P0877 on 2016-2021 Mazda CX-3: Transmission Pressure Switch Causes and Fixes

What's Unique About the 2016-2021 Mazda CX-3

The Mazda CX-3, along with other models using the SKYACTIV-Drive 6-speed automatic transmission (model FW6A-EL), has a well-documented issue with this code. Mazda has issued multiple Technical Service Bulletins (TSBs) acknowledging that contamination in the transmission fluid can cause internal pressure switches to stick. Unlike a simple external sensor replacement, this problem requires accessing the internal valve body, making the repair more involved than on many other vehicles where a pressure sensor might be externally mounted.

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
• Automatic Transaxle (AT) warning light is on
• Harsh or delayed gear shifts
• Vehicle may enter "limp mode," getting stuck in a single gear
• Transmission slipping between gears
• Erratic RPM behavior

Most Likely Causes

1. Stuck/Failed Internal Transmission Oil Pressure Switches 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSBs state this is a known issue caused by contamination in the transmission fluid that causes the switches (Switch A and Switch B) on the valve body to stick.
   How to confirm: A professional technician will confirm by scanning for P0877 and its common companions (P0842, P0847, P0872, P0780, P1738). Diagnosis is primarily based on the presence of these codes as described in Mazda TSB #05-002-23.
   Typical fix: Replace both Oil Pressure Switch A and Oil Pressure Switch B, which are located on the transmission's internal valve body. This also requires draining the fluid, removing the oil pan, and refilling with fresh fluid. Often the transmission strainer/filter is replaced at the same time.
   Est. part cost: $100-$200 for the set of switches and new fluid.

Rare But Worth Checking

• Wiring Harness or Connector Issue: While the TSB points to the switches, damage to the internal wiring harness leading to the switches could cause similar symptoms. This is typically checked during the switch replacement.
• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly A failing TCM is a rare possibility. The TSB for the CX-3 prioritizes replacing the switches over reprogramming or replacing the TCM, making it an unlikely root cause. However, since the TCM is integrated into the valve body on the FW6A-EL transmission, a major failure could require replacing the entire unit.

Diagnosis Steps

1. Verify the Check Engine Light and AT warning light are on.
2. Use an OBD-II scanner capable of reading manufacturer-specific codes to confirm P0877 is present. Check for any other codes from the common group (P0842, P0847, P0872, P0780, P1738).
3. Refer to Mazda TSB #05-002-23, which confirms the likely cause is stuck pressure switches for this vehicle.
4. Check the transmission fluid level and condition. While low or dirty fluid can cause issues, the TSB indicates a component failure is the direct cause of this code.
5. Plan for the replacement of the internal oil pressure switches. This involves draining the transmission fluid and removing the transmission oil pan.
6. Visually inspect the wiring to the switches and valve body once the pan is removed.
7. Replace both Oil Pressure Switch A and Oil Pressure Switch B.
8. Consider replacing the transmission strainer (filter) while the pan is off, especially if there is visible debris.
9. Reinstall the transmission oil pan with new sealant (Form-in-Place Gasket) as required by Mazda.
10. Refill the transmission with the correct type and amount of Mazda SKYACTIV-Drive ATF FZ fluid.
11. Clear the trouble codes and perform the "Initial Learning Procedure" as specified in the workshop manual to reset transmission adaptations.
12. Perform a test drive to ensure the issue is resolved.

