P0780 on 2021 Mazda CX-9: Shift Malfunction Causes and Fixes

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

For the 2021 CX-9 and its SKYACTIV-Drive automatic transmission, the P0780 code is not a generic mystery. Mazda has explicitly addressed it in Technical Service Bulletin #05-002-23. 🎬 Watch: A breakdown of the TSB and the required fix. The bulletin states the cause is contamination in the transmission fluid causing internal oil pressure switches to stick. Unlike some other Mazda models where a TCM software update is the first step, the TSB's diagnostic chart explicitly directs technicians working on a 2021 CX-9 to proceed directly with replacing the faulty oil pressure switches.

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 and/or AT (Automatic Transaxle) warning light is on.
• Harsh or erratic shifting, including aggressive downshifts when braking.
• Transmission gets stuck in one gear (limp-home mode, often 3rd gear).
• Delayed engagement when shifting from Neutral to Drive.
• Slipping between gears.
• Reduced power and acceleration, sometimes requiring a restart to temporarily fix.
• Whining noise from the transmission.

Most Likely Causes

1. Sticking Internal Oil Pressure Switches 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSB #05-002-23 explicitly identifies this as the primary cause. Contamination in the automatic transmission fluid (ATF) causes oil pressure switches A and/or B in the control valve body to get stuck, sending incorrect data to the TCM. The TSB's diagnostic flowchart for the 2021 CX-9 specifically points to 'Oil Pressure Switch Replacement' as the correct procedure.
   How to confirm: A technician will confirm the presence of P0780, often with related codes like P0842, P0847, P0872, P0877, or P1738. Following the TSB for a 2021 CX-9, the diagnostic procedure points directly to replacing the switches.
   Typical fix: Replace the oil pressure switch A and B kit inside the transmission valve body. This service includes draining the fluid, dropping the pan, replacing the switches, and refilling with new Mazda FZ ATF. An 'Initial Learning' procedure must be performed afterward using a Mazda-compatible diagnostic tool.
   Est. part cost: $90-$180

Rare But Worth Checking

• Low or Dirty Transmission Fluid: → Shop Transmission Assembly While the TSB points to contamination causing switches to stick, severely low or degraded fluid can cause a variety of shifting problems on its own. It's always a fundamental check, though less likely to be the sole cause on this specific vehicle for this code. A transmission flush was part of a successful repair for a similar issue on a CX-5.
• Transmission Control Module (TCM) or Wiring Issue: → Shop Transmission Assembly A wiring fault to the transmission or a failure of the TCM itself is possible but very rare compared to the known pressure switch issue. This should only be considered after the TSB-specified repair has been performed without success. An older TSB for similar codes did involve replacing the entire valve body which contains the TCM, but this has been superseded.

Diagnosis Steps

1. Verify the battery is fully charged and the charging system is working correctly. Some owners report various electronic malfunctions, including limp mode, from a weak battery.
2. Use an OBD-II scanner capable of reading transmission codes to confirm P0780 and check for any other related DTCs (e.g., P0842, P0847, P0872, P0877, P1738).
3. Consult Mazda Technical Service Bulletin (TSB) #05-002-23.
4. For a 2021 CX-9, follow the TSB's diagnostic chart which directs the repair to 'Oil Pressure Switch Replacement'.
5. Drain the automatic transmission fluid (ATF).
6. Remove the transmission oil pan.
7. Locate and replace the Oil Pressure Switch A and B assembly on the control valve body.
8. Inspect the bottom of the oil pan for excessive metal debris. If a large amount is found, the oil strainer should also be replaced.
9. Install a new transmission pan gasket and reinstall the pan, torquing bolts to manufacturer specifications (typically 9-10 N-m or 80-88 in-lbf).
10. Refill the transmission with the correct type and amount of new Mazda FZ Automatic Transmission Fluid.
11. Perform the 'Initial Learning' procedure for the transmission as specified by Mazda service information using a tool like the Mazda IDS.
🎬 See how to perform a transmission reset and learning procedure.
12. Clear all DTCs and perform a test drive to verify the repair.

