P0877 on 2014-2017 Mazda MAZDA6: Causes and Fixes for Stuck Pressure Switch

What's Unique About the 2014-2017 Mazda MAZDA6

The 2014-2017 Mazda 6 uses the SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL). This transmission has a known issue, documented in multiple Technical Service Bulletins (TSBs), where internal oil pressure switches can get stuck. The cause is explicitly stated as contamination within the transmission fluid that physically enters the switch passage and jams it. Unlike a simple sensor failure, this problem is a specific mechanical issue within the transmission's hydraulic control system, and Mazda has issued specific repair procedures and updated parts to address it.

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 (A/T) warning light is on
• Harsh or erratic shifting
• Delayed gear engagement
• Vehicle may be stuck in one gear (limp mode)
• Smell of overheated transmission fluid

Most Likely Causes

1. Stuck Internal Oil Pressure Switch 🔴 High Probability → Shop Engine Oil Pressure Switch As documented in Mazda TSBs #05-002-23 and its predecessors, contamination in the transmission fluid can cause the oil pressure switches (A and B) on the valve body to stick. This is the most common and well-documented failure for this code on this platform.
   How to confirm: A technician will confirm by retrieving the P0877 code and following the diagnostic procedure outlined in the relevant TSB, which points directly to this failure if the TCM software is already up to date.
   Typical fix: Replace both oil pressure switches (A and B) on the control valve body, replace the transmission fluid and filter, and perform the TCM Initial Learning procedure.
   Est. part cost: $100-$200 for both OEM switches.
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Mazda TSBs specify that before replacing parts, the technician should check if a TCM software update is available. The update can change the diagnostic logic to be less sensitive to momentary sticking, potentially preventing the code from being set.
   How to confirm: A Mazda dealership or qualified shop with a Mazda Diagnostic System (MDS) can check the current software calibration against the TSB's list of updated files.
   Typical fix: Reprogram the TCM to the latest software version. If the code returns, parts replacement is necessary.
   Est. part cost: $0 (if covered) or 1-2 hours of labor.
3. Faulty Control Valve Body ⚪ Low Probability → Shop Transmission Valve Body If the contamination is severe or if the hydraulic channels within the valve body are scored or blocked, replacing the switches alone may not be sufficient. In some cases, the TCM is integrated into the valve body, requiring replacement as a unit.
   How to confirm: This is typically diagnosed after replacing the pressure switches and the problem persists. A technician may inspect the valve body for scoring or blockages during the initial repair.
   Typical fix: Replacement of the entire control valve body assembly, which includes the TCM.
   Est. part cost: $1000-$1500 for the part.

Diagnosis Steps

1. Connect an OBD-II scanner to confirm the P0877 code and check for any other related transmission codes (P0842, P0847, P0872, P0780, P1738).
2. Check for applicable Technical Service Bulletins (TSBs) for the vehicle's VIN. Specifically, TSB #05-002-23 or its successors.
3. If a TCM software update is indicated by the TSB for your VIN, perform the reprogram first.
4. If software is up to date or the problem persists, proceed with parts replacement.
5. Drain the automatic transmission fluid.
6. Remove the transmission oil pan.
7. It is recommended to also remove and replace the transmission oil strainer (filter) at this stage.
8. Locate and disconnect the electrical connectors for oil pressure switches A and B.
9. Remove the bolts for both switches. Install the new pressure switches and torque the bolts to 9-10 N·m (80-88 in·lbf).
10. Reinstall the valve body components, a new oil pan gasket, and the oil pan.
11. Refill the transmission with the correct type and amount of Mazda FZ ATF.
12. Perform the 'Initial Learning' procedure using a compatible diagnostic tool (M-MDS) or the manual method as specified by Mazda. This step is mandatory. Failure to do so can result in poor shifting and may leave a P06B8 code stored.
13. Clear all DTCs and perform a test drive to verify the repair.

