault in the Transmission Fluid Pressure Sensor/Switch “B” circuit. In practical terms, the control module has detected an electrical circuit problem related to the “B” pressure sensor/switch signal path rather than confirming a mechanical transmission failure. The exact sensor type (analog pressure transducer vs. on/off pressure switch), its location, and how the module runs the monitor can vary by vehicle, so use the correct service information and wiring diagrams for your application. Your goal in diagnosis is to verify the integrity of the circuit (power, ground, signal, and connectors) and confirm whether the sensor/switch itself responds correctly under operating conditions.

What Does P0845 Mean?

P0845 – Transmission Fluid Pressure Sensor/Switch “B” Circuit means the vehicle’s control module has detected a circuit fault associated with the transmission fluid pressure sensor/switch identified as “B.” Per SAE J2012 DTC structure conventions, this code points to an electrical problem in the sensor/switch circuit (such as wiring, connections, power/ground, or the signal path), not a guaranteed hydraulic pressure issue. Because “B” is an identifier used to distinguish multiple pressure inputs, you must reference service information to determine which specific pressure sensor/switch is designated “B,” what circuits serve it, and what conditions must be present for the monitor to set the code.

Quick Reference

• Subsystem: Transmission fluid pressure sensor/switch “B” electrical circuit (signal, power feed, and ground as applicable).
• Common triggers: Open/short in the sensor/switch wiring, poor connector contact, loss of sensor power/ground, or an out-of-range/invalid circuit signal detected by the module.
• Likely root-cause buckets: Harness/connector faults, sensor/switch internal electrical failure, shared power/ground distribution issues, or (less often) module terminal/pin issues.
• Severity: Varies by vehicle; may cause harsh/abnormal shifting, fail-safe operation, reduced performance, or warning indicators; treat as potentially drivability-impacting.
• First checks: Scan for related transmission electrical codes, inspect connector/harness for damage/fluid intrusion, verify power/ground integrity, and review live data for the “B” input behavior.
• Common mistakes: Replacing transmission components or fluid based on the code alone, skipping circuit testing, or ignoring shared power/ground problems affecting multiple sensors.

Theory of Operation

The transmission fluid pressure sensor/switch “B” provides an electrical indication of hydraulic pressure or pressure state to the control module. Depending on design, it may be a pressure transducer producing a varying signal or a pressure switch reporting an on/off state. The module uses this input for control decisions and diagnostics, often comparing the input against expected behavior during commanded shifts, line pressure changes, or specific operating states.

P0845 sets when the module determines the “B” circuit is not electrically valid. That determination is typically based on circuit rationality checks (signal present and stable), detection of opens/shorts, or a response that cannot be achieved because the signal is missing or not interpretable. Because the monitor logic varies by vehicle, confirm which circuit terminals, shared references, and enabling conditions apply before testing.

Symptoms

• Warning light: Check engine light or transmission-related warning indicator illuminated.
• Shift quality: Harsh shifts, delayed engagement, or abnormal shift timing.
• Fail-safe mode: Transmission may default to a limited gear range or protective operating strategy.
• Performance: Reduced acceleration response due to limited shifting or torque management.
• Driveability: Intermittent symptoms that change with bumps, vibration, heat, or moisture affecting connectors.
• Scan data: “B” pressure input appears fixed, missing, erratic, or not changing when operating conditions change.

Common Causes

• Open circuit, short to ground, or short to power in the Transmission Fluid Pressure Sensor/Switch “B” signal circuit
• High resistance in the sensor signal circuit (damaged conductor, corrosion, poor splice, partial break)
• Connector issues at the sensor or control module (backed-out pin, poor terminal tension, moisture intrusion, fretting)
• Power supply or ground fault for the sensor/switch circuit (missing feed, shared ground offset, ground path resistance)
• Transmission Fluid Pressure Sensor/Switch “B” internal electrical fault (skewed output or failed switch element)
• Harness damage near heat/vibration points (chafing, pinched loom, contact with brackets or transmission case)
• Control module input circuit concern (rare; only after power/ground and wiring integrity are proven)
• Incorrect installation or disturbed connectors after recent service (connector not fully seated, terminals spread during probing)

Diagnosis Steps

Tools that help: a scan tool capable of reading transmission-related data and viewing freeze-frame, a digital multimeter, and back-probing or terminal-test adapters suited to the connector style. A wiring diagram and connector pinout are essential because circuit routing varies by vehicle. If available, use a breakout lead to avoid damaging terminals, and be prepared to perform voltage-drop tests under load.

