P0842 on 2021 Mazda CX-9: Transmission Pressure Switch Fault Causes and Fixes

What's Unique About the 2021-2021 Mazda CX-9

The 2021 Mazda CX-9 uses the SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL family), which has a well-documented history of this specific fault across multiple Mazda models. The issue is so common that Mazda issued Technical Service Bulletin (TSB) #05-002-23, which directly addresses P0842 and a cluster of related codes (P0847, P0872, etc.). Unlike a simple external sensor failure, this code points to a known internal mechanical issue with the pressure switches. The TSB notes that mass production changes were made to reduce contamination, but earlier models remain susceptible.

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 is on
• Automatic Transaxle (AT) warning light is on
• Harsh or erratic shifting
• Transmission may be stuck in a single gear (limp mode)
• Manual shift mode is disabled
• Reduced fuel economy

Most Likely Causes

1. Sticking Internal Transmission Oil Pressure Switch(es) 🔴 High Probability → Shop Engine Oil Pressure Switch This is the primary cause identified in Mazda TSB #05-002-23. The bulletin explicitly states the cause is the oil pressure switch (A and/or B) getting stuck due to contamination entering the passage.
   How to confirm: A technician will follow the diagnostic procedure in TSB #05-002-23. This involves checking for a specific cluster of codes (P0842, P0847, P0872, etc.) and may involve a TCM software update before parts replacement.
   Typical fix: Replace the internal oil pressure switches (Switch A and Switch B) located on the transmission valve body. This requires removing the transmission oil pan.
   Est. part cost: $50-$150
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Mazda has released software updates to change the diagnostic logic for these codes, making the system less sensitive to momentary sticking of the switches. The TSB often lists this as a required first step for certain VIN ranges to avoid unnecessary parts replacement.
   How to confirm: A Mazda dealer or a shop with Mazda Diagnostic and Repair Software (MDARS) can check the current software version against available updates listed in the TSB.
   Typical fix: Reprogram the TCM to the latest software version. If the code returns, the pressure switches must be replaced.
   Est. part cost: $0
3. Contaminated or Low Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While low or old fluid is the source of the contamination that causes the switches to stick, it's rarely the sole cause if the code is already present. The damage to the switch is typically already done. However, checking it is a fundamental diagnostic step.
   How to confirm: A physical inspection of the transmission fluid level and condition is required. This transmission has no dipstick, so it must be checked by a technician via the fill plug at a specific temperature.
   Typical fix: Perform a transmission fluid drain and fill using the correct Mazda ATF-FZ fluid. This is always done in conjunction with replacing the pressure switches.
   Est. part cost: $80-$150

Rare But Worth Checking

• Faulty Control Valve Body: → Shop Transmission Valve Body If the transmission fluid is heavily contaminated with debris or if replacing the pressure switches does not resolve the issue, the entire valve body assembly may be faulty or too clogged to function properly. Mazda's own diagnostic tree for the CW6A-EL transmission points to replacing the control valve body if other steps fail.
• TCM Malfunction: While a possible cause listed in generic service manuals, a hardware failure of the TCM is very rare compared to the known mechanical issue with the pressure switches. It should only be considered after all other possibilities are exhausted.

Diagnosis Steps

1. Scan the TCM for all stored trouble codes. Confirm if P0842 is present with any of the related codes listed in TSB #05-002-23 (P0847, P0872, P0877, P0780, P1738).
2. Check for available TSBs for the vehicle's VIN. A dealer will determine if a TCM software update is the first required step based on the TSB's flowchart.
3. Have a technician check the transmission fluid level and condition. This requires the vehicle to be level and the fluid to be at a specific temperature (40-110 °C), as there is no dipstick.
4. If directed by the TSB or if a software update does not fix the issue, proceed with replacing the internal transmission oil pressure switches.
5. This involves draining the fluid, removing the transmission oil pan, and accessing the control valve body to replace the switches.
6. Inspect the oil pan and magnets for excessive metal debris, which could indicate a more severe transmission problem beyond the scope of this repair.
7. After replacing the switches, install a new pan gasket/seal, refill with genuine Mazda ATF-FZ fluid to the correct level, clear all codes, and perform an 'Initial Learning' procedure.
8. The Initial Learning procedure forces the TCM to relearn the hydraulic properties of the new components. This can be done with a Mazda scan tool or via a specific sequence of shifter movements and pedal presses. A final road test verifies the repair is complete.

