P0872 on 2016-2020 Mazda CX-9: Transmission Pressure Switch Causes and Fixes

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

The 2016-2020 CX-9 uses Mazda's SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL family). For this specific transmission family, Mazda has issued multiple Technical Service Bulletins (TSBs), most recently #05-002-23, acknowledging that internal oil pressure switches can get stuck due to contamination in the fluid. This makes the problem a well-documented pattern failure for this vehicle and its platform mates (CX-5, Mazda6, etc.). The official repair path is to replace only the small switches, which is far less expensive than replacing the entire valve body, a common misdiagnosis.

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
• AT (Automatic Transaxle) warning light is on
🎬 Watch: How to fix the AT light and transmission codes.
• Harsh, erratic, or delayed gear shifts
• Transmission may feel like it's slipping or holding gears too long
• Vehicle may enter 'limp mode', limiting available gears
• In some cases, no noticeable driving issues other than the warning lights

Most Likely Causes

1. Failed Internal Transmission Oil Pressure Switch 🔴 High Probability Mazda has identified this as a pattern failure in TSB #05-002-23, stating that contamination in the transmission fluid can cause the switches to stick. The switches are sensitive to microscopic debris that can impede their mechanical movement.
   How to confirm: A technician will confirm the code with a scan tool. The diagnosis is confirmed by following the procedure in the TSB, which may involve a TCM software update first, followed by parts replacement if the code returns or the software is current.
   Typical fix: Replace both oil pressure switches ('A' and 'B') on the transmission valve body. This includes draining the fluid, dropping the oil pan, replacing the switches, reinstalling the pan with a new gasket, and refilling with new Mazda FZ automatic transmission fluid.
   Est. part cost: $80-$150
2. Contaminated or Low Transmission Fluid 🟡 Medium Probability The pressure switches are sensitive to debris. While Mazda transmissions are sealed ('lifetime fluid') and don't have a dipstick for easy checking, fluid can degrade or become contaminated over time from normal wear, leading to this issue. The TSB notes contamination as the root cause of the switches sticking.
   How to confirm: When the transmission pan is removed to inspect the switches, the technician will inspect the condition of the fluid and check for excessive metal shavings or clutch material on the pan magnets.
   Typical fix: Perform a transmission fluid drain and fill. If significant contamination is found, the transmission oil strainer (filter) should also be replaced. This is done as part of the pressure switch replacement.
   Est. part cost: $100-$200
3. Outdated Transmission Control Module (TCM) Software ⚪ Low Probability Mazda released software updates to improve the diagnostic logic and sensitivity for these codes. An older software version might trigger the code prematurely. However, by now, most vehicles have either received this update or the issue is a hard part failure.
   How to confirm: A Mazda dealer or a shop with a compatible scan tool (like Mazda IDS) can check the current software calibration against the latest version listed in the TSB.
   Typical fix: Reprogram the TCM to the latest software calibration. Per the TSB, this is the first step for some vehicles, but if the code returns, the switches must be replaced.
   Est. part cost: $0-$200

Rare But Worth Checking

• Wiring Harness Issue: While less common than the switch failure itself, damage to the internal transmission wiring harness leading to the pressure switches could cause a similar electrical fault. A wiring recall in 2019 for some CX-9s addressed different symptoms but highlights potential harness vulnerability.
• Failed Transmission Control Module (TCM): In very rare cases, the TCM itself could be faulty. This should only be considered after all other possibilities, especially the pressure switches and wiring, have been ruled out.

Diagnosis Steps

1. Verify the code P0872 and any other related codes are present in the TCM using an OBD-II scanner.
2. Check for applicable Technical Service Bulletins (TSBs). For this code, TSB #05-002-23 is the primary document.
3. Following the TSB, a technician may first attempt to reprogram the TCM to the latest software version, depending on the vehicle's current software calibration.
4. If the code returns or the software is already current, the next step is to drain the transmission fluid and remove the transmission oil pan.
5. Inspect the fluid and pan magnets for signs of excessive contamination or large metal debris. If significant debris is found, it may indicate a more severe internal failure.
6. Locate and replace both oil pressure switches (A and B) on the valve body.
7. It is highly recommended to replace the transmission oil strainer (filter) at the same time, especially if the fluid showed signs of contamination.
8. Reinstall the oil pan with a new gasket or sealant (Mazda specifies sealant TB1217E or equivalent), and refill the transmission with the correct type and amount of Mazda FZ automatic transmission fluid.
9. Perform the 'Initial Learning' procedure for the transmission as specified by Mazda, which allows the TCM to adapt to the new components.
10. Clear all DTCs and perform a road test to verify the repair and ensure the code does not return.