1. Confirm the DTC. Record all stored and pending codes and capture freeze-frame data. If other transmission electrical codes are present, address power supply or network-related issues first because they can influence circuit monitoring.
2. Check for symptoms and fail-safe behavior. With the scan tool, note whether the transmission is in a default strategy, and document any abnormal shift behavior. Do not assume a mechanical pressure problem; keep the focus on the “B” circuit until testing proves otherwise.
3. Clear codes and perform a short key-on/engine-on check. See if P0845 resets immediately (suggesting a hard electrical fault) or only after driving/commanded shifts (suggesting intermittent wiring/connector sensitivity or a monitor that runs under specific conditions).
4. Visually inspect the sensor/switch “B” circuit. Inspect the harness routing at the transmission case, brackets, and heat sources. Look for chafing, pinches, melted insulation, and areas where the loom could rub through and contact ground or power.
5. Inspect connectors and terminals. Unplug the sensor/switch connector and the related control module connector (as applicable). Check for corrosion, moisture, damaged seals, bent pins, or terminals that appear spread. Ensure connectors fully latch and that terminal tension is adequate.
6. Perform a wiggle test with live data logging. Reconnect components, then monitor the pressure sensor/switch “B” parameter and DTC status while gently moving the harness and connectors. Log the data so you can correlate any signal dropouts/spikes with harness movement.
7. Verify sensor circuit power and ground integrity. Using the wiring diagram, identify the sensor feed and ground circuits. With the circuit powered, check for an appropriate supply presence and a stable ground. Perform voltage-drop testing on the ground path (and feed path if applicable) while the circuit is active to reveal hidden resistance that a static ohms test may miss.
8. Check the signal circuit for opens/shorts. With the ignition off and the connectors unplugged, test continuity end-to-end for the signal circuit between the sensor and the module. Then test for unwanted continuity from the signal wire to ground and to power/other circuits. Flex the harness during testing to catch intermittent opens.
9. Validate sensor/switch response (electrical). If the wiring and connector checks pass, evaluate whether the sensor/switch output changes logically as operating conditions change (varies by vehicle). Use scan data trends and service information test procedures. If a bench or in-vehicle functional test is specified by service information, follow it rather than guessing expected values.
10. Check module-side input plausibility (only after wiring is proven). If the circuit integrity, feeds/grounds, and sensor behavior are verified yet P0845 persists, verify the module connector pins for fitment and contact. If available, compare the module’s interpreted data to direct electrical measurements at the signal wire to see if the module input is inconsistent.
11. Confirm the repair. After corrective action, clear codes and complete the drive cycle/conditions required for the monitor to run (varies by vehicle). Recheck for pending codes and review live data to ensure the signal remains stable under vibration, temperature, and load changes.

Professional tip: When chasing an intermittent “circuit” fault, prioritize finding what changes with movement and temperature. A short live-data log during a controlled wiggle test, combined with voltage-drop testing of the sensor ground under load, often reveals high-resistance terminals or broken conductors that look normal in a quick visual inspection.

Possible Fixes & Repair Costs

Repair costs vary widely because P0845 can be caused by anything from a connector issue to a sensor or control-module circuit concern. The final cost depends on confirmed test results, parts access, fluid handling requirements, and labor time for circuit diagnosis and verification.

• Repair wiring damage: Locate and repair opens/shorts in the Transmission Fluid Pressure Sensor/Switch “B” signal, power, or ground circuits; restore proper routing and protection.
• Clean and secure connectors: Remove corrosion/contamination, correct poor terminal tension, and ensure connectors are fully seated and locked at the sensor and control module.
• Verify and restore power/ground integrity: Repair shared splices, grounds, or power feeds; correct excessive resistance found during voltage-drop testing.
• Replace the pressure sensor/switch (if proven): Replace only after confirming the circuit is healthy and the sensor output remains implausible or nonresponsive.
• Address fluid intrusion: If fluid has migrated into a connector or harness, clean/replace affected terminals and repair the harness as needed to prevent repeat faults.
• Control module circuit repair or replacement (rare): Consider only after all external circuit and sensor checks pass and the fault can be reproduced at the module pins.

