OBD-II trouble code
P0848: Transmission Fluid Pressure Sensor/Switch "B" Circuit High
Before anything else, find out whether your "B" tap is an analogue sensor or a simple pressure switch. On a switch, "high" means the contacts never closed — which turns this into an entirely different fault from the one the name suggests.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $130 – $1,800
- DIY difficulty
- Advanced DIY
What does P0848 mean?
P0848 sets when the second transmission fluid pressure channel reports a voltage or state above anything the module accepts as valid. The standard writes this family as "Sensor/Switch" for a reason that matters more here than on any other member of the group: the "B" tap is not always the same kind of device. On some transmissions it is a genuine analogue pressure transducer producing a voltage proportional to pressure. On a great many others — particularly older domestic units and several GM six-speeds — it is a pressure switch, a simple set of contacts that close when pressure in one circuit exceeds a threshold and open when it does not.
That distinction reverses the meaning of the word "high". On an analogue sensor, a high reading is a voltage sitting near the reference supply, and the usual cause is a short to voltage or an open in the ground return that lets the signal float up. On a switch, there is no proportional value at all: the circuit is either pulled down by closed contacts or sitting at the pull-up voltage because the contacts are open. A "high" on a switch therefore means the contacts never closed when the module expected them to. Nothing shorted to anything. The switch simply did not see the pressure, or could not report that it had. Buying a pressure sensor for a vehicle fitted with a pressure switch is the most common wasted purchase on this code, and it is avoidable by looking up which device the vehicle actually has before ordering.
On switch-based designs there is a specific and well-known internal failure worth knowing about. The switch element is a small rubber or elastomer diaphragm that lifts against a contact when pressure builds behind it. Diaphragms harden with heat and age, and once one tears the contact can no longer be made regardless of what the hydraulics are doing. Where the switch is packaged inside a larger electro-hydraulic control unit rather than sold as a separate part, the repair is the whole assembly, and the price gap between that and a bolt-in sensor is the reason this identification step is worth doing first.
The timing of the fault is the other thing that separates P0848 from the circuit-high code on the main sensor. The main sensor watches line pressure, which is present whenever the engine runs, so a high reading there is judged continuously. The "B" tap generally watches a circuit that is only pressurised during an event — one clutch or band applying, or the converter clutch engaging. That means the module frequently records the impossible value at the moment the circuit should have been at its quietest, and it also means a bench check with the engine idling proves very little. Look at freeze frame and note what the transmission was being asked to do when the code stored.
One wiring failure is characteristic of this channel and has a findable location. Inside the pan, the sensor or switch lead runs in the same internal harness as the solenoid drive wires, which carry battery-level voltage in pulses. Where that harness has chafed against a casting edge or been pinched by the pan lip, the low-level signal line can be shorted to a solenoid feed. The tell is that the reported value moves in step with solenoid commands rather than with pressure, which is visible on a scan tool graph and is not something a static resistance check will ever show.
Common causes
- Signal wire shorted to a voltage source, frequently to a solenoid drive line inside the internal transmission harness
- Open or high-resistance ground return, letting an analogue signal float toward the reference voltage
- Torn or hardened diaphragm inside a pressure switch, so the contacts can no longer close
- Open circuit between the switch or sensor and the module, which on a switched circuit reads as permanently high
- Connector terminal backed out or not fully seated at the transmission case connector
- Failed pressure sensor with its output latched at the top of its range
- Internal harness chafed against a casting edge or pinched by the pan flange during earlier service
- Electro-hydraulic control unit fault where the switch is integrated into a larger assembly rather than sold separately
- Corrosion in the case connector from fluid or moisture intrusion
- Module reference voltage supply fault, which is uncommon and should be confirmed rather than assumed
Symptoms
- Check engine light with P0848 stored and often no drivability change at all
- Transmission defaulting to a fixed line pressure strategy, giving noticeably firm or harsh shifts
- Torque converter clutch not engaging, with a small rise in cruising engine speed
- Transmission entering limp or fail-safe mode and holding one gear
- Shift quality varying between cold and warm operation
- Reduced fuel economy on the motorway where converter lock-up is inhibited
- Code stored as history only, with the vehicle behaving normally most of the time
- Other pressure or solenoid codes stored alongside it where the internal harness is damaged
Diagnostic steps
- 1.Look up whether this vehicle's "B" tap is an analogue pressure sensor or a pressure switch before ordering any part. The two fail differently, test differently and cost differently, and the code number does not tell you which you have.
- 2.Read freeze frame and note what the transmission was commanding when the code stored. This channel is only meaningful during the event that pressurises it, so the conditions at capture narrow the search more than any bench test will.
- 3.Graph the reported value against solenoid commands during a road test. A value that steps in time with a solenoid rather than with pressure indicates the signal line is being fed from a solenoid drive inside the internal harness.
- 4.On a switched circuit, jumper the signal line to ground at the case connector and confirm the module sees the state change. If it does, the wiring back to the module is sound and the fault is at or beyond the connector.
- 5.On an analogue circuit, disconnect the sensor and check whether the signal stays high. Still high with the sensor removed points at a short to voltage; falling to zero points at the sensor.
- 6.Measure the ground return with a voltage-drop test while the circuit is live rather than checking resistance at rest, since a partially open ground is the classic cause of a floating high reading.
- 7.Inspect the case connector for fluid inside the shell and for terminals that have backed out of their locks, and treat fluid intrusion as a seal fault rather than something to dry and reassemble.
- 8.If the vehicle has an integrated electro-hydraulic control unit, confirm whether the switch is separately serviceable before quoting the repair, because the answer changes the price by an order of magnitude.
- 9.Verify the repair with a road test that performs the specific event that pressurises the monitored circuit, not by clearing the code and idling in the bay.
Repair cost
$130 – $1,800
An external wiring or connector repair is $130 to $400. A separately serviceable pressure switch or sensor mounted on the outside of the case runs $150 to $450 fitted. Where the device lives inside the pan on the valve body, add pan-off labour and fresh fluid, so $350 to $800 fitted is realistic. Repairing a chafed internal harness is usually the same pan-off labour plus a harness at $80 to $300, so $400 to $900. The expensive outcome is a transmission where the pressure switch is packaged inside an integrated electro-hydraulic control unit and is not sold on its own: replacing and programming that assembly is commonly $900 to $1,800. Establishing which of those three cases applies is worth the $90 to $150 diagnostic fee before any part is ordered.
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 advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.