P0780 on 2016-2020 Mazda CX-9: Shift Malfunction Causes and Fixes

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

For the 2016-2020 CX-9 and other Mazdas using the SKYACTIV-Drive 6-speed automatic, the P0780 code is specifically addressed in Technical Service Bulletin #05-002-23. The bulletin explains that contamination in the transmission fluid can cause internal oil pressure switches (Switch A and/or B) to stick temporarily or permanently. Unlike a generic shift solenoid failure, the primary documented fix for this platform often starts with a software update (TCM reprogram) to improve the diagnostic logic before any parts are replaced. The transmission itself, the FW6A-EL, is an in-house Mazda design that combines features of traditional automatic, CVT, and dual-clutch transmissions, with an internal TCM.

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 illuminated.
• Harsh, erratic, or delayed shifting.
• Transmission slipping (engine RPMs rise without an increase in vehicle speed).
• Vehicle entering "limp-home" mode, often stuck in a single gear.
• Failure to shift into or out of a specific gear.
• A feeling of the transmission being 'clumsy' or shuddering at low speeds.

Most Likely Causes

1. Sticking Oil Pressure Switch due to Fluid Contamination 🔴 High Probability Mazda issued TSB #05-002-23 specifically for this issue, stating that contamination in the passage can cause the oil pressure switch A and/or B in the control valve body to stick. These switches detect engagement pressure for various clutches and brakes within the transmission.
   How to confirm: A Mazda dealer or qualified shop will first check for the presence of P0780 and related codes (P0842, P0847, P0872, P0877, P1738). Following the TSB, they will attempt a TCM reprogram. If the issue persists after the software update, the switches are the confirmed cause.
   Typical fix: The first step is reprogramming the TCM with updated software. If that does not resolve the issue, the oil pressure switches A and B inside the transmission valve body must be replaced. This requires removing the valve body.
   Est. part cost: $50-$150 for the switches
2. Outdated Transmission Control Module (TCM) Software 🔴 High Probability The official TSB repair path starts with a software update, indicating Mazda has improved the diagnostic logic to be more tolerant or to better manage the conditions that cause the code. This suggests the original software was too sensitive.
   How to confirm: A Mazda dealership can check the current software version of the TCM and determine if an update is available per TSB #05-002-23.
   Typical fix: Reprogramming the TCM using Mazda's official diagnostic tool (IDS). A Reddit user with a 2015 CX-5 reported a successful fix with just the reprogram and a transmission flush, costing $170 for the diagnosis/reprogram.
   Est. part cost: $0 (labor cost only, typically $150-$300 for diagnosis and reprogram)
3. Low or Dirty Transmission Fluid 🟡 Medium Probability While the TSB points to contamination being the root cause, low or degraded fluid can cause similar symptoms like poor hydraulic pressure and shifting issues in any automatic transmission. The SKYACTIV-Drive transmission is specified to use Mazda ATF-FZ, a low-viscosity fluid, and using incorrect fluid can cause issues.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level, bluish (for FZ type), and not smell burnt. Note: The FW6A-EL transmission does not have a traditional dipstick, requiring a specific procedure involving a check plug and specific temperatures to verify the level. 🎬 See how to correctly change Mazda Skyactiv ATF FZ fluid.
   Typical fix: Perform a transmission fluid drain and fill or a full flush using the correct Mazda FZ specification fluid. A flush was recommended by a dealer in one owner's repair story.
   Est. part cost: $150-$300 for fluid and filter
4. Failed Shift Solenoid ⚪ Low Probability While possible, this is less likely to be the primary cause on this specific vehicle than the TSB-documented issue with pressure switches. A shift solenoid failure would typically be accompanied by a more specific solenoid code (e.g., P0751-P0758). However, some owners have experienced solenoid-related codes alongside general shifting issues.
   How to confirm: Test the resistance and operation of individual shift solenoids. This requires accessing the transmission's valve body and a specialized scan tool.
   Typical fix: Replace the faulty shift solenoid. Note that Mazda does not sell individual solenoids for the FW6A-EL as they are calibrated to the TCM; they are replaced as part of the valve body assembly.
   Est. part cost: $50-$100 per solenoid

Rare But Worth Checking

• Internal Mechanical Transmission Failure: This is a worst-case scenario. If the transmission fluid contains a significant amount of metal shavings, it could indicate a failure of clutches, bands, or other internal components requiring a transmission rebuild or replacement. The average lifespan of a CX-9 transmission is estimated to be between 130,000-180,000 miles.
• Faulty Transmission Control Module (TCM): While a software issue is common, a complete hardware failure of the TCM itself is rare but possible. The TCM is located inside the transmission on the FW6A-EL. This should only be considered after all other possibilities, including wiring, have been exhausted. Replacement can cost over $1,000.