Parts You'll Likely Need

• Transmission Oil Pressure Switch 'A' (OEM #FZ01-21-2C0) — This is the primary switch indicated by code P0872. It is the most likely component to have failed.
  Trusted brands: Mazda (OEM)
  OEM price range: $40-$70
  Aftermarket price range: $25-$45
• Transmission Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — Mazda's TSB recommends replacing both switches 'A' and 'B' at the same time, as they are prone to the same failure.
  Trusted brands: Mazda (OEM)
  OEM price range: $40-$70
  Aftermarket price range: $25-$45
• Automatic Transmission Fluid (Mazda FZ) (OEM #0000-FZ-113E-01) — The transmission must be drained to perform the repair and refilled with the correct OEM fluid. Approximately 4-5 quarts are needed for a drain and fill.
  Trusted brands: Mazda (OEM)
  OEM price range: $18-$25 per quart
  Aftermarket price range: $12-$20 per quart
• Transmission Oil Pan Gasket (OEM #FZU1-21-51X (This is the pan, gasket may be separate or RTV is used)) — The gasket should be replaced whenever the transmission oil pan is removed to ensure a proper seal. Some applications may use sealant instead of a pre-formed gasket.
  Trusted brands: Mazda (OEM), Fel-Pro TOS 18731
  OEM price range: $20-$40
  Aftermarket price range: $15-$30
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — Recommended to be replaced during the service, as contamination is the root cause of the switch failure.
  Trusted brands: Mazda (OEM)
  OEM price range: $45-$60

Related Codes That Often Appear With This One

• P0842 — This code is for 'Oil Pressure Switch No. 1 (Switch B) Stuck On' and is caused by the same issue. Mazda's TSB recommends replacing both switches at the same time.
• P0847 — This code is for 'Oil Pressure Switch No. 2 (Switch A) Stuck On' and is part of the same family of faults.
• P0877 — This code is for 'Oil Pressure Switch No. 4 (Switch B) Stuck On' and is also caused by the same contamination issue.
• P0780 — This code indicates a 'Gear Shifting Malfunction' and can be triggered as a direct result of the pressure switch providing incorrect information to the TCM.
• P1738 — This is a manufacturer-specific code for an 'Automatic Transaxle Internal Malfunction' that Mazda groups with the pressure switch codes in its TSB.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: The most recent TSB, issued May 17, 2023, covering 2016-2021 CX-9 and other models. It identifies DTCs P0842, P0847, P0872, P0877, P0780, and P1738 as being caused by sticking oil pressure switches due to contamination.
• 05-002/15: An older version of the TSB from 2015 that details the same issue and repair procedure, including diagrams for replacing the switches on the valve body.

Platform-Specific Known Issues

• A known issue documented in Mazda TSB #05-002-23 involves contamination causing oil pressure switches to stick, triggering this code and others. This TSB has been updated multiple times and covers a wide range of Mazda models with the SKYACTIV-Drive 6-speed AT from 2012-2021.

Mechanic-Grade Diagnostic Values

• Continuity/Resistance of Pressure Switch Circuit — expected: Below 1 Ω. Failure: Resistance higher than 1 Ω indicates an open or high resistance in the circuit.
• Short to Ground Test of Pressure Switch Circuit — expected: 10 kΩ or higher. Failure: Resistance lower than 10 kΩ indicates a short to ground.

Hidden / Shadow Codes Worth Checking

• P06B8:00: This DTC indicates that the 'Initial Learning' procedure has not been successfully completed after a relevant part replacement. (see via This code is stored in the TCM and is visible with a dealer-level scan tool like the Mazda M-MDS. It serves as a flag to the technician that a required procedural step was missed.)

Scan Tool Commands That Help

• Mazda M-MDS (or equivalent): Initial Learning — This procedure is mandatory after replacing the oil pressure switches or the control valve body. It allows the TCM to adapt to the new components and prevent issues like harsh shifting or engine racing. Failure to perform this step will often result in poor shift quality and may set code P06B8:00.
• Mazda M-MDS / GTS (or equivalent): Active Test: Control the Shift Position — This function allows a technician to manually command gear shifts while monitoring the live data for the pressure switches (e.g., 'TPS 1 Switch'). It can be used to verify if the switch is stuck ON or OFF under specific gear commands, helping to confirm the failure before disassembly.

Wiring & Ground Locations

• Main Transmission Ground — On top of the transmission case. A main ground cable from the negative battery terminal bolts to a bracket on the transmission.. A poor ground connection at this location can cause a host of electrical issues for the transmission, including erratic sensor readings and incorrect solenoid operation. While not the most common cause for P0872, it's a fundamental check for any electrical fault.
• F73 Connector — The main transmission wire harness connector.. Pin 17 of this connector is the circuit for the pressure switch. It is a key test point for checking continuity and shorts to ground between the transmission and the TCM without having to access the internal components first.

OEM Part Supersession History

• fza1212c0 → FZ01-21-2C0 — Part revision or number system update.
  Heads up: The new part number FZ01-21-2C0 is the correct replacement.

Model Year Variations Within This Range

• 2020: For the 2020 model year, Mazda increased the engine's torque output by 10 lb-ft (to 320 lb-ft) when using 93-octane premium fuel. This was a software and engine calibration change and did not involve mechanical changes to the FW6A-EL transmission itself. The P0872 fault remains common across the entire 2016-2020 range.