P0873 – Transmission Fluid Pressure Sensor/Switch “C” Circuit High

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

Definition source: SAE J2012/J2012DA (industry standard)

P0873 indicates the powertrain control system has detected a high electrical condition in the Transmission Fluid Pressure Sensor/Switch “C” circuit. “Circuit High” is a signal-level fault: the module is seeing a voltage or input state higher than expected for the sensor/switch circuit, not a confirmed mechanical transmission failure by itself. The exact sensor type, wiring layout, and how the module interprets the signal vary by vehicle, so always verify component location, pinout, and test specifications in the appropriate service information before probing circuits. A thorough diagnosis focuses on the electrical path (power, ground, signal, and connectors) and on confirming the fault with scan data and repeatable testing.

What Does P0873 Mean?

P0873 – Transmission Fluid Pressure Sensor/Switch “C” Circuit High means the control module has identified an abnormally high input on the circuit associated with the transmission fluid pressure sensor/switch designated as “C.” Per SAE J2012 DTC conventions, this is categorized as a “circuit high” fault, which typically points to an electrical condition such as a short to voltage, an open in a ground/return path, an open signal circuit that allows the input to float high, or an internally failed sensor/switch that outputs a high signal. The code describes what the module detected electrically; it does not, by itself, confirm fluid pressure is actually high.

Quick Reference

• Subsystem: Transmission fluid pressure sensor/switch “C” electrical circuit (signal, power/feed, and ground/return as applicable).
• Common triggers: Signal short-to-power, open ground/return, unplugged/poor connector contact, chafed harness causing high input, or sensor/switch internal high output.
• Likely root-cause buckets: Wiring/connector faults; sensor/switch fault; power/ground integrity issue; less commonly module input circuit issue (varies by vehicle).
• Severity: Often moderate to high; may cause protective transmission strategy, harsh shifts, limited gear operation, or reduced drivability depending on strategy.
• First checks: Confirm code is current, review freeze-frame, inspect connector/harness at sensor/switch “C,” check for corrosion/fluid intrusion, and verify power/ground integrity.
• Common mistakes: Replacing the sensor/switch without testing the circuit for a short-to-voltage or open ground; assuming the code proves a hydraulic pressure problem.

Theory of Operation

The transmission fluid pressure sensor/switch “C” provides the control module with an electrical signal representing a pressure-related state or measurement used for shift control and transmission protection. Depending on vehicle design, it may be a pressure transducer (analog signal) or a pressure switch (discrete on/off input). The module supplies a defined electrical environment (such as a reference and return, or a pull-up/pull-down strategy) and monitors the resulting signal.

For a “circuit high” determination, the module detects the input staying higher than expected for the operating conditions or higher than the allowed electrical range. This can occur when the signal is shorted to a voltage source, when a return/ground path opens causing the signal to float high, when connector contact intermittently opens, or when the sensor/switch fails in a way that drives the output high. The monitor may require the condition to persist for a calibrated time before setting the DTC.

Symptoms

• Warning light: Check engine light or powertrain warning illuminated.
• Shift quality: Harsh, delayed, or inconsistent shifts.
• Fail-safe mode: Transmission may enter a protective strategy or limited operating mode (varies by vehicle).
• Gear limitation: Reduced ability to upshift/downshift normally or restricted gear range.
• Driveability: Sluggish acceleration or reduced performance due to transmission control limitations.
• Intermittent behavior: Symptoms may come and go with vibration, heat, or harness movement.
• Additional DTCs: Related transmission pressure, solenoid, or sensor circuit codes may also be present.

Common Causes

• Wiring short-to-power on the Pressure Sensor/Switch “C” signal circuit causing an abnormally high input to the control module
• Open ground or high resistance in the sensor/switch ground circuit (including poor ground point contact) that makes the signal read high
• Connector damage such as spread terminals, corrosion, fluid intrusion, broken locks, or partial engagement at the sensor/switch or module
• Signal circuit open (broken conductor, poor pin fit) that allows the signal to float high depending on circuit design
• Reference/feed circuit issue (varies by vehicle) where an abnormal high on the reference or supply skews the sensor/switch output high
• Sensor/switch “C” internal fault that outputs an erroneously high signal under normal operating conditions
• Harness routing problem where chafing against brackets/heat sources intermittently drives the circuit high
• Module input fault (less common) such as a biased or damaged input channel after wiring/sensor integrity has been verified

Diagnosis Steps

Tools typically needed include a scan tool capable of reading transmission-related live data and freeze-frame, a digital multimeter, back-probing leads, and basic hand tools for access and connector inspection. Depending on vehicle design, you may also need a wiring diagram and connector pin-out from service information to identify the Pressure Sensor/Switch “C” circuits and the correct module pins to test.

