P0877 on 2016-2020 Mazda CX-9: Transmission Pressure Sensor Fault Causes and Fixes

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

The 2016-2020 Mazda CX-9 uses the SKYACTIV-Drive 6-speed automatic transmission, model FW6A-EL. This transmission has a known issue, documented in Mazda Technical Service Bulletin (TSB) 05-002-23, where internal contamination can cause one or more oil pressure switches to stick. This leads to a pattern of pressure switch codes, including P0877. 🎬 See a detailed breakdown of the P0877 diagnostic code. Mazda's official repair procedure has evolved, starting with a software update to make the TCM less sensitive to erratic signals before moving to hardware replacement. The issue is not unique to the CX-9 and affects other Mazda models with the same transmission.

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.
• Harsh or delayed gear shifts.
• Transmission may feel like it's slipping.
• Vehicle may enter 'limp mode', getting stuck in a single gear.
• Engine revs higher than usual before shifting.
• Vehicle shudders, especially on inclines at moderate speeds.

Most Likely Causes

1. Sticking Oil Pressure Switches due to Contamination 🔴 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. Tiny metallic particles or other debris in the transmission fluid can cause the internal oil pressure switches (A and/or B) to stick temporarily or permanently.
   How to confirm: A technician will confirm this by checking for the specific pattern of DTCs listed in the TSB (P0842, P0847, P0872, P0877, P0780, P1738) and following the TSB's diagnostic procedure, which prioritizes a software check before hardware replacement.
   Typical fix: The first step is to update the Transmission Control Module (TCM) software. If the code returns, the next step is to drop the transmission pan, access the valve body, and replace both oil pressure switches A and B. Mazda recommends replacing them as a pair. After replacement, an 'Initial Learning' procedure must be performed.
   Est. part cost: $100-$200 for the switches
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Mazda released software updates to improve the diagnostic logic and make the TCM less sensitive to minor, temporary fluctuations from the pressure switches. The TSB explicitly lists TCM reprogramming as the first step for certain VIN ranges before parts replacement.
   How to confirm: A Mazda dealer or a shop with a J2534 pass-thru device and Mazda Modular Diagnostic System (MDARS) software can check the current software calibration version against the latest version available from Mazda.
   Typical fix: Reprogramming the TCM to the latest software version. This is often the first step in the official repair procedure and may resolve the issue without hardware changes.
   Est. part cost: $0 (parts), labor for programming varies, typically 1-2 hours at a dealership.
3. Low or Dirty Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While possible, it's less common than the specific TSB-related issue. Low fluid can cause general pressure drops, and dirty fluid is the source of the contamination that makes the switches stick. The SKYACTIV-Drive transmission is sensitive to fluid quality.
   How to confirm: Check the transmission fluid level and condition according to the manufacturer's procedure. The fluid should be at the correct level and not appear dark, burnt, or contain excessive debris. This transmission does not have a traditional dipstick, so checking requires lifting the vehicle and using a check plug.
   Typical fix: Perform a transmission fluid drain-and-fill or flush using the specified Mazda FZ fluid. If the fluid is very dirty, the pan should be dropped for cleaning and inspection of the filter and magnets.
   Est. part cost: $75-$150 for fluid

Rare But Worth Checking

• Wiring Harness or Connector Issue: Damage to the wiring leading to the transmission pressure switch can cause a 'Circuit Low' code. This could be from chafing, corrosion at the connector, or rodent damage. The TSB repair procedure involves inspecting these connections.
• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly In rare cases, the TCM itself can fail, causing it to misinterpret signals from a perfectly good sensor. This is usually diagnosed only after all other possibilities, including software and sensors, have been eliminated.
• Internal Valve Body Malfunction: → Shop Transmission Valve Body Beyond the pressure switches, the valve body itself can have issues with solenoids or clogged passages that could contribute to this code, though this is less common than the switches themselves. In some older, related TSBs, valve body replacement was the recommended fix.

Diagnosis Steps

1. Scan the TCM for all stored diagnostic trouble codes. Note if P0877 appears with other codes listed in TSB 05-002-23.
2. Check the transmission fluid level and condition. The fluid should be at the proper level and not appear burnt or contaminated. This requires raising the vehicle level and removing the check/fill plug.
3. Using a diagnostic scan tool (like the Mazda MDARS), check the current TCM software version. Compare it to the latest version specified by Mazda in the TSB.
4. If the software is outdated, perform the TCM reflash as per TSB 05-002-23.
5. Clear the codes and perform a test drive, including idling for 10 seconds in Park with ATF temp above 68°F (20°C), to see if the code returns.
6. If the code returns, the next step is to inspect the wiring and connector to the transmission for any visible damage or corrosion.
7. If wiring is okay, proceed with replacing the internal oil pressure switches (A and B) located on the valve body, which requires draining the fluid and removing the transmission pan.
8. While the pan is off, inspect for excessive metal debris on the magnets, which could indicate more severe internal wear.
9. After replacement, refill with the correct type of Mazda transmission fluid (Type FZ), clear codes, and perform an initial learning procedure for the TCM as specified in the workshop manual.

