OBD-II trouble code
P0845: Transmission Fluid Pressure Sensor/Switch "B" Circuit
The existence of a second pressure tap tells you something about your gearbox before you test anything — and whether the fault names only 'B' or names 'A' as well is the fastest way to split a shared-circuit problem from a sensor problem.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $750
- DIY difficulty
- Intermediate DIY
What does P0845 mean?
P0845 is the general circuit fault for the second transmission fluid pressure sensor or switch — the one the standard labels "B". Before any of the electrical detail matters, the fact that your vehicle can set this code at all is itself information: it means the transmission was designed with more than one pressure tap. A great many transmissions have exactly one, monitoring main line pressure, and on those vehicles there is no "B" and this code cannot exist. Where a second one is fitted, it is generally there because the designers wanted visibility of a specific hydraulic circuit rather than only the overall supply — a particular clutch or band apply circuit, a converter clutch circuit, or a secondary pressure in a continuously variable or dual-clutch design.
That distinction shapes what the code means. Where the "A" sensor usually tells the module whether the transmission has pressure at all, the "B" sensor usually tells it whether pressure arrived at one particular place. Losing "A" undermines the module's confidence in the whole hydraulic system; losing "B" undermines its confidence in one part of it. Vehicles therefore often behave less dramatically with a "B" fault, which is a reason it goes unattended, not a reason it does not matter.
The most useful diagnostic consequence of having two sensors is a cross-check that a single-sensor transmission cannot offer. Both sensors are normally fed from the same module, share a common reference or ground, and reach the transmission through the same case connector. So the pattern of codes tells you where to start before a probe touches anything. A fault naming only "B" points at the branch that is unique to "B" — its own signal wire, its own pin in the connector, the sensor itself. A fault naming both "A" and "B" together points instead at what they have in common: the shared ground, the shared reference feed, the case connector, or the module. Two sensors failing independently at the same moment is possible but unlikely, and treating a paired code set as two separate faults leads to two replaced sensors and an unchanged problem.
The environment is the same for both, and it is a hostile one. The connector where the harness enters the transmission case sits in heat, vibration and fluid, and a failing pass-through seal lets fluid into a connector that was never intended to hold any. Corroded and spread terminals, fluid-wicked wiring and chafe against the case account for far more of these codes than failed sensors do — which is worth knowing before ordering a part that, on many designs, lives inside the pan on the valve body and cannot be reached without dropping fluid.
One caution specific to "B": because the module frequently compares the two sensors against each other, a drifted or degraded "A" can be what causes a complaint about "B". Confirming that "A" is reading correctly is part of diagnosing "B", not a separate job.
Common causes
- Corroded, spread or backed-out terminal on the "B" signal pin in the transmission case connector
- Chafed or broken wiring on the branch that is unique to the second sensor
- Failed transmission fluid pressure sensor or switch "B"
- Fluid intrusion into the case connector through a failed pass-through seal, wicking along the wires
- Degraded shared ground or reference feed, which typically brings an "A" code along with it
- Short to ground or to voltage in the sensor circuit
- Contamination or debris around an in-pan sensor mounted on the valve body
- Low fluid level or badly degraded fluid producing genuinely abnormal pressure in the monitored circuit
- Connector left unlatched or a pin displaced during previous transmission work
- Transmission control module input failure, which is uncommon and should be proved rather than assumed
Symptoms
- Check engine light with P0845 stored and often little else to notice
- Shift quality changes affecting one particular gear change rather than all of them
- Torque converter lock-up behaving oddly, engaging late, early or not at all
- Transmission entering a fail-safe or limp mode on some vehicles
- Slightly harsh or delayed engagement when selecting drive or reverse
- Reduced fuel economy where lock-up is being suppressed
- Companion transmission codes appearing at the same time, which is itself diagnostic information
- No perceptible drivability complaint at all on many vehicles
Diagnostic steps
- 1.Record every stored code before clearing anything, and note specifically whether "A" codes are present alongside this one. That pattern is the fastest split available between a shared-circuit fault and a fault in the "B" branch.
- 2.Check fluid level and condition first. A genuinely abnormal pressure reported honestly by a healthy sensor is a different problem from an electrical fault and is cheaper to find early.
- 3.Establish from service information what the "B" sensor actually monitors on this specific transmission — a clutch apply circuit, converter clutch pressure or a secondary circuit — since that determines when the module can evaluate it and what a wrong reading implies.
- 4.Compare the "A" and "B" live values against each other with the engine idling and again under load. Two readings that move together sensibly point away from the sensors and toward the common circuit.
- 5.Verify the "A" sensor is reading correctly before condemning "B". Where the module compares the two, a drifted "A" produces a complaint about "B".
- 6.Inspect the transmission case connector for fluid intrusion, corrosion and spread terminals, and treat fluid inside the connector as a finding that needs the seal addressed rather than just the pins cleaned.
- 7.Back-probe the "B" signal with the connector mated and measure voltage drop under the module's own reference current, rather than checking resistance on a disconnected circuit.
- 8.Check the shared ground and reference feed under load, since a marginal ground can skew one channel more than the other and mimic a single failed sensor.
- 9.Confirm actual hydraulic pressure with a mechanical gauge if the electrical side tests clean, so that a real pressure problem is not repeatedly blamed on the sensor reporting it.
Repair cost
$100 – $750
Diagnosis is typically $90 to $200. Repairing a connector terminal or a chafed wire is $120 to $400 and is the most common genuine fix on these circuits. A transmission fluid pressure sensor is $45 to $320 in parts with 0.5 to 3.5 hours of labour, so roughly $150 to $750 fitted — and the wide spread is almost entirely about location, because a sensor mounted externally on the case is a short job while one mounted inside the pan on the valve body means dropping the fluid, a new pan gasket and usually a filter at the same time. A fluid and filter service is $150 to $350 and is occasionally the whole repair. If diagnosis shows genuinely wrong pressure rather than a wrong signal, costs follow the solenoid or valve-body repair required and are considerably higher.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission fluid pressure sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.