P0842 indicates a fault in the Transmission Fluid Pressure Sensor/Switch C Circuit High. In most cases the issue is sensor or wiring related, though some vehicles may require transmission service if the problem persists after basic fixes.
What P0842 means
P0842 is a generic OBD-II code that points to sensor C in the transmission’s hydraulic pressure monitoring system. The “Circuit High” designation means the TCM (transmission control module) detects a signal value that’s higher than expected for the C-sensor, which can trigger shifting issues or a fail-safe mode. Exact sensor naming and pressure ranges vary by manufacturer, so always check the vehicle’s service manual for the precise definition and specs.
Common symptoms
The following signs may accompany P0842, though they can appear differently depending on the vehicle:
- Illuminated check engine light or transmission warning light
- Harsh, delayed, or erratic upshifts or downshifts
- Transmission entering a failsafe or limp mode to protect itself
- Inconsistent or abnormal transmission data on a scan tool
These symptoms suggest a problem with the sensor, its wiring, or the transmission’s hydraulic circuit, but fluid condition and external wiring should be ruled out first.
Diagnostic steps
Use this structured approach to confirm the cause of P0842. The steps progress from quick checks to electrical testing and may require a professional scan tool.
- Verify the code and review freeze-frame data with a scan tool to confirm P0842 and look for related codes (for example, sensor A/B issues or circuit faults).
- Check the transmission fluid level and condition. Low, dirty, or burnt fluid can affect pressure readings and overall transmission performance.
- Inspect the wiring harness and connector to the transmission fluid pressure sensor C. Look for damaged insulation, corrosion, bent pins, or a loose connection.
- Test the sensor circuit with a multimeter or oscilloscope: confirm there is a proper 5-volt reference from the PCM, a solid ground, and that the sensor’s output signal responds to pressure changes as specified by the service manual.
- Check for shorts or opens in the circuit (to power or to ground). Inspect for water intrusion or damaged shielding that could cause erroneous readings.
- If feasible, swap in a known-good sensor or use a known-good harness to determine whether the fault follows the sensor or the circuit.
- Review live data from the scan tool while operating the transmission (in a safe, controlled environment) to see if the C-sensor reading goes abnormally high or remains out of range.
- If sensor and wiring test normal but the code persists, consider internal transmission issues or TCM software faults per the manufacturer’s guidance and service bulletin history.
After completing these steps, you should have a clearer picture: a faulty sensor or wiring is most common, while gear-valve or pump issues are more involved and may require professional service.
Recommended fixes
Based on diagnostic results, apply the most likely repair first. The following repairs are typical for P0842 with varying vehicle makes and transmissions.
- Replace the transmission fluid pressure sensor C if it’s faulty or out of spec.
- Repair or replace damaged wiring to the sensor, including connectors, pins, and protective harness sleeves; reseal connectors as needed to prevent water intrusion.
- Correct any transmission fluid issues: top up to the proper level with the recommended fluid, or replace the fluid if it’s contaminated or burnt.
- Update or reprogram the TCM/ECU if a software fault is contributing to incorrect sensor readings (factory updates or recalls may apply).
- If there is evidence of an internal hydraulic issue (blockages, sticking valves, or pressure regulation problems), follow the manufacturer’s service procedure for valve body inspection or replacement.
Concluding the fixes: Start with the least invasive repair (sensor or wiring) and progress to more extensive transmission work only if the fault persists or other related codes indicate a deeper issue.
Safety and when to seek professional help
Working on transmissions involves fluid under pressure, heavy components, and electrical systems. If you’re not comfortable performing electrical tests, or if the code returns after a sensor/wiring repair, consult a qualified technician or a dealer. A professional can perform controlled pressure tests, valve-body inspections, and software updates with OEM tools and procedures.
Preventive tips
To minimize future P0842 risk, maintain proper transmission fluid level and condition, inspect the sensor and wiring during regular maintenance, and address any transmission hesitation or warning lights promptly. Using the correct fluid type and following the manufacturer's service intervals can help prevent hydraulic pressure issues that trigger sensor circuit faults.
Summary
P0842 points to a high circuit condition in Transmission Fluid Pressure Sensor C. The most common fixes involve replacing the C-sensor or repairing damaged wiring and connectors, followed by ensuring the transmission fluid is clean and at proper level. If the sensor and wiring checks pass but the code remains, more advanced diagnostics—such as TCM software updates or valve-body-related service—may be required. A careful, stepwise approach, starting with the simplest fixes, will usually resolve P0842 without unnecessary transmission teardown.