P0872 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, equipped with the SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL family) 🎬 Watch: A detailed overview of the Skyactiv Drive transmission system., is subject to a well-documented issue outlined in multiple Technical Service Bulletins. The problem is rarely a wiring fault or a simple sensor failure. Instead, it's a pattern issue where contamination inside the transmission's control valve body causes one or more oil pressure switches to stick. This is why P0872 is frequently seen with a cluster of other pressure switch and gear ratio codes (like P0842, P0847, P0780), pointing to a systemic internal fault rather than a simple electrical problem. The official Mazda repair involves replacing the switches, not the entire valve body, making it a more targeted and less expensive fix than a common misdiagnosis might suggest.

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 ON
• Automatic Transaxle (AT) warning light ON
• Harsh or erratic shifting
• Transmission stuck in one gear (limp mode)
• Delayed gear engagement
• Transmission shifts to a high gear and will not downshift

Most Likely Causes

1. Sticking Internal Transmission Oil Pressure Switch 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSB #05-002-23 explicitly states this is caused by contamination entering a passage in the control valve body, causing the oil pressure switches (A and/or B) to stick temporarily or permanently. This is a known pattern failure for the SKYACTIV-Drive 6-speed transmission (FW6A-EL) across many Mazda models.
   How to confirm: The presence of P0872 along with other codes mentioned in the TSB (P0842, P0847, P0877, P0780, P1738) is a strong confirmation of this specific fault.
   Typical fix: Replace both oil pressure switches (A and B) located on the transmission valve body. An 'Initial Learning' procedure with a Mazda diagnostic tool is required after replacement. A transmission fluid and filter change is also performed as part of the service.
   Est. part cost: $150-$250
2. Faulty Transmission Control Module (TCM) Software ⚪ Low Probability → Shop Transmission Assembly Older versions of the TSB for this issue included a TCM software update as a potential first step for some vehicles to change the diagnostic logic. While less common for a 2021 model which should have newer software, it's a possibility that a dealer would check for available updates first.
   How to confirm: A Mazda dealership or qualified shop can check the current TCM calibration file against the latest version specified in TSB #05-002-23.
   Typical fix: Reprogram the TCM to the latest software version. If the code returns, the pressure switches must be replaced.
   Est. part cost: $0

Rare But Worth Checking

• Wiring Harness or Connector Issue: While the TSB points to an internal issue, it's always wise to rule out external problems. A damaged harness or corroded connector could mimic the symptoms of a failed switch. The internal transmission wiring harness could also be a point of failure.
• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly A hardware failure in the TCM itself is rare but possible. This should only be considered after all other possibilities, including the TSB-documented switch failure, have been exhausted.

Diagnosis Steps

1. Scan the Transmission Control Module (TCM) for all stored codes. Note if P0872 is accompanied by P0842, P0847, P0877, P0780, or P1738, as this cluster strongly indicates the TSB-related issue.
2. Check for applicable Technical Service Bulletins. TSB #05-002-23 is the primary document for this issue.
3. Based on the TSB, determine if the vehicle requires a TCM reprogram or the oil pressure switch replacement. For a 2021 CX-9, the TSB points toward switch replacement.
4. Check the transmission fluid level and condition. While unlikely to be the root cause, contaminated fluid is the reason the switches fail and indicates the need for service.
5. If proceeding with the TSB repair, remove the transmission oil pan to access the control valve body.
🎬 See this walkthrough on how to remove a valve body.
6. Carefully remove and replace both oil pressure switches (A and B) on the valve body as per the TSB instructions. The switches are held by bolts torqued to 9-10 N·m (80-88 in-lbf).
7. Reinstall the valve body and oil pan with a new gasket and a new transmission filter/strainer.
8. Refill the transmission with the correct fluid (Mazda ATF-FZ).
9. Perform the 'Initial Learning' procedure using a compatible diagnostic tool (like the Mazda Modular Diagnostic System - MDARS) to allow the TCM to adapt to the new components.
10. Clear all DTCs and perform a test drive to verify the repair.