Parts You'll Likely Need

• Transmission Oil Pressure Switch Kit (A and B) (OEM #FZ01-21-2C0) — This is the component identified by Mazda in TSB #05-002-23 as the primary cause of the P0780 fault due to sticking from contamination. This part number supersedes FZA1-21-2C0.
  Trusted brands: Mazda OE
  OEM price range: $90-$130
  Aftermarket price range: $40-$120
• Automatic Transmission Fluid (ATF FZ) (OEM #0000-FZ-113E-01) — New fluid is required when replacing internal transmission components. Using the correct Mazda FZ fluid is critical for the performance and longevity of the SKYACTIV-Drive transmission.
  Trusted brands: Mazda OE, Idemitsu, Ravenol
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart
• Transmission Oil Pan Gasket (OEM #FZ01-19-4J0 (example, verify by VIN)) — The gasket should always be replaced when the transmission oil pan is removed to ensure a proper seal and prevent leaks. The exact part number can vary.
  Trusted brands: Mazda OE
  OEM price range: $25-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0842 — This code is for 'Transmission Fluid Pressure Sensor/Switch A Circuit Low', which is one of the switches identified as faulty in the TSB.
• P0847 — This code is for 'Transmission Fluid Pressure Sensor/Switch B Circuit High', another switch implicated in the TSB. A Reddit user with a CX-5 reported this code alongside P0780.
• P0872 — This code relates to an oil pressure switch being stuck on, pointing to the same valve body assembly fault.
• P0877 — This code also relates to an oil pressure switch fault, further confirming the issue described in the TSB.
• P1738 — This Mazda-specific code is for a clutch pressure control solenoid valve or 'Automatic Transaxle Internal Malfunction', which is part of the same hydraulic control system and is explicitly listed in TSB 05-002-23.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND A/T WARNING LIGHT ON WITH ONE OR MORE OF THESE DTCS (P0842, P0847, P0872, P0877, P0780, P1738) STORED IN TCM MEMORY.

Platform-Specific Known Issues

• Mazda has issued Technical Service Bulletin #05-002-23 which directly addresses this code on the 2021 CX-9. It states the cause is contamination causing oil pressure switches in the control valve body to stick and advises replacing the switches as the corrective action for this model year, bypassing the software update step recommended for other models/years.

Mechanic-Grade Diagnostic Values

• On/Off Solenoid Resistance — expected: 11-14 ohms. Failure: A reading outside this range suggests a faulty solenoid.
• Pressure Control Solenoid Resistance — expected: 5-7 ohms. Failure: A reading outside this range suggests a faulty solenoid.
• Oil Pressure Switch Continuity Test — expected: Infinite resistance (no continuity) in Park; 0 ohms (continuity) in Drive.. Failure: Readings that do not change between Park and Drive indicate a stuck switch.
• ATF Temperature for Initial Learning — expected: 60—110 °C (140—230 °F). Failure: The initial learning procedure will not start or will abort if the fluid temperature is outside this range.
• Oil Pressure Switch Bolt Torque — expected: 9-10 N·m (80-88 in-lbf). Failure: Incorrect torque can cause leaks or damage to the valve body or switch.

Hidden / Shadow Codes Worth Checking

• P06B8:00: Non-Volatile Random Access Memory (NVRAM) Error. This code can be set if the 'Initial Learning' procedure for the transmission fails or aborts. (see via Standard OBD-II scanner capable of reading transmission control module codes.)

Scan Tool Commands That Help

• Mazda M-MDS / IDS: Initial Learning Procedure — Required after replacing the oil pressure switches or valve body. This function allows the TCM to learn the hydraulic response times of the new components to ensure smooth shifting.
• No Scan Tool Required: Manual Initial Learning Procedure — Can be used to reset and relearn transmission shift adapts if a dealer tool is unavailable. It involves a specific sequence of shifting and engine start/stop events with the brake pedal depressed and ATF at operating temperature (60-110°C).
• Mazda M-MDS / IDS: Module Reprogramming (TCM) — This is the primary first step for this code on many Mazda models, but TSB #05-002-23 directs technicians to skip this for the 2021 CX-9 and proceed to parts replacement.

Wiring & Ground Locations

• Main Transmission Ground — Located on a bracket on top of the transmission housing, where the main ground cable from the negative battery terminal connects.. A poor or corroded ground at this location can cause a wide range of electrical issues for the transmission, including erratic sensor readings and incorrect solenoid operation, which could potentially trigger a P0780 code or other related faults.

OEM Part Supersession History

• FZA1-21-2C0 → FZ01-21-2C0 — Part revision or consolidation. The FZ01-21-2C0 part is a pressure sensor assembly that contains two switches.
  Heads up: The newer part number should be used. Older TSBs sometimes listed individual switches 'A' (FZ01-21-2C0) and 'B' (FZ01-21-2J0), but the current fix typically involves replacing the single assembly.
• FZ01-21-2C0 (as Switch A) → FZ01-21-2C0 (as assembly) — This part, described as 'Pressure sensor [A]', monitors the pressure in the 2-6 Brake and 3-5-Reverse clutch packs.
  Heads up: This provides detail on the function of one of the switches within the assembly.