1. Confirm the code and capture data: Verify P0873 is present as current or pending. Record freeze-frame and any transmission-related companion codes. Clear codes and see if P0873 returns under similar conditions to confirm it is repeatable.
2. Check data plausibility (high input): On the scan tool, view the Pressure Sensor/Switch “C” parameter(s) and observe if the reading is stuck high, intermittently high, or spikes high during certain events. Log data during a short road test if it is safe to do so.
3. Perform a quick visual inspection: With key off, inspect the sensor/switch and harness area for damaged insulation, crushed sections, recent repairs, contact with sharp edges, or evidence of fluid intrusion into connectors. Address obvious physical issues before deeper electrical testing.
4. Connector integrity checks: Disconnect the sensor/switch connector and inspect for bent pins, pushed-out terminals, corrosion, or loose terminal tension. Verify the connector locks fully and that the harness has proper strain relief. Repeat at the module-side connector if accessible and safe per service procedures.
5. Wiggle test for intermittents: With the scan tool graphing the Pressure Sensor/Switch “C” signal, wiggle the harness along its routing and gently manipulate connectors. If the value jumps high or the code sets, isolate the exact section/connector that triggers the change.
6. Check for short-to-power on the signal circuit: Using the wiring diagram, identify the signal wire. With the sensor/switch disconnected, measure the signal circuit relative to ground and relative to the reference/supply circuits as appropriate. If the signal is being driven high when it should not be, trace for contact with a powered circuit or internal short in a shared loom.
7. Verify ground quality with voltage-drop testing: Under operating conditions (as allowed by service info), perform a voltage-drop test on the sensor/switch ground path between the sensor ground pin and a known-good ground. Excessive drop indicates high resistance (corrosion, loose ground point, damaged wire) that can bias the input high.
8. Verify reference/supply integrity: If the sensor/switch uses a reference or regulated feed (varies by vehicle), confirm it is stable and not abnormally high relative to system expectations. Compare readings at the sensor connector and at the module pin to detect harness resistance or incorrect backfeed.
9. Continuity and isolation checks: With power off and connectors disconnected, check continuity end-to-end for the signal, ground, and any reference/feed circuits. Then check for unwanted continuity between the signal and power circuits, and between the signal and other adjacent circuits, indicating insulation damage or pin-to-pin bridging.
10. Sensor/switch evaluation: If wiring, connectors, power, and ground test good, evaluate the sensor/switch “C” per service information (functional test method varies by vehicle). If the output remains high or the switch state indicates high input when the circuit conditions are correct, suspect the sensor/switch.
11. Module-side confirmation: If possible, verify the signal behavior at the module connector while the circuit is connected (back-probe carefully). If the signal is normal at the sensor but high at the module, suspect harness issues between points. If it is high at both with known-good wiring, consider a module input concern only after all other causes are eliminated.

Professional tip: A “circuit high” fault is often caused by a wiring condition (short-to-power, open ground, or signal floating high) rather than a mechanical transmission problem. Prioritize proving the electrical integrity with connector inspection, wiggle testing, and voltage-drop checks before replacing the sensor/switch or condemning the control module.

Possible Fixes & Repair Costs

Repair cost for P0873 varies widely by vehicle and depends on what testing confirms: a simple connector issue, a wiring repair, a sensor replacement, or a control-module power/ground concern. Labor time also changes based on access to the transmission harness and required verification steps.

• Clean, reseat, and secure the transmission fluid pressure sensor/switch “C” connector; correct poor pin fit, corrosion, or moisture intrusion found during inspection
• Repair or replace damaged wiring in the sensor/switch “C” signal, reference, or ground circuits (chafing, melted insulation, stretched wiring, or broken conductors)
• Correct a short-to-power condition on the sensor/switch “C” signal circuit (pinched harness, contact with a powered circuit, or internal harness damage)
• Restore proper ground integrity (repair ground splice/eyelet, address high resistance, and confirm voltage-drop results meet service information requirements)
• Replace the transmission fluid pressure sensor/switch “C” only after circuit integrity and correct power/ground are verified
• Address module-side issues only after all external circuit and component tests pass (verify connector condition, terminal tension, and module power/ground before considering replacement or reprogramming, as applicable)

Can I Still Drive With P0873?