Parts You'll Likely Need

• Oil Pressure Switch 'A' (OEM #FZ01-21-2C0) — This is one of the two switches identified in Mazda TSBs that are prone to sticking. It monitors pressure for the 2-6 Brake and 3-5-R Brake clutch packs. It is recommended to replace both switches simultaneously.
  Trusted brands: Mazda OE
  OEM price range: $50-$100
  Aftermarket price range: $30-$60
• Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — This is the second of the two switches identified in Mazda TSBs. It monitors pressure for the High Clutch (4-5-6 gears) and Low Clutch (1-2-3-4 gears). P0877 specifically points to a fault in this switch.
  Trusted brands: Mazda OE
  OEM price range: $50-$100
  Aftermarket price range: $30-$60
• Automatic Transmission Fluid FZ (OEM #0000-FZ-113E-01) — The transmission must be drained and refilled with this specific fluid as part of the repair.
  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 of the failure is contamination, replacing the internal transmission filter (strainer) is a critical step to prevent a repeat failure.
  Trusted brands: Mazda OE
  OEM price range: $45-$60
  Aftermarket price range: $20-$40
• Transmission Oil Pan Gasket — The gasket should be replaced when the oil pan is removed to prevent leaks. Some applications may use sealant instead of a pre-formed gasket.
  Trusted brands: Mazda OE, Fel-Pro
  OEM price range: $20-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0842 — Stands for 'Oil pressure switch No. 1 (oil pressure switch B) stuck on'. It's part of the same family of faults documented in Mazda TSB 05-002-23.
• P0847 — Stands for 'Oil pressure switch No. 2 (oil pressure switch A) stuck on'. It's part of the same family of faults documented in Mazda TSB 05-002-23.
• P0872 — Stands for 'Oil pressure switch No. 3 (oil pressure switch A) stuck on'. It's part of the same family of faults documented in Mazda TSB 05-002-23.
• P0780 — Indicates a 'Gear shifting malfunction'. This is a general code that can be triggered by the specific pressure switch failures.
• P1738 — A manufacturer-specific code for 'Automatic transaxle internal malfunction', often set alongside the pressure switch codes.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND A/T WARNING LIGHT ON WITH ONE OR MORE OF THESE DTCS (P0842, P0847, P0872, P0877, P0780, P1738). This is the latest version, superseding previous bulletins.
• 05-002/15: Check Engine light and A/T Warning light on with one or more of these DTS (P0842, P0847, P0872, P0877, P0780, P1738) Stored in TCM Memory. An older version of the bulletin with similar repair instructions.

Platform-Specific Known Issues

• Mazda has released multiple superseding TSBs over many years for this exact issue across various models using the SKYACTIV-Drive 6AT, indicating a persistent design or contamination control problem.
• The TSB explicitly states the cause is 'contamination entering the passage' of the oil pressure switch, making fluid condition and filtration paramount in these transmissions.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity to Ground — expected: Infinite resistance (no continuity) in Park or Neutral. 0 ohms (continuity) in Drive.. Failure: Continuity in Park/Neutral or no continuity in Drive indicates a stuck or failed switch.
• ATF Temperature for Diagnostics — expected: 55 °C (131 °F) or more.. Failure: Attempting on-board diagnostic tests below this temperature may yield incorrect results.
• ATF Temperature for Initial Learning Procedure — expected: Between 60-110 °C (140-230 °F).. Failure: The learning procedure will abort if the fluid temperature is outside this range.
• Oil Pressure Switch Bolt Torque — expected: 9-10 N·m (80-88 in·lbf).. Failure: Incorrect torque can lead to leaks or damage to the valve body or switch.

Hidden / Shadow Codes Worth Checking

• P06B8:00: Internal Control Module Non-Volatile Random Access Memory (NVRAM) Error. On this platform, this code specifically indicates that the mandatory 'Initial Learning' procedure for the transmission has not been performed, has been interrupted, or has failed. (see via Standard OBD-II scanner after replacing valve body components or attempting the learning procedure.)

Scan Tool Commands That Help

• Mazda M-MDS: Initial Learning — This is a mandatory procedure after replacing the oil pressure switches or the control valve body. It allows the TCM to learn the hydraulic characteristics of the new components.
• FORScan: TCM - Reset all default adaptations — Used to clear the learned shift adaptations in the Transmission Control Module, forcing it to relearn. This can resolve some shifting issues and is a key part of the overall repair process.
• Manual Procedure (No Scan Tool): Initial Learning Mode Activation — For DIYers without access to M-MDS or a capable scan tool. It involves a specific sequence of starting the engine, shifting the lever (e.g., from N to M-), and pressing pedals to force the TCM into its learning cycle.

Wiring & Ground Locations

• Main Transmission Ground — A ground cable from the negative battery terminal is bolted to a bracket on top of the transmission case.. While not a documented common cause for P0877 specifically, a poor main ground for the transmission can cause a wide range of erratic electronic behaviors and shifting problems. Ensuring this connection is clean and tight is a fundamental check during any transmission-related electrical diagnosis.

Real Owner Repair Stories

• Reddit user 'redcx5' in r/CX5 (2018 Mazda CX-5 (uses same FW6A-EL transmission family)) — P0877 code stored.
  ❌ Tried (didn't work) Owner was advised by a mechanic to replace the entire transmission.
  ✅ What actually fixed it The expert advice, which aligns with Mazda's TSBs, was to reject the transmission replacement diagnosis. The correct, and much cheaper, procedure is to first test and likely replace the specific pressure switch ('B' for P0877). If that fails, the next step is to replace the valve body assembly, which is still significantly less expensive than a full transmission replacement.

OEM Part Supersession History

• Multiple older TSBs (e.g., 05-005/13, 05-002/15) → 05-002-23 — The repair procedure and, most importantly, the TCM software calibration files have been repeatedly updated. The updates aim to reduce contamination and improve the diagnostic logic to be less sensitive to momentary switch sticking.
  Heads up: Using an outdated repair procedure or failing to update to the latest TCM software may result in the code returning even after parts are replaced.

Model Year Variations Within This Range

• 2014-2017: No significant mechanical differences related to this fault are documented within this year range for the Mazda 6. The primary variation is the specific TCM software calibration file required, which can differ by model year. The correct file is automatically selected by the Mazda M-MDS tool based on the vehicle's VIN.