P0845

Transmission Fluid Pressure Sensor/Switch B Circuit

P0845 is a generic OBD-II powertrain diagnostic trouble code: Transmission Fluid Pressure Sensor/Switch B Circuit. It is logged by the engine control unit when the trans monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0845
Group
Powertrain
System
Trans
Severity
Warning (MIL on, possible limp mode)
Need P0845 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P0845 means

P0845 is stored when the PCM or TCM detects a general fault condition on the Transmission Fluid Pressure Sensor/Switch "B" circuit — the signal voltage is outside the expected operating range without a clear high or low direction. The sensor converts hydraulic line pressure in the transmission into a voltage signal that the control module uses to verify clutch engagement and time gear changes correctly.

The code covers both sensor-side and wiring-side electrical faults: an intermittent open, a damaged connector, internal sensor failure, or even contaminated fluid affecting sensor response. It is distinct from P0846 (rationality failure), P0847 (circuit low/stuck low), and P0848 (circuit high/stuck high) — P0845 acts as the parent circuit code when no more specific fault direction is determined.

Left unresolved, the control module may default to a fixed pressure assumption that causes harsh, delayed, or missed shifts. In severe cases the transmission enters fail-safe (limp) mode, limiting gear selection to protect the unit from overload. Address the code promptly to prevent secondary hydraulic damage.

Common causes

Most-frequently reported root causes when P0845 is logged.

  • 1
    Defective transmission fluid pressure sensor/switch B (internal open or signal failure).
  • 2
    Damaged, corroded, or backed-out wiring harness connector at the sensor.
  • 3
    Chafed or broken wire in the sensor signal, reference voltage, or ground circuit.
  • 4
    Low or severely contaminated transmission fluid affecting hydraulic pressure and sensor response.
  • 5
    Faulty electronic pressure control solenoid causing abnormal line pressure readings.
  • 6
    Cracked or missing check balls in the valve body producing erratic line pressure.
  • 7
    Stuck or sticking pressure regulator valve in the valve body.
  • 8
    PCM/TCM internal fault or software error misreading the sensor circuit.

Symptoms drivers notice

Check engine or transmission warning light is illuminated.
Harsh, delayed, or erratic gear shifts under normal driving conditions.
Transmission slipping between gears or failing to hold the engaged gear.
Vehicle entering limp (fail-safe) mode with limited gear availability.
Elevated transmission fluid temperature due to improper clutch engagement.
Reduced fuel economy from the transmission operating outside its optimal pressure range.

How to diagnose P0845

A typical diagnostic flow when this code is present.

  1. 1
    Check transmission fluid level and condition — low or dirty fluid is a common contributing factor; correct before further diagnosis.
  2. 2
    Connect a scan tool, record freeze-frame data, and note any additional transmission codes stored alongside P0845.
  3. 3
    Inspect the wiring harness and connector at the transmission fluid pressure sensor B for corrosion, spread terminals, or physical damage.
  4. 4
    With the connector unplugged, measure reference voltage (typically 5 V) and ground integrity at the harness side using a DMM.
  5. 5
    Measure sensor signal voltage with the connector reinstalled and the ignition on — compare against manufacturer specification for idle/operating conditions.
  6. 6
    If wiring and supply voltage are within spec, substitute or bench-test the transmission fluid pressure sensor B.
  7. 7
    If sensor and wiring pass, inspect valve body for stuck valves or missing check balls before suspecting PCM/TCM.

Related powertrain codes

Frequently asked questions

Can low transmission fluid alone trigger P0845?

Yes. Insufficient fluid reduces hydraulic pressure below the sensor's normal operating range, which can push the signal voltage out of specification and set the code. Always check fluid level and condition as the first step.

Is P0845 the same as P0840 (Sensor A)?

No. P0840–P0843 relate to Transmission Fluid Pressure Sensor/Switch "A" circuit, while P0845–P0848 cover the "B" circuit. On many vehicles, each circuit monitors a different hydraulic control circuit within the transmission.

Will my car go into limp mode with P0845?

It depends on severity and vehicle make. Many TCMs will restrict gear selection to a fixed gear (commonly 3rd) to protect the transmission when pressure data is unreliable. Intermittent faults may not trigger limp mode immediately.

Can I drive with P0845 active?

Short distances at low speed may be possible, but continued driving risks transmission overheating and accelerated clutch wear due to incorrect line pressure. The fault should be diagnosed and repaired without unnecessary delay.

Disabling P0845 in software

RaceTune can permanently disable P0845 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

Got P0845 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file