P0847 on 2021 Mazda CX-9: Transmission Pressure Switch Fault Causes and Fixes

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

For the 2021 CX-9 and other Mazdas with the SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL), P0847 is rarely a simple sensor failure. Mazda has issued Technical Service Bulletin (TSB) #05-002-23 which directly addresses this code, along with others like P0842 and P0780. This TSB is the latest in a long line of superseded bulletins (including #05-003/20 and #05-002/15), indicating Mazda has been addressing this issue for many years. The TSBs state the cause is metallic contamination within the transmission fluid that causes the internal oil pressure switches to stick. Because of this known issue, the diagnostic path is unique: it starts with a software update for the TCM before any parts are replaced.

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
🎬 Watch: What to do when your AT light comes on
• Harsh or erratic shifting
• Transmission may get stuck in one gear (fail-safe mode, often 3rd gear)
• Delayed engagement when shifting from Park or Neutral into Drive
• Manual shift mode may be disabled
• Vehicle may suddenly disengage the transmission while driving, requiring a restart to function again

Most Likely Causes

1. Sticking Internal Oil Pressure Switches 🔴 High Probability Mazda TSB #05-002-23 and its predecessors explicitly state this is the primary cause. Contamination in the transmission fluid passage causes the switch (specifically Oil Pressure Switch A for P0847), which is mounted on the control valve body, to stick.
   How to confirm: This is confirmed by following the diagnostic procedure in the TSB. If a TCM software update does not resolve the code, the switches are deemed faulty. The TSB recommends replacing both switches A and B at the same time.
   Typical fix: Replacement of the internal oil pressure switches (A and B) located on the valve body. This requires draining the fluid and removing the transmission oil pan. 🎬 See this walkthrough on replacing the internal pressure switches It is also highly recommended to replace the transmission oil filter (strainer) at the same time.
   Est. part cost: $100-$350
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability Mazda released software updates to change the diagnostic logic, making the system less sensitive to minor contamination and preventing the code from being triggered incorrectly. This is the official first step in the repair process.
   How to confirm: A Mazda dealer or a qualified shop with access to Mazda's diagnostic software (MDARS) can check the current TCM calibration file against the latest version listed in the TSB.
   Typical fix: Reprogramming the TCM to the latest software version. A Reddit user with a 2015 CX-5 reported this alone fixed their P0847 and P0780 codes for a cost of $170.
   Est. part cost: $0 (labor only)
3. Low or Contaminated Transmission Fluid ⚪ Low Probability While the root cause is contamination causing a part to stick, the problem is exacerbated by old, degraded fluid that has not been serviced at recommended intervals. The contamination itself is the problem, and the fluid is the carrier.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level, pink/red in color, and not smell burnt. A fluid analysis can provide a definitive answer on its condition.
   Typical fix: Perform a transmission fluid drain and fill or flush. This is often done in conjunction with other repairs, such as the TCM reprogram or switch replacement.
   Est. part cost: $75-$150

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): While possible, this is the least likely cause. All other possibilities, especially those outlined in the TSB, should be exhausted before considering TCM replacement.
• Wiring Harness Issue: A short to power in the circuit between the TCM and the valve body connector could theoretically cause a 'circuit high' fault, but this is uncommon compared to the internal switch issue documented in the TSB.

Diagnosis Steps

1. Verify the code with an OBD-II scanner. Check for any other related codes, particularly P0842, P0780, 🎬 Watch: Understanding the P0847 fault code and sensor circuit P1738, P0872, and P0877.
2. Consult Mazda Technical Service Bulletin #05-002-23 for the official diagnostic flow for your vehicle's VIN.
3. Have a Mazda dealer or qualified independent shop check the TCM software version. If it is outdated, perform the TCM reprogram as the first step.
4. Clear the codes and perform a test drive. If the code does not return, the issue is resolved.
5. If the code returns, the next step is to replace the internal oil pressure switches (A and B).
6. This involves draining the transmission fluid, removing the transmission oil pan, and accessing the switches on the valve body.
7. While the pan is off, inspect for a large amount of foreign material (debris). If found, the TSB recommends replacing the transmission oil filter (strainer).
8. After replacing the switches and filter (if needed), reinstall the pan with a new gasket or sealant, and refill with the correct specification Mazda FZ automatic transmission fluid.
9. Perform the 'Initial Learning' procedure with a compatible scan tool (e.g., Mazda MDARS).
10. Verify the repair with a final test drive to ensure smooth shifting and no warning lights.

