P0842 on 2019-2020 Subaru Ascent: Transmission Pressure Sensor Circuit Low Causes and Fixes

What's Unique About the 2019-2020 Subaru ASCENT

For the 2019-2020 Ascent, this code is the subject of multiple manufacturer documents, including a safety recall (WUV-07/WRK-21). 🎬 See why Subaru is paying millions in CVT class action lawsuits The issue is rarely the sensor itself, but rather the transmission wiring harness. The original harness was built with dissimilar metals (tin-plated terminals and copper wires) at a mid-joint connection, which can oxidize and create high electrical resistance. This increased resistance causes an incorrect, higher-than-actual pressure reading, leading the TCM to command lower pressure, which can cause the CVT drive chain to slip and potentially break. Subaru has an updated harness and specific diagnostic procedures to address this known defect.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light illuminated
• AT OIL TEMP warning light flashing
• Irregular noise or vibration while driving
• Shuddering, especially in slow traffic
• Harsh or delayed gear shifting
• Vehicle enters 'limp mode' (stuck in one gear)
• Sudden loss of power while driving, especially under acceleration
🎬 Watch: 5 warning signs your Subaru CVT is about to fail

Most Likely Causes

1. Faulty Transmission Wiring Harness 🔴 High Probability A known manufacturing defect where a mid-joint connection was made with dissimilar materials (tin-plated terminals and copper wires) causes oxidation and high resistance. This fools the TCM into thinking pressure is high, causing it to lower pressure and induce chain slip. This is documented in Subaru TSB 16-128-20R and recall WRK-21. The updated harness was incorporated into Ascent production starting with VIN L3441927.
   How to confirm: Follow the diagnostic procedure outlined in Subaru's service bulletins. After checking for other codes and performing a stall test, if P0842 is present, the official procedure directs the replacement of the harness. Diagnosis should rule out the sensor itself before condemning the harness.
   Typical fix: Replace the original transmission wiring harness with the updated part. The original replacement was P/N 24031AA850, which was superseded by P/N 24031AA851.
   Est. part cost: $70-$120
2. Faulty Transmission Fluid Pressure Sensor ('A') ⚪ Low Probability
   How to confirm: The sensor can be tested with a multimeter for correct resistance and voltage readings per the factory service manual. However, TSB 16-128-20R was created specifically for cases where diagnosis determines the sensor is OK but the harness is faulty.
   Typical fix: Replace the transmission fluid pressure sensor.
   Est. part cost: $50-$100

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): A possible but very rare cause. The TCM should only be considered after the harness and sensor have been definitively ruled out.
• Low or Contaminated Transmission Fluid: While low or dirty fluid can cause pressure issues, the 'circuit low' nature of P0842 points specifically to an electrical fault, making this less likely to be the root cause for this specific code.

Diagnosis Steps

1. Check for any active recalls (like WUV-07/WRK-21) related to the transmission. The recall procedure involves reprogramming the TCM and, if chain slip is detected, inspecting internal components and potentially replacing the entire CVT assembly.
2. Using a professional scan tool (like the Subaru Select Monitor), check for P0842 in the TCM's 'Current' or 'History' memory.
3. Follow the diagnostic flowchart in Subaru's service bulletins. TSB 16-122-19 provides an updated procedure.
4. The procedure generally involves checking fluid, performing a stall test, and then inspecting the harness circuit for resistance.
5. If resistance measurements on the harness are out of specification, replace the transmission wiring harness. Do not attempt to repair the harness.
6. If the harness tests correctly, proceed with testing the sensor itself, though this is the less likely failure point on this vehicle.
7. After the repair, clear all DTCs and perform a test drive and CVT relearn procedure as specified by the service manual.

Parts You'll Likely Need

• Transmission Wiring Harness (OEM #24031AA851 (supersedes 24031AA850)) — This is the updated part designed by Subaru to fix the manufacturing defect that causes code P0842.
  Trusted brands: Subaru (OEM)
  OEM price range: $70-$120

Technical Service Bulletins (TSBs) & Recalls

• 16-128-20R: Announcing the availability of a new transmission harness for use when diagnosis of DTC P0842 determines the sensor is OK but the harness is NG. It also identifies affected 2020 Legacy and Outback models.
• 16-122-19: Provides an updated diagnostic procedure for DTC P0842, instructing that the harness must be replaced, not repaired, if found to be out of specification.
• WUV-07 / WRK-21: Safety recall campaign addressing the faulty transmission harness. The remedy involves TCM reprogramming and inspection, which can lead to full transmission replacement if damage is found.

Platform-Specific Known Issues

• A safety recall (WUV-07, superseded by WRK-21) was issued for 2019-2020 models due to a faulty transmission harness connection causing incorrect pressure readings, which could lead to drive chain slip and loss of power.
• Subaru released TSB 16-128-20R to announce the availability of an updated harness (initially P/N 24031AA850, now 24031AA851) specifically to fix DTC P0842 when the sensor is found to be OK.
• Owner experiences on forums like Reddit describe the sudden loss of power as terrifying, with one user's vehicle dropping to 16 MPH when trying to merge onto a highway.
• The WRK-21 recall involves a multi-step process: first, reprogramming the TCM, then checking for stored slip history. If slip is detected, the dealer may inspect the transmission with a videoscope and potentially replace the entire CVT assembly.
🎬 Watch: Why some owners say the CVT recall fix failed

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or equivalent: AT Learning Mode (or Compulsory Learning Mode) — Required after replacing the transmission harness, valve body, TCM, or entire CVT assembly. This procedure adapts the TCM to the electrical and hydraulic characteristics of the new components. It must be performed with the vehicle lifted and the ATF temperature between 40-65°C (104-149°F).
• Subaru Select Monitor (SSM) or equivalent: Clear AT Learning Value — This function is used to erase the transmission's learned adaptive values before performing a new 'AT Learning Mode' procedure. It is a prerequisite step to ensure the new learning cycle starts from a clean slate.

Wiring & Ground Locations

• T4 / B11 Connector (Grey) — On top of the transmission housing, this is the main external connector for most solenoids and sensors.. This connector contains the signal wire for the pressure sensor. Testing can be performed at this connector's pins to check the circuit's integrity from the TCM to the transmission.
• T4 Connector, Pin 14 — Within the grey T4/B11 connector on top of the transmission.. This specific pin is for the Secondary Pressure Sensor Signal. A technician can use this pin to test for voltage or continuity when diagnosing a P0842 code, helping to isolate the fault between the TCM, external wiring, and the internal transmission harness.
• T9 (Transmission Radio Ground Cord) — Located on the converter case of the TR690 transmission.. This is a specified ground point directly on the transmission assembly. A poor ground connection here can cause erratic behavior from various transmission sensors and solenoids, so it should be checked for cleanliness and security during diagnosis.

OEM Part Supersession History

• 24031AA850 → 24031AA851 — The original harness was manufactured with a mid-joint connection using dissimilar metals (tin-plated terminals and copper wires), which could oxidize and cause high resistance. The updated part corrects this manufacturing defect.
  Heads up: Subaru explicitly warns in campaign WUV-70R to NEVER use the old part number (24031AA850) for this repair and to scrap any remaining inventory.

Model Year Variations Within This Range

• 2019-2020: The updated transmission wiring harness (P/N 24031AA851) was incorporated into factory production starting with Ascent VIN L3441927. Vehicles produced before this VIN have the potentially faulty original harness.