Can I Still Drive With P0845?

Driving with P0845 may be possible, but it is not recommended if you notice harsh shifting, slipping, delayed engagement, or the transmission going into a protective mode. If the vehicle shows reduced power, warning lights that affect braking/steering systems, abnormal noises, or loss of propulsion, do not drive—stop safely and arrange service. Even when the vehicle seems to drive normally, continued operation can worsen shift quality and increase the chance of additional transmission-related faults, so verify the circuit and sensor operation promptly using service information.

What Happens If You Ignore P0845?

Ignoring P0845 can lead to recurring warning lights, repeated protective strategies (such as limited shifting), and progressive drivability issues. A persistent circuit fault can cause incorrect pressure-related inputs to the control strategy, increasing the risk of harsh or delayed shifts and unnecessary heat buildup. Over time, continued operation with poor control inputs may contribute to accelerated wear and may complicate diagnosis by adding secondary codes related to shift performance or pressure control.

Related Pressure Transmission Codes

Compare nearby pressure transmission trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P0875 – Transmission Fluid Pressure Sensor/Switch “D” Circuit
• P0870 – Transmission Fluid Pressure Sensor/Switch “C” Circuit
• P0840 – Transmission Fluid Pressure Sensor/Switch “A” Circuit
• P0878 – Transmission Fluid Pressure Sensor/Switch “D” Circuit High
• P0877 – Transmission Fluid Pressure Sensor/Switch “D” Circuit Low
• P0873 – Transmission Fluid Pressure Sensor/Switch “C” Circuit High

Key Takeaways

• P0845 is a circuit DTC: It indicates an electrical issue in the Transmission Fluid Pressure Sensor/Switch “B” circuit, not a confirmed mechanical failure.
• Test the circuit before replacing parts: Wiring, connector, power, and ground faults are common root causes for circuit DTCs.
• Use pin-level checks: Verify integrity from sensor to module, including wiggle testing and voltage-drop testing under load.
• Driveability can degrade: The transmission may shift harshly or enter a protective mode depending on vehicle strategy.
• Confirm the fix: Clear codes and validate with live data and a road test to ensure the monitor passes and the fault does not return.

Vehicles Commonly Affected by P0845

• Vehicles with electronically controlled automatic transmissions
• Vehicles using multiple transmission pressure sensors/switches
• Applications with external transmission harnesses exposed to heat and road splash
• High-mileage vehicles with harness fatigue near the transmission case
• Vehicles with recent transmission service where connectors may be disturbed
• Vehicles operating in high-heat or stop-and-go conditions
• Vehicles with underbody impact history affecting wiring routing
• Vehicles with prior electrical repairs near powertrain grounds or splices

FAQ

Does P0845 mean my transmission is bad?

No. P0845 indicates the control module detected a fault in the Transmission Fluid Pressure Sensor/Switch “B” circuit. It does not, by itself, confirm internal transmission damage; the circuit, connectors, and sensor must be tested to determine the root cause.

Can low or dirty transmission fluid cause P0845?

P0845 is categorized as a circuit fault, so the primary focus is electrical: wiring, connector integrity, power/ground, and sensor circuit behavior. Fluid condition can contribute indirectly (for example, contamination leading to connector issues during service), but it should not be assumed as the cause without electrical testing.

What should I check first for P0845?

Start with a visual inspection of the sensor and transmission harness, looking for damage, chafing, fluid intrusion, corrosion, and loose connectors. Then verify power and ground integrity and check the signal circuit continuity and shorts using the correct wiring diagram and service procedures.

Will replacing the pressure sensor/switch fix P0845?

Sometimes, but only if testing proves the sensor/switch is faulty and the wiring and connectors are known-good. Replacing the sensor without verifying circuit integrity can result in the code returning if the real issue is an open, short, poor terminal fit, or a shared power/ground problem.

How do I confirm the repair is successful?

After the verified repair, clear the code, then perform a drive cycle while monitoring relevant live data for the pressure sensor/switch “B” circuit behavior. Confirm the DTC does not reset and that shifting behavior is normal under the conditions that previously triggered the fault.

For best results, confirm the exact connector pinout, circuit routing, and test method in the correct service information, since the “B” circuit definition and monitoring strategy can vary by vehicle.