Diagnosis Steps

1. Scan for Codes: Use an OBD-II scanner that can read transmission codes. Note P0780 and any other codes present, especially those listed in TSB #05-002-23.
2. Check for TSBs: Specifically search for TSB #05-002-23 for your vehicle's VIN. This is the most critical step. Note that this TSB supersedes several older ones.
3. Check Transmission Fluid: Have a shop check the transmission fluid level and condition. The correct procedure is critical as there is no dipstick. The fluid should be bluish (Mazda FZ) and not burnt or full of debris.
4. Follow TSB #05-002-23: The recommended procedure is to first have a Mazda dealer perform a TCM software update. This requires the Mazda Diagnostic and Repair Software (MDARS) or IDS.
5. Test Drive: After the reprogram, the technician should perform the 'Initial Learning' procedure as specified by Mazda and test drive the vehicle to see if the shifting issue is resolved.
6. Replace Oil Pressure Switches: If the reprogram does not fix the problem, the next step according to the TSB is to replace oil pressure switches A and B. This involves removing the transmission oil pan and control valve body.
7. Inspect Wiring: If the TSB does not apply or does not fix the issue, inspect the transmission wiring harness for any signs of damage, corrosion, or loose connections between the TCM and the main harness.
8. Further Mechanical/Electrical Diagnosis: If the problem persists, more advanced diagnostics are needed to test the valve body, or check for internal mechanical problems, which may require a transmission specialist.

Parts You'll Likely Need

• Transmission Oil Pressure Switch (OEM #FZ01-21-2C0 (Example for Switch A, verify by VIN)) — Identified in Mazda TSB #05-002-23 as the likely hardware cause if a TCM reprogram doesn't work. They get stuck due to contamination.
  Trusted brands: Mazda (OEM)

Related Codes That Often Appear With This One

• P0842 — Transmission Fluid Pressure Sensor/Switch 'A' Circuit Low. All are mentioned in the same TSB as being caused by sticking pressure switches.
• P0847 — Transmission Fluid Pressure Sensor/Switch 'B' Circuit High. All are mentioned in the same TSB as being caused by sticking pressure switches.
• P0872 — Transmission Fluid Pressure Sensor/Switch 'C' Circuit Low. All are mentioned in the same TSB as being caused by sticking pressure switches.
• P0877 — Transmission Fluid Pressure Sensor/Switch 'D' Circuit High. All are mentioned in the same TSB as being caused by sticking pressure switches.
• P1738 — A manufacturer-specific code for 'Automatic Transmission Internal Malfunction' also listed in TSB #05-002-23, indicating it's related to the same underlying fault.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23
• 05-004/18
• 05-002/15

Platform-Specific Known Issues

• TSB #05-002-23: Mazda Technical Service Bulletin #05-002-23 (and its predecessors like 05-004/18 and 05-002/15) explicitly details the cause for P0780 and related codes on 2016-2019 and 2021 CX-9 models. It points to contamination causing oil pressure switches to stick and outlines a two-pronged repair: first, reprogram the TCM, and second, if the problem persists, replace the switches.

Mechanic-Grade Diagnostic Values

• On/Off Solenoid Resistance — expected: 11-14 ohms. Failure: A reading outside this range indicates a faulty solenoid.
• Pressure Control Solenoid Resistance — expected: 5-7 ohms. Failure: A reading outside this range indicates a faulty solenoid.
• Oil Pressure Switch A and B Bolt Torque — expected: 9-10 N·m {80-88 in·lbf}. Failure: Incorrect torque can lead to leaks or damage.

Scan Tool Commands That Help

• Mazda M-MDS (IDS): Initial Learning Procedure — Required after replacing the valve body, oil pressure switches, or the entire transmission assembly to calibrate the TCM to the new components' hydraulic characteristics. The procedure involves a specific sequence of shifter movements and engine states to allow the TCM to learn clutch engagement points.
• Mazda M-MDS (IDS): Module Reprogramming (TCM) — This is the first step in the repair procedure outlined by TSB #05-002-23 and its successors. It updates the diagnostic logic to prevent false positives for P0780 and related codes.

Wiring & Ground Locations

• Main Transmission Ground — On a bracket on top of the transmission housing. The main ground cable from the negative battery terminal connects here.. A poor ground connection at this point can cause a variety of electrical issues for the transmission, including erratic sensor readings and incorrect solenoid operation, potentially leading to a P0780 code.

Real Owner Repair Stories

• Reddit user in r/mazda (2015 Mazda CX-5 with ~100k miles) — Vehicle went into limp mode with Check Engine Light and AT warning light on. Codes P0780 & P0847 were stored.
  ✅ What actually fixed it The user followed TSB 05-002/15. The dealership performed a TCM reprogram and a transmission flush, which resolved the issue. The cost was $170 for diagnosis and reprogramming, and $270 for the flush.
• Reddit user in r/mazda (2017 Mazda CX-5 with 120k miles) — Check Engine Light with code P0847 (often seen with P0780).
  ❌ Tried (didn't work) Dealer quoted $1040 to replace only Switch B, and over $8700 for a full transmission replacement, without suggesting a fluid change.
  ✅ What actually fixed it The owner noted that the TSB specifies replacing BOTH switches (A and B), not just one. The final resolution was not posted, but the story highlights a common misdiagnosis/incomplete repair attempt by a dealer, which is to replace only one of the two switches implicated in the TSB.

OEM Part Supersession History

• FZA1-21-2C0 → FZ01-21-2C0 — Part number update/revision. FZ01-21-2C0 is listed as the replacement for the older number.

Model Year Variations Within This Range

• 2014+: An older TSB (05-003/14) from 2014 for P0780 on early SKYACTIV models recommended replacing the entire valve body assembly. Later TSBs (like 05-002/23) for newer models, including the 2016-2020 CX-9, revised this procedure to start with a TCM reprogram and, if needed, replacement of only the two oil pressure switches, which is a much less expensive repair.