Parts You'll Likely Need

• Transmission Oil Pressure Switch A (OEM #FZ01-21-2C0) — This is one of the two switches (also called 'Switch A Oil Pressure') identified in Mazda TSBs as the point of failure due to internal contamination. P0872 specifically points to a fault in this switch assembly.
  Trusted brands: Mazda OE
  OEM price range: $70-$120
  Aftermarket price range: $50-$90
• Transmission Oil Pressure Switch B (OEM #FZ01-21-2J0) — This is the second of two switches (also called 'Switch B Oil Pressure') that must be replaced according to the TSB repair procedure, as it is susceptible to the same contamination.
  Trusted brands: Mazda OE
  OEM price range: $70-$120
  Aftermarket price range: $50-$90
• Automatic Transmission Fluid (OEM #K020-W0-049F (ATF-FZ)) — The transmission fluid must be drained to perform the repair and refilled with the correct type, which is blue-colored Mazda ATF-FZ.
  Trusted brands: Mazda ATF-FZ
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart
• Transmission Oil Pan Gasket (OEM #FW01-21-51X) — The oil pan must be removed to access the valve body, and the gasket should be replaced to prevent leaks.
  Trusted brands: Mazda OE
  OEM price range: $30-$50
  Aftermarket price range: $20-$40
• Transmission Oil Filter (Strainer) (OEM #FZ01-21-500) — Since contamination is the root cause, replacing the internal filter/strainer during the service is a critical step.
  Trusted brands: Mazda OE
  OEM price range: $40-$60
  Aftermarket price range: $25-$45

Related Codes That Often Appear With This One

• P0842 — Relates to Oil Pressure Switch No. 1 (Switch B) sticking on, caused by the same contamination issue.
• P0847 — Relates to Oil Pressure Switch No. 2 (Switch A) sticking on, part of the same known fault.
• P0877 — Relates to Oil Pressure Switch No. 4 (Switch B) sticking on, also caused by contamination in the valve body.
• P0780 — A general 'Gear Shifting Malfunction' code that is triggered as a result of the pressure switches providing incorrect data to the TCM.
• P1738 — An 'Automatic Transaxle Internal Malfunction' code that often accompanies the specific pressure switch codes.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND AT WARNING LIGHT ON WITH DTC P0842 / P0847 / P0872 / P0877 / P0780 / P1738. This TSB confirms the cause is sticking oil pressure switches and provides the repair procedure for a wide range of Mazda vehicles, including the CX-9.

Platform-Specific Known Issues

• A known issue documented in TSB #05-002-23 affects 2016-2021 CX-9 models, stating that contamination in the control valve body can cause oil pressure switches to stick, triggering P0872 and other related codes.
• A Reddit user on r/mazda with a similar vehicle experienced a cluster of warning lights (Check Engine, AT, Traction Control) and a code P0877, where another user correctly identified it as the known issue covered by TSB #05-002-23 requiring pressure switch replacement.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity to Ground (Engine Running) — expected: Infinite resistance (no continuity) in Park or Neutral; 0 ohms (continuity) in Drive.. Failure: Continuity to ground while in Park or Neutral indicates a stuck-on switch.
• ATF Temperature for Diagnostic Condition — expected: 20 °C {68 °F} or more.. Failure: The self-test for P0872 will not run if the transmission fluid is colder than this temperature.

Hidden / Shadow Codes Worth Checking

• P06B8:00: Internal Control Module Non-Volatile Random Access Memory (NVRAM) Error. This code can appear if the mandatory 'Initial Learning' procedure has not been successfully completed after replacing the pressure switches or valve body. (see via A professional scan tool like the Mazda M-MDS will retrieve this code from the TCM.)
• P0872:00: The ':00' is a Failure Type Code (FTC) used by Mazda, indicating a component or system fault. It confirms the specific 'stuck on' nature of the fault described in the TSB. (see via Visible on Mazda's dealer-level diagnostic systems (MDARS/M-MDS) and some advanced aftermarket scanners.)

Scan Tool Commands That Help

• Mazda M-MDS (or MDARS): Initial Learning — This procedure is mandatory after replacing the oil pressure switches or the control valve body. It allows the TCM to learn the hydraulic characteristics of the new components. Failure to perform this step can result in harsh shifting and may set code P06B8:00.
• Mazda M-MDS (or MDARS): Retrieve CMDTCs / Clear DTCs — Used as part of the 'Initial Learning' procedure. The technician must clear codes at a specific point in the process to initiate the learning sequence.

OEM Part Supersession History

• FZA1-21-2C0 → FZ01-21-2C0 — Part revision or supplier change. The new part number is the currently supplied service part.
• FZA1-21-2J0 → FZ01-21-2J0 — Part revision or supplier change. The new part number is the currently supplied service part.

Model Year Variations Within This Range

• 2016-2021: According to TSB 05-002-23, the repair path depends on the model year. For a 2021 CX-9, the procedure directs the technician to 'Oil Pressure Switch Replacement' rather than attempting a 'TCM Reprogramming' first, which was an option for some older models in the TSB's range.