Parts You'll Likely Need

• Transmission Oil Pressure Switch (OEM #FZ01-21-2J0) — This is the component that gets stuck and causes the code, as identified by Mazda's TSB. It's recommended to replace both switches (A and B) at the same time. This part is also called 'Switch B Oil Pressure'.
  Trusted brands: Mazda (OEM)
  OEM price range: $75-$110 per switch
  Aftermarket price range: $30-$60 per switch
• Mazda SKYACTIV-Drive Automatic Transmission Fluid FZ (OEM #0000-FZ-113E-01) — The transmission must be refilled with this specific low-viscosity blue fluid after the pan is dropped for the repair. Approximately 3.5 quarts are needed.
  Trusted brands: Mazda (OEM)
  OEM price range: $20-$25 per quart
  Aftermarket price range: $15-$20 per quart
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — Since the root cause is contamination, replacing the filter is a critical part of the repair to ensure longevity.
  Trusted brands: Mazda (OEM)
  OEM price range: $45-$60
  Aftermarket price range: $20-$40
• Transmission Oil Pan Gasket / Seal (OEM #FZ01-21-51X (example, verify by VIN)) — A new gasket or sealant is required to prevent leaks when reinstalling the transmission oil pan. Mazda specifies using silicone sealant TB1217E or an equivalent.
  Trusted brands: Mazda (OEM)
  OEM price range: $25-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0847 — This code is for 'Oil pressure switch No. 2 (oil pressure switch A) stuck on'. Both switches are on the valve body and prone to the same contamination issue. They are often replaced as a pair.
• P0872 — This code is for 'Oil pressure switch No. 3 stuck on'. It is part of the same family of codes indicating a stuck pressure switch, as listed in TSB #05-002-23.
• P0877 — This is another related pressure switch code often found with P0842, as noted in TSB #05-002-23.
• P0780 — This is a 'Shift Malfunction' code that is triggered as a result of the invalid pressure signals from the sticking switches. It is explicitly mentioned in the TSB.
• P1738 — This is a manufacturer-specific code for 'Automatic Transmission Internal Malfunction' that is also listed in TSB #05-002-23 and points towards the same internal valve body fault.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23

Platform-Specific Known Issues

• TSB #05-002-23: Mazda issued this bulletin for the 2021 CX-9 (among other models) which explicitly states that codes P0842, P0847, P0872, P0877, P0780, or P1738 are caused by contamination causing the oil pressure switches on the control valve body to stick.

Mechanic-Grade Diagnostic Values

• On-Vehicle Oil Pressure Switch Continuity Test — expected: In Park or Neutral with the engine running, there should be infinite resistance (no continuity to ground). In Drive with the engine running, there should be continuity to ground (0 ohms).. Failure: Continuity to ground while in Park or Neutral indicates a 'stuck on' switch, which is the direct cause of code P0842.
• Oil Pressure Switch Bolt Tightening Torque — expected: 9-10 N·m (80-88 in·lbf). Failure: Improper torque could lead to leaks or damage to the switch or valve body.
• Diagnostic Trigger Condition — expected: The code sets when the Automatic Transmission Fluid (ATF) temperature is 20°C (68°F) or more and the TCM detects the switch is stuck on.. Failure: N/A

Scan Tool Commands That Help

• Mazda Diagnostic and Repair Software (MDARS): Module Reprogramming (TCM) — This is the first step in the official TSB repair procedure for some VINs to update the TCM with improved diagnostic logic before replacing any parts. The 2021 CX-9 requires the MDARS software, as older IDS/M-MDS systems are not compatible.
• Mazda Diagnostic and Repair Software (MDARS): Initial Learning [FW6A-EL] — This procedure is mandatory after replacing the oil pressure switches or the control valve body. It allows the TCM to relearn the hydraulic characteristics of the new components. Failure to perform this can result in continued harsh shifting.

Wiring & Ground Locations

• Transmission Connector (C-33) — Main external electrical connector on the automatic transaxle case.. This is the primary connector where the internal transmission wiring harness, including the circuits for the pressure switches, interfaces with the main vehicle harness going to the TCM. A visual inspection for corrosion or damage here is a key external check, though internal issues are far more common for P0842. Wiring diagrams for the 2021 CX-9 are available for detailed pinout tracing.

OEM Part Supersession History

• fza1212j0 → FZ01-21-2J0 — Part number update/revision. The reason for the change is not officially documented, but it is common for supersessions to address reliability improvements or manufacturing changes.

Model Year Variations Within This Range

• 2021+: The 2021 Mazda CX-9 requires the use of the newer Mazda Diagnostic and Repair Software (MDARS) for any TCM reprogramming or special functions. The older Mazda Modular Diagnostic System (M-MDS) or IDS is not compatible with this model year.