P0842 on 2016-2020 Mazda CX-9: Transmission Pressure Switch Fault Causes and Fixes

What's Unique About the 2016-2020 Mazda CX-9

The 2016-2020 Mazda CX-9, along with other models using the Skyactiv-Drive 6-speed automatic transmission (FW6A-EL) 🎬 Watch: A detailed overview of the Skyactiv-Drive transmission system., is subject to a specific known issue. Mazda has issued Technical Service Bulletins (TSBs), including the most recent #05-002-23, explaining that internal contamination can cause the oil pressure switches to stick. As a result, P0842 rarely appears alone 🎬 Watch: Understanding the P0842 code and pressure switch circuit faults. and is often accompanied by a cluster of other pressure switch codes (like P0847, P0872, P0877), pointing directly to this internal fault rather than a simple wiring or isolated sensor failure.

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
• Automatic Transaxle (AT) warning light illuminated
🎬 See this walkthrough on fixing the AT warning light issue.
• Harsh, jerky, or erratic shifting
• Transmission may not shift properly or get stuck in one gear (limp mode)
• Manual shift mode may be inhibited
• Increased fuel consumption
• Shuddering at certain speeds, especially on inclines

Most Likely Causes

1. Stuck Internal Oil Pressure Switch(es) 🔴 High Probability → Shop Engine Oil Pressure Switch As documented in Mazda TSB #05-002-23, contamination in the transmission fluid can cause the oil pressure switches on the valve body to stick in the 'on' or 'off' position. Mazda has updated procedures over the years, starting with TCM reprogramming and escalating to parts replacement if the issue persists.
   How to confirm: Confirm by checking for the presence of related codes (P0847, P0872, P0877, P0780, P1738) and following the diagnostic procedure outlined in the TSB. A professional scan tool can monitor the live data from the pressure switches to see if their state matches the expected state for the current gear.
   Typical fix: Replace both oil pressure switches (A and B) located on the valve body. This requires removing the transmission oil pan and oil strainer. It is critical to maintain cleanliness to avoid further contamination.
   Est. part cost: $50-$150 for the switches
2. Faulty Control Valve Body 🟡 Medium Probability → Shop Transmission Valve Body The pressure switches are mounted on the valve body. In some cases, the issue is with the valve body's hydraulic circuits or integrated solenoids, or it is too contaminated to be cleaned. Mazda's own diagnostic tree points to replacing the valve body if basic checks fail or if P1738 is also present. In many cases, the valve body is sold as a pre-programmed mechatronic unit that includes the TCM and solenoids, as the solenoids are calibrated to the specific TCM.
   How to confirm: This is typically diagnosed after replacing the pressure switches doesn't solve the problem, or if a transmission specialist identifies wear, blockage, or a large amount of debris in the valve body itself upon inspection.
   Typical fix: Replace the entire control valve body assembly. This part often comes as a mechatronic unit including the TCM and solenoids. This is a more definitive but also more expensive repair.
   Est. part cost: $950-$1800
3. Low or Dirty Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While Mazda has called the fluid 'lifetime', owner and mechanic experiences suggest that fluid breaks down over time, leading to harsh shifting and potential contamination that causes this code. Using any fluid other than the specified blue Mazda ATF-FZ can also cause severe shifting problems.
   How to confirm: Check the transmission fluid level and condition according to the manufacturer's procedure. Fluid should be at the correct level, be blue in color, and not appear dark, burnt, or contain excessive debris.
   Typical fix: Perform a transmission fluid drain and fill using only Mazda ATF-FZ. A full flush is generally discouraged; multiple drain-and-fills are preferred if the fluid is very dirty.
   Est. part cost: $100-$300 for fluid
4. Faulty Transmission Control Module (TCM) Software ⚪ Low Probability → Shop Transmission Assembly Older TSBs for this issue on other Mazda models sometimes listed a TCM software update as a potential first step to change the diagnostic logic and prevent false positives. The latest TSBs for the CX-9 focus more on mechanical replacement.
   How to confirm: A Mazda dealership or a shop with a compatible scan tool (like Mazda's IDS) can check the current TCM calibration file against the latest version available from Mazda.
   Typical fix: Reprogram the TCM to the latest software version. This is often the first step a dealer will take.
   Est. part cost: $0 (if under warranty) or $100-$250 for labor

Rare But Worth Checking

• Wiring Harness or Connector Issue: While the TSBs point to an internal issue, it's always possible for the wiring harness leading to the transmission to become damaged, or for the main connector to have corrosion or a loose pin. This should be inspected before condemning internal parts.

Diagnosis Steps

1. Scan the TCM for all stored trouble codes. Note if P0842 is present with any of the codes listed in TSB #05-002-23 (P0847, P0872, P0877, P0780, P1738).
2. Check the transmission fluid level and condition. The fluid should be clean, at the proper level, and not smell burnt. It should be the correct blue Mazda ATF-FZ fluid.
3. Visually inspect the transmission's main electrical connector and wiring harness for any signs of damage, corrosion, or loose connections.
4. Refer to Mazda TSB #05-002-23. The bulletin provides a specific diagnostic flow chart based on which codes are present.
5. If directed by the TSB, a technician will drop the transmission oil pan to access the control valve body. This requires draining the fluid.
6. Inspect the valve body for sludge or metallic debris. A large amount of debris may indicate the need to replace the valve body or the entire transmission.
7. Following the TSB, replace both oil pressure switches (A and B) located on the valve body.
8. If the problem persists or if significant contamination is found, replacement of the entire control valve body may be necessary.
9. After repairs, install a new oil pan gasket, refill with Mazda Skyactiv-Drive ATF-FZ, clear all codes, and perform a road test to verify the fix.