Parts You'll Likely Need

• Transmission Oil Pressure Switch A (OEM #FZ01-21-2C0) — This is the specific switch ('Switch No. 2') identified by Mazda's TSB as getting stuck and causing code P0847.
  Trusted brands: Mazda OE
  OEM price range: $50-$80
  Aftermarket price range: $40-$60
• Transmission Oil Pressure Switch B (OEM #FZ01-21-2J0) — The TSB recommends replacing both pressure switches (A and B) at the same time to prevent a comeback for a related code like P0842.
  Trusted brands: Mazda OE
  OEM price range: $50-$80
  Aftermarket price range: $40-$60
• Transmission Oil Filter (Strainer) (OEM #FZ01-21-500) — Highly recommended to be replaced when the oil pan is removed, especially since contamination is the root cause of the switch failure.
  Trusted brands: Mazda OE
  OEM price range: $30-$50
  Aftermarket price range: $20-$40
• Mazda FZ Automatic Transmission Fluid (OEM #0000-FZ-113E-01) — This specific fluid is required to refill the transmission after removing the pan to replace the internal switches. Using the correct fluid is critical for SKYACTIV transmissions.
  Trusted brands: Mazda OE

Related Codes That Often Appear With This One

• P0842 — Transmission Fluid Pressure Sensor/Switch 'A' Circuit Low (per generic definition), which Mazda defines as 'Oil pressure switch No. 1 (oil pressure switch B) stuck on'. This code is listed in the same TSB and is caused by the same issue of sticking internal pressure switches.
• P0780 — Shift Malfunction. This is a general shift error code that is often triggered as a result of the incorrect pressure readings from the faulty switches. It is frequently seen with P0847 in owner forums.
• P1738 — Automatic Transaxle Internal Malfunction. This is a manufacturer-specific code that Mazda's TSB links directly to the same sticking pressure switch problem.
• P0872 — Transmission Fluid Pressure Sensor/Switch 'C' Circuit Low. Mazda defines this as 'Oil pressure switch No. 3 (oil pressure switch A) stuck on'. Also listed in TSB 05-002-23 as being related to the same root cause.
• P0877 — Transmission Fluid Pressure Sensor/Switch 'D' Circuit High. Mazda defines this as 'Oil pressure switch No. 4 (oil pressure switch B) stuck on'. Also listed in TSB 05-002-23 as being related to the same root cause.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND A/T WARNING LIGHT ON WITH DTCs (P0842, P0847, P0872, P0877, P0780, P1738). This is the primary TSB for the 2021 CX-9, identifying sticking oil pressure switches due to contamination as the cause.

Platform-Specific Known Issues

• Mazda has a well-documented history of this issue across multiple models using the SKYACTIV-Drive 6-speed automatic transmission. TSB #05-002-23, which applies to the 2021 CX-9, supersedes numerous older bulletins (like 05-003/20, 05-002/15, etc.) for the same problem on other models, indicating a persistent design or contamination issue that Mazda has addressed through both software and hardware changes.

Mechanic-Grade Diagnostic Values

• ATF Temperature for DTC Detection — expected: 20 °C {68 °F} or more. Failure: The TCM will not run the diagnostic test for P0847 if the transmission fluid is below this temperature.
• ATF Temperature for Post-Repair On-Board Diagnostic Test — expected: 55 °C {131 °F} or more. Failure: The on-board diagnostic test to verify the repair will not run until the fluid reaches this temperature.
• Oil Pressure Switch Fitting Bolt Torque — expected: 9—10 N·m {80—88 in·lbf}. Failure: Incorrect torque can lead to leaks or damage to the switch or valve body.
• Oil Pan Bolt Torque — expected: 8—10 N·m {71—88 in·lbf}. Failure: Incorrect torque can cause transmission fluid leaks.

Scan Tool Commands That Help

• Mazda M-MDS / FORScan: Initial Learning Procedure — This procedure must be performed after replacing the oil pressure switches to calibrate the new components with the TCM.
• Mazda M-MDS / FORScan: Clear TCM Adaptive Tables — After a repair, clearing the learned shift adaptations can help the transmission relearn shift points with the new components, potentially resolving lingering shift quality issues.
• Mazda M-MDS / FORScan: TCM Reprogramming — This is the first step in the official TSB repair procedure to update the diagnostic logic, which may resolve the code without parts replacement.

Wiring & Ground Locations

• Under-hood Fuse Box — Located in the engine bay on the driver's side, near the 12-volt battery.. This fuse box contains fuses and relays for the main vehicle electrical systems, including power feeds to the TCM. While a fuse is not a common cause for P0847, it's a primary location to check for any related electrical issues.
• Transmission Control Module (TCM) — The TCM for the Skyactiv-Drive transmission is integrated into the valve body assembly, inside the transmission oil pan.. Since the TCM is internal, the wiring from the pressure switches to the module is not exposed to external damage. This reinforces the TSB's conclusion that the cause is almost always internal (stuck switch, software, or the TCM itself) rather than an external wiring fault.

Real Owner Repair Stories

• Reddit user in /r/mazda (2015 Mazda CX-5 Touring, FWD, ~100k miles) — Vehicle went into limp mode, Check Engine Light and A/T light came on.
  ✅ What actually fixed it The dealer performed a TCM reprogram as per the TSB. This alone resolved the P0780 and P0847 codes. The cost was $170 for the diagnosis and reprogram, plus an additional charge for a transmission flush.

OEM Part Supersession History

• fza1212c0 → FZ01-21-2C0 — Part number format update and potential internal revisions.