Parts You'll Likely Need

• Transmission Oil Pressure Switch Kit (A and B) (OEM #FZ01-21-2J0B (supersedes FZ01-21-2J0A)) — This is the component kit that fails due to contamination, as identified in Mazda's TSBs. It contains both required switches.
  Trusted brands: Mazda OE
  OEM price range: $80-$150
  Aftermarket price range: $50-$100
• Mazda SKYACTIV-Drive Automatic Transmission Fluid (ATF FZ) (OEM #0000-FZ-113E-01) — The transmission must be drained to perform the repair and refilled with approximately 3.5-4 quarts of the correct OEM fluid.
  Trusted brands: Mazda OE
  OEM price range: $15-$25 per quart
  Aftermarket price range: $10-$20 per quart
• Transmission Oil Strainer / Filter (OEM #FZ01-21-500) — Since the cause is contamination, it is highly recommended to replace the internal filter when the oil pan is removed.
  Trusted brands: Mazda OE
  OEM price range: $45-$60
  Aftermarket price range: $25-$45
• Transmission Oil Pan Sealant — The oil pan must be removed and requires a new bead of Form-in-Place Gasket (FIPG) sealant upon reinstallation to prevent leaks. Mazda does not use a pre-formed gasket for this pan.
  Trusted brands: Permatex Ultra Black, Mazda OE Silicone Gasket
  OEM price range: $20-$30
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0842 — Relates to Oil Pressure Switch No. 1 (Switch B) stuck on, part of the same system.
• P0847 — Relates to Oil Pressure Switch No. 2 (Switch A) stuck on, part of the same system.
• P0872 — Relates to Oil Pressure Switch No. 3 (Switch A) stuck on, part of the same system.
• P0780 — A general code for a gear shifting malfunction, triggered by the failure of the pressure switches.
• P1738 — A Mazda-specific code for an automatic transaxle internal malfunction, which is set because the pressure switches are not responding correctly.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND A/T WARNING LIGHT ON WITH DTCS (P0842, P0847, P0872, P0877, P0780, P1738) STORED IN TCM MEMORY. This is the most current TSB describing the cause and repair procedure.
• 05-002-15: An earlier version of the TSB, also covering the same set of codes and cause, which has since been superseded by 05-002-23.

Platform-Specific Known Issues

• The SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL) in this vehicle is subject to TSB #05-002-23, which specifically identifies P0877 as being caused by internal pressure switches sticking due to contamination.
• This is not an isolated issue for the CX-3; it affects a wide range of Mazda vehicles from 2012 onwards that use the same family of transmissions.

Mechanic-Grade Diagnostic Values

• Automatic Transmission Fluid (ATF) Temperature for Initial Learning — expected: 60–110° C (140–230° F). Failure: The initial learning procedure will abort or fail to start if the ATF temperature is outside this range.

Hidden / Shadow Codes Worth Checking

• P0780: Gear Shifting Malfunction. This is a general performance code that is set as a direct result of the pressure switches failing to report correct pressures, preventing the TCM from confirming proper gear changes. (see via Standard OBD-II scanner capable of reading transmission codes.)
• P1738: Automatic Transaxle Internal Malfunction. This is a Mazda-specific code that accompanies the pressure switch codes, reinforcing that the fault is internal to the transmission assembly. (see via Standard OBD-II scanner capable of reading manufacturer-specific (P1xxx) codes.)

Scan Tool Commands That Help

• Manual Procedure (No Scan Tool): Initial Learning Procedure — Required after replacing the valve body or the internal pressure switches to allow the TCM to relearn the hydraulic response times of the new components. This procedure calibrates the clutch application pressures. The procedure will not work if ATF temperature is outside the 60-110°C range. The steps must be followed exactly.
• Mazda Modular Diagnostic System (M-MDS): Initial Learning — This is the dealer/shop method to initiate the same relearn process required after replacing the pressure switches. It is a menu-driven function within the TCM service routines.

Wiring & Ground Locations

• TCM (Transmission Control Module) — Integrated into the control valve body assembly, inside the transmission oil pan.. The TCM is a 'mechatronic' unit, combined with the valve body. This means there are no external wiring harnesses or connectors between the TCM and the pressure switches. This architecture is why a 'circuit low' code almost always points to an internal component failure (the switch) rather than an external wiring problem.
• Pressure Switches A and B — Bolted directly onto the control valve body, inside the transmission oil pan.. These are the components that fail. Their internal location necessitates removing the transmission pan for access, making the repair more complex than an external sensor.

OEM Part Supersession History

• FZ01-21-2J0A → FZ01-21-2J0B — Component revision. The TSBs indicate Mazda implemented changes to reduce contamination issues, which may be reflected in the updated part.

Model Year Variations Within This Range

• 2017-2021: For the 2017 model year, Mazda introduced G-Vectoring Control (GVC), which provides more integrated control over the engine, transmission, and chassis to improve handling. However, the underlying FW6A-EL transmission hardware and the specific P0877 fault remained the same. This software/dynamics change does not alter the diagnosis or repair procedure for the pressure switch failure.