Parts You'll Likely Need

• Transmission Control Valve Body (OEM #FZ21-21-100J) — This is the complete assembly that houses the problematic pressure switches and solenoids. Mazda's diagnostic procedure often leads to replacing the whole unit, especially if multiple codes are present or switch replacement fails.
  Trusted brands: Mazda OE
  OEM price range: $950-$1800
  Aftermarket price range: $800-$1400
• Transmission Oil Pressure Switch 'A' (OEM #FZ01-21-2C0) — One of the two switches inside the transmission that commonly fails due to contamination, as per Mazda TSBs. This part number may be listed as 'Switch A Oil Pressure' or 'Pressure Sensor'.
  Trusted brands: Mazda OE
  OEM price range: $40-$70
• Transmission Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — The second of two switches inside the transmission that commonly fails. TSBs recommend replacing both at the same time.
  Trusted brands: Mazda OE
  OEM price range: $40-$70
• Mazda Skyactiv-Drive ATF FZ (OEM #0000-FZ-113E-01) — Correct low-viscosity, blue-colored fluid is required for this transmission. Using any other fluid can cause performance issues and damage.

Related Codes That Often Appear With This One

• P0847 — Fault with Oil Pressure Switch No. 2 (Switch A). Often set together with P0842 due to the common cause of contamination.
• P0872 — Fault with Oil Pressure Switch No. 3 (Switch A). Often set together with P0842 due to the common cause of contamination.
• P0877 — Fault with Oil Pressure Switch No. 4 (Switch B). Often set together with P0842 due to the common cause of contamination.
• P0780 — Indicates a gear shifting malfunction, a direct symptom of the pressure switch failure.
• P1738 — A manufacturer-specific code for an automatic transaxle internal malfunction, often triggered by the same event and pointing towards a more severe internal issue.

Technical Service Bulletins (TSBs) & Recalls

• TSB #05-002-23: Check Engine Light and AT Warning Light ON with DTCs P0842, P0847, P0872, P0877, P0780, P1738.
• TSB #05-003/20: Superseded bulletin covering the same DTC cluster for a wide range of Mazda models including the CX-9.
• TSB #05-002/15: An earlier version of the bulletin with similar diagnostic advice.

Platform-Specific Known Issues

• A known issue documented in Mazda TSB #05-002-23 describes how contamination in the transmission can cause one or more internal oil pressure switches to become stuck. The bulletin states: 'This is caused by the oil pressure switch A and/or B at the control valve body temporarily or permanently getting stuck due to contamination entering the passage.' This leads to a specific cluster of DTCs, with P0842 being one of the most common.

Mechanic-Grade Diagnostic Values

• DTC Detection Condition — expected: The code sets when the engine is running, ATF temperature is 20°C (68°F) or more, and the TCM detects that oil pressure switch No. 1 is stuck on.. Failure: The presence of code P0842 indicates the condition has been met.
• Post-Repair Verification Temperature — expected: After repairs, the ATF temperature must be verified to be 55°C (131°F) or more before performing the on-board diagnostic test to confirm the fix.. Failure: N/A - This is a required condition for testing.

Scan Tool Commands That Help

• Mazda IDS (or equivalent): On-Board Diagnostic Test — This function is used after a repair, such as replacing the valve body or pressure switches, to allow the TCM to run its internal checks and verify that the fault has been cleared. The ATF temperature must be above 55°C (131°F) to run the test.
• Mazda IDS (or equivalent): Log Viewer / TCM File Name Confirmation — When replacing a valve body, the IDS can be used to confirm the existing TCM file name to ensure the correct pre-programmed replacement valve body is ordered.

Wiring & Ground Locations

• Transmission Connector — The main electrical connector is located on the exterior of the transmission housing. The internal harness connects the TCM and valve body components to this external connector.. While the fault is almost always internal, a visual inspection of the main external connector for corrosion, damage, or pushed-out pins is a mandatory first step before condemning internal components.
• Pressure Switch 'A' (FZ01-21-2C0) Wiring — Inside the transmission, on the valve body. It has a two-pin connector with red and white wires in the internal harness.. This switch monitors pressure for the 2-6 Brake and 3-5-R Brake clutch packs. A fault here directly relates to shift quality for gears 2, 3, 5, 6, and Reverse. The code P0842 specifically points to a problem with this switch's circuit.

Real Owner Repair Stories

• Reddit user in r/mazda (2017 Mazda 3 with FW6A-EL transmission, 115k miles) — Valve body going out, shifting hard, kicking during downshifts.
  ❌ Tried (didn't work) A transmission shop was hesitant to rebuild the valve body, suggesting a full used transmission replacement was cheaper/easier for them.
  ✅ What actually fixed it Another user in the thread with a 2019 Mazda 3 had the TCM and Control Valve Body replaced at a dealership for ~$2400, which resolved their similar issue. The expert consensus in the thread was that replacing the entire valve body assembly (which comes as a pre-programmed mechatronic unit with TCM and solenoids) is the standard, reliable fix.

OEM Part Supersession History

• FZ21-21-100 (blank valve body) → Various pre-programmed part numbers (e.g., PEY0-21-C00, PEX1-21-C00, etc.) — Mazda phased out the 'blank' valve body that required programming. New service parts come pre-programmed with the correct TCM calibration for a specific vehicle configuration.
  Heads up: A blank valve body is no longer available. When replacing, the correct pre-programmed part number must be selected based on the vehicle's VIN and original TCM file name. Installing the wrong pre-programmed unit will result in malfunction.
• fza1212c0 → FZ01-21-2C0 — Standard part number revision or supplier change.