P0877 on 2021 Mazda CX-9: Transmission Pressure Switch Fault Causes and Fixes

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

The 2021 CX-9 uses the SKYACTIV-Drive 6-speed automatic transmission (model FW6A-EL or GW6A-EL). A widely documented issue for this transmission family is contamination from tiny metallic particles or other debris causing internal oil pressure switches to stick. Mazda has acknowledged this problem in Technical Service Bulletin (TSB) 05-002-23, which directly applies to the 2021 CX-9 and lists P0877 as a primary fault code. The TSB outlines a specific repair procedure that, depending on the vehicle's history, may start with a TCM software update before progressing to the replacement of the internal switches, making it a more involved repair than on many other vehicles.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Generation note: This issue is not unique to the 2021 model. It affects a wide range of Mazda vehicles equipped with the SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL family) from roughly 2012 onwards. TSB 05-002-23 and its predecessors cover models including the Mazda3, Mazda6, CX-3, and CX-5 across various model years.

Symptoms You May Notice

• Check Engine Light is on
• AT (Automatic Transaxle) warning light is on
• Harsh, erratic, or delayed gear shifts
• Transmission may feel like it's slipping or hesitating
• Vehicle may enter 'limp mode', stuck in a single gear (often 3rd)
• Jerking during acceleration

Most Likely Causes

1. Sticking Internal Oil Pressure Switches 🔴 High Probability → Shop Engine Oil Pressure Switch This is a well-documented issue for Mazda's SKYACTIV transmissions, as noted in TSB 05-002-23. Contamination in the transmission fluid, such as fine metallic debris from normal wear, can cause the small mechanical switches on the valve body to physically stick.
   How to confirm: A professional technician will confirm by checking for the presence of P0877 along with other related codes (P0842, P0847, P0872, etc.) and following the diagnostic procedure outlined in TSB 05-002-23, which prioritizes a software check before hardware replacement.
   Typical fix: Replace both oil pressure switches (A and B) located on the control valve body inside the transmission. This also involves draining the fluid, removing the oil pan, and replacing the pan gasket. An 'Initial Learning Procedure' must be performed afterward.
   Est. part cost: $100-$250
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Mazda has issued software updates to make the TCM's diagnostic logic less sensitive to minor, temporary fluctuations from the pressure switches. An older software version might trigger the code prematurely. The TSB specifies that for some vehicles, a software update is the first and sometimes only required step.
   How to confirm: A Mazda dealership or a shop with a Mazda-specific scan tool (MDARS) can check the current software calibration against the latest version specified in the TSB for the vehicle's VIN.
   Typical fix: Reprogram the TCM to the latest software version. Per TSB 05-002-23, this is the first step for certain VINs and may resolve the issue without hardware replacement.
   Est. part cost: $0-$200
3. Low or Dirty Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While possible, it's a less common primary cause for this specific code compared to the known TSB issue. However, degraded or contaminated fluid is the root cause of the switches sticking. Using incorrect, non-FZ fluid can also cause problems.
   How to confirm: Checking the fluid on the SKYACTIV-Drive transmission is complex as there is no traditional dipstick. It requires raising the vehicle level, removing a check plug, and verifying the level at a specific temperature (50°C / 122°F) using a scan tool. The fluid should be its original blue color (Mazda FZ) and not brown, black, or have a burnt smell.
   Typical fix: Perform a transmission fluid drain and fill using the correct Mazda FZ-type automatic transmission fluid. This is done as part of the pressure switch replacement.
   Est. part cost: $80-$150

Rare But Worth Checking

• Faulty Control Valve Body: → Shop Transmission Valve Body In some cases, the passages within the valve body itself may be blocked or damaged, or a solenoid may have failed. If replacing the pressure switches doesn't resolve the issue, the entire valve body may need replacement, which is significantly more expensive. Some earlier TSBs recommended replacing the entire valve body assembly.
• Wiring Harness Damage: Damage to the internal transmission wiring harness leading to the pressure switches could cause a low voltage signal. The connector for switch B uses red and white wires. This is unlikely unless the transmission has been recently serviced improperly.

Diagnosis Steps

1. Scan the TCM for all stored trouble codes. Note if P0877 is present with other codes listed in TSB 05-002-23 (P0842, P0847, P0872, P0780, P1738).
2. Using a Mazda-specific scan tool (MDARS), check the current TCM software calibration file against the latest version listed in the TSB for the vehicle's VIN.
3. If an update is available and indicated by the TSB as the first step, perform the TCM reprogramming. Clear codes and test drive to see if the fault returns.
4. If software is up to date or the code returns, proceed with hardware replacement.
5. Drain the transmission fluid and remove the transmission oil pan.
6. Inspect the valve body and internal wiring for any visible damage. The connector for switch B has red and white wires.
7. Replace both oil pressure switches (A and B) as recommended by the TSB. Torque the new switch bolts to 9-10 N·m (80-88 in·lbf).
8. It is recommended to also replace the transmission oil strainer (filter) at this time.
9. Reinstall the transmission pan with a new gasket or sealant as specified, refill with genuine Mazda FZ ATF, and verify the correct fluid level at operating temperature.
10. Perform the mandatory 'Initial Learning' procedure with a scan tool to allow the TCM to relearn clutch pressures and shift points.