You may be able to drive short distances, but it’s not recommended unless operation is normal and no warning indicators suggest reduced control. A “circuit high” fault can cause the control module to use a default value, potentially leading to harsh or delayed shifts and reduced drivability. If you notice slipping, severe shift issues, reduced-power behavior, or any safety-related warnings, stop driving and have the vehicle diagnosed to avoid transmission damage and unsafe operation.

What Happens If You Ignore P0873?

Ignoring P0873 can lead to ongoing incorrect pressure feedback and control strategies that may increase heat and wear inside the transmission. Over time, this can worsen shift quality, trigger additional transmission-related DTCs, and potentially contribute to internal damage. The longer the circuit-high condition persists, the greater the chance of secondary issues caused by the system operating in a fail-safe or default mode.

Related Pressure Transmission Codes

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

• P0878 – Transmission Fluid Pressure Sensor/Switch “D” Circuit High
• P0848 – Transmission Fluid Pressure Sensor/Switch “B” Circuit High
• P0843 – Transmission Fluid Pressure Sensor/Switch “A” Circuit High
• P0877 – Transmission Fluid Pressure Sensor/Switch “D” Circuit Low
• P0875 – Transmission Fluid Pressure Sensor/Switch “D” Circuit
• P0872 – Transmission Fluid Pressure Sensor/Switch “C” Circuit Low

Key Takeaways

• P0873 indicates the transmission fluid pressure sensor/switch “C” circuit is reading high electrically, not a confirmed mechanical pressure problem.
• Most fixes start with connector and harness inspection, then circuit testing for shorts-to-power, reference issues, and poor grounds.
• Replacing the sensor without verifying wiring, power, and ground integrity is a common misstep.
• Driving may be possible, but shift quality and transmission protection strategies can be affected.
• Confirm the correct sensor/switch “C” circuit and test procedure using the vehicle’s service information.

Vehicles Commonly Affected by P0873

• Vehicles with electronically controlled automatic transmissions using pressure feedback for shift control
• Vehicles with external transmission harness routing near hot components or moving brackets (higher chafe risk)
• Higher-mileage vehicles with aged connectors, brittle insulation, or prior harness repairs
• Vehicles operated in high-heat or high-vibration environments that accelerate connector and wiring degradation
• Vehicles with recent transmission, engine, or subframe service where connectors may be disturbed or pinched during reassembly
• Vehicles frequently used for heavy-duty operation where underbody wiring may be exposed to debris and impacts
• Vehicles with underbody exposure to water, road salt, or contamination that can promote terminal corrosion
• Vehicles with prior fluid leaks that can wick into harness coverings and connectors over time

FAQ

Does P0873 mean the transmission fluid pressure is definitely too high?

No. P0873 specifically indicates a “circuit high” electrical condition for the transmission fluid pressure sensor/switch “C” circuit. The module is detecting a higher-than-expected signal electrically; actual hydraulic pressure is not confirmed without separate testing.

What electrical problems most commonly cause a “circuit high” on this sensor?

Common electrical causes include a short-to-power on the signal circuit, an open or high-resistance ground that lets the signal float high, terminal corrosion creating abnormal signal behavior, or an incorrect reference/feed condition (varies by vehicle). Testing should confirm the exact failure mode.

Should I replace the transmission fluid pressure sensor/switch “C” right away?

Not until basic circuit checks are completed. A wiring or connector fault can mimic a failed sensor. Verify the connector condition, harness routing, and module-side power/ground integrity, then confirm signal behavior with scan data and electrical testing before replacing parts.

Will clearing the code fix P0873?

Clearing the code only resets the stored fault; it does not correct the underlying circuit-high condition. If the fault is still present, the monitor will typically fail again during the next self-test or driving conditions that run the diagnostic.

Can low transmission fluid cause P0873?

Low fluid is not a direct cause of a “circuit high” electrical fault. While fluid level issues can create shift concerns, P0873 is set due to an electrical signal being high in the sensor/switch “C” circuit. If fluid condition is suspected, address it separately, but do not treat it as the primary explanation without confirming circuit results.

Always verify the exact sensor/switch “C” circuit identification, connector pinout, and test procedure in the correct service information for your vehicle before repairs.