Parts You'll Likely Need

• Transmission Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — This is the specific switch ('No. 4' or 'B') that triggers the P0877 code. It is highly recommended to replace both switches A and B at the same time.
  Trusted brands: Mazda OE
  OEM price range: $50-$125 each
  Aftermarket price range: $30-$80 each
• Transmission Pan Gasket — The transmission pan must be removed to access the pressure switches, and the gasket must be replaced upon reassembly to prevent leaks. Some applications use sealant instead of a pre-formed gasket.
  Trusted brands: Mazda OE, Fel-Pro
  OEM price range: $25-$40
  Aftermarket price range: $15-$30
• Mazda SKYACTIV-Drive FZ ATF (OEM #0000-FZ-113E-01) — The correct fluid is required when refilling the transmission after the repair. Using a non-specified fluid can cause further damage. Approximately 3.5-4 quarts are needed for a pan drop.
  Trusted brands: Mazda OE, Aisin MFZ
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart

Related Codes That Often Appear With This One

• P0842 — Relates to Oil Pressure Switch 'A' (No. 1), which is prone to the same sticking issue and is often replaced at the same time.
• P0847 — Relates to Oil Pressure Switch 'B' (No. 2), also susceptible to the same contamination problem.
• P0872 — Relates to Oil Pressure Switch 'C' (No. 3), another switch in the same system that can fail for the same reason.
• P0780 — Indicates a generic shift malfunction, which is a direct symptom of the pressure switches failing to report correct pressures for gear changes.
• P1738 — A manufacturer-specific code indicating an automatic transmission internal problem, often logged alongside the 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 is the primary document identifying the cause as sticking pressure switches due to contamination and providing the repair flow.
• 05-004/18: An earlier version of the TSB covering the same issue on 2016-2018 models, showing the history of this known problem.
• 05-003/20: A related TSB that also lists the same set of DTCs for a wide range of Mazda vehicles with the SKYACTIV 6AT, confirming it's a platform-wide issue.

Platform-Specific Known Issues

• The 2021 Mazda CX-9 is explicitly mentioned in Mazda TSB #05-002-23, which details the cause and repair procedure for DTC P0877 and related codes. The bulletin points to contamination causing internal oil pressure switches to stick.
• For the 2021 CX-9, the TSB indicates that a technician should proceed directly to 'Oil Pressure Switch Replacement' rather than attempting a TCM reprogram first, unlike some other models/years.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity Test (On-Vehicle) — expected: With the engine running, there should be infinite resistance (no continuity) between the switch terminal and ground when the transmission is in Park or Neutral. When shifted to Drive, there should be 0 ohms (continuity to ground).. Failure: If the switch shows continuity to ground while in Park/Neutral, it indicates the switch is 'stuck on', which is the specific failure condition for code P0877.
• Pressure Switch Bolt Torque — expected: 9-10 N·m (80-88 in·lbf). Failure: Improper torque can lead to leaks or damage to the new switch or valve body.
• ATF Temperature for Diagnostics — expected: The TCM requires the ATF temperature to be 20 °C {68 °F} or more to run the diagnostic that sets P0877. The 'Initial Learning' procedure requires the temperature to be between 40-110 °C {72-198 °F}.. Failure: Diagnostics or procedures may not run if the fluid is too cold or too hot.

Scan Tool Commands That Help

• Mazda MDARS (Mazda Diagnostic and Repair Software): TCM Reprogramming — This is the first step in the diagnostic procedure outlined by TSB 05-002-23 for certain VINs. It updates the TCM's diagnostic logic to be less sensitive to momentary switch sticking and may resolve the code without hardware replacement.
• Mazda MDARS (or equivalent like FORScan): Initial Learning Procedure — This is a mandatory service function that must be performed after replacing the oil pressure switches or the valve body. It allows the TCM to learn the hydraulic characteristics of the new components to ensure proper shift quality.

Wiring & Ground Locations

• Oil Pressure Switch 'B' (No. 4) Connector — Inside the transmission oil pan, on the control valve body. The switch itself has a two-pin connector.. The connector for switch 'B' uses red and white wires. Damage to these wires within the internal transmission harness could cause the 'Circuit Low' fault, though this is much rarer than the switch itself failing.

Real Owner Repair Stories

• Reddit user with a 2019 CX-5 (2019 Mazda CX-5 (shares the same FW6A-EL transmission and is covered by the same TSB)) — AT Transmission Malfunction light illuminated on the dash.
  ❌ Tried (didn't work) Not applicable, as the owner took it directly to the dealer.
  ✅ What actually fixed it The Mazda dealer performed the repair outlined in TSB 05-002-23 under warranty, which involves checking software and replacing the internal pressure switches. This confirms the TSB is the correct and effective fix for these symptoms.

OEM Part Supersession History

• fza1212j0 → FZ01-21-2J0 — Part number revision by the manufacturer. This is common for parts that have known issues, suggesting a potential internal design update.

Model Year Variations Within This Range

• 2021: According to Mazda's official service information, the 2021 CX-9 is one of the models that requires the newer Mazda Diagnostic and Repair Software (MDARS), whereas older models may use the legacy IDS software. This is critical for any shop attempting a TCM software update.