Parts You'll Likely Need

• Transmission Oil Pressure Switch Kit (A and B) (OEM #FZ01-21-2J0 (This is for Switch B, often sold with Switch A as a set)) — These switches are the primary hardware failure point for this code on this vehicle, as identified by Mazda's TSB. They are replaced as a pair.
  Trusted brands: Mazda OE
  OEM price range: $100-$200
  Aftermarket price range: $70-$150
• Automatic Transmission Fluid (OEM #0000-FZ-113E-01 (or equivalent Mazda Type FZ)) — Fluid must be drained to access the internal switches and should be replaced with new, manufacturer-specified fluid (Mazda FZ). Approximately 4-5 quarts are needed for a pan drop.
  Trusted brands: Mazda OE, Idemitsu Type-M, Ravenol ATF FZ
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart
• Transmission Pan Gasket (OEM #FZ01-21-520) — The transmission pan on the FW6A-EL uses sealant from the factory, but a replacement gasket is often used for easier service. If reusing sealant, a high-quality, oil-resistant RTV is required. New pan bolts are also recommended.
  Trusted brands: Mazda OE, Fel-Pro, Mahle
  OEM price range: $20-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0842 — This code for 'Oil pressure switch No. 1 stuck on' is part of the same known issue described in TSB 05-002-23.
• P0847 — This code for 'Oil pressure switch No. 2 stuck on' is part of the same known issue described in TSB 05-002-23.
• P0872 — This code for 'Oil pressure switch No. 3 stuck on' is part of the same known issue described in TSB 05-002-23.
• P0780 — This code for 'Gear shifting malfunction' often appears with the pressure switch codes as it indicates a general shifting problem detected by the TCM.
• P1738 — This code for 'Automatic transaxle internal malfunction' is a more serious code that can be triggered alongside the pressure switch faults.

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) STORED IN TCM MEMORY.
• 05-004/18: An earlier version of the TSB covering the same issue on 2016-2018 models.
• 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.

Platform-Specific Known Issues

• Mazda Technical Service Bulletin 05-002-23 directly addresses P0877, along with other codes, on 2016-2021 CX-9 models. It identifies the cause as contamination causing oil pressure switches to stick and provides a repair path starting with a software update and progressing to switch replacement.
• A Reddit user with a 2019 CX-5 (which shares the transmission) experienced the 'AT Transmission Malfunction' light, which was resolved under warranty by the dealer performing the TSB, highlighting the prevalence of this issue.

Mechanic-Grade Diagnostic Values

• Transmission Fluid Temperature (ATF) — expected: Above 20 °C (68 °F). Failure: The DTC test for P0877 runs when the ATF temperature is above this threshold. A cold transmission may not exhibit the fault.
• Transmission Fluid Pressure Sensor (TFPS) Reference Voltage — expected: 5V or 12V (Varies by specific circuit design). Failure: If voltage at the sensor connector (with key on, engine off) is incorrect or missing, it indicates a wiring or PCM/TCM issue rather than a sensor failure.

Scan Tool Commands That Help

• Mazda Modular Diagnostic System (MDARS): Initial Learning — This procedure is mandatory after replacing the oil pressure switches or valve body to make the TCM relearn clutch engagement points and pressures. Failure to perform this can result in harsh shifting or persistent codes.
• Mazda Modular Diagnostic System (MDARS): Module Reprogramming — Used to update the TCM software to the latest calibration. This is the first step in the official TSB repair procedure and may fix the code without replacing any parts.
• Advanced OBD-II Scanner (e.g., YOUCANIC): Active Test: Pressure Control Solenoid — Allows a technician to command individual solenoids on and off to verify their electrical function and listen for an audible click from the transmission, helping to isolate a faulty component.

Wiring & Ground Locations

• Oil Pressure Switch B Connector — Inside the transmission, on the valve body. Requires removing the transmission oil pan to access.. This is the direct connector for the sensor that triggers P0877. It is a two-pin connector with red and white wires. Checking for corrosion or a secure connection here is a key diagnostic step once the pan is off.

Real Owner Repair Stories

• Reddit user on r/mazda (2017 Mazda CX-5 with 120k miles) — Check Engine Light on, no noticeable driving issues. Code P0847 (Transmission Fluid Pressure Sensor/Switch B Circuit Low) was stored.
  ❌ Tried (didn't work) The owner took the car to a Mazda dealership for diagnosis.
  ✅ What actually fixed it The issue was not yet fixed, but the story is a valuable cautionary tale. The dealer quoted $1040 to replace only Switch B, but warned it would likely fail and a full $8768 transmission replacement would be needed due to 'contamination'. The owner found the TSB which recommends replacing BOTH switches (A and B) as the proper fix, and was questioning the dealer's high-pressure up-sell to a full transmission replacement.
• Reddit user on r/CX5 (2018 Mazda CX-5 FWD with 80k miles) — Check Engine Light with code P0877.
  ❌ Tried (didn't work) A mechanic immediately suggested replacing the entire transmission.
  ✅ What actually fixed it The final fix was not posted, but forum advice steered the owner away from a full transmission replacement. The consensus was to follow the proper diagnostic path: test the specific solenoid/switch ('B'), the TCM, and the wiring first, as replacing just the switch or the valve body w/TCM is far cheaper and the more likely solution. This highlights the common misdiagnosis of jumping to the most expensive repair.

OEM Part Supersession History

• fza1212c0 → FZ01-21-2C0 — This is the part number for Oil Pressure Switch 'A'. It is relevant because Mazda's TSB for P0877 (caused by Switch 'B') recommends replacing both switches A and B as a set to prevent a comeback.

Model Year Variations Within This Range

• 2016-2020: Mazda issued multiple software updates for the Transmission Control Module (TCM) over the years to improve the diagnostic logic for pressure switch codes. An earlier model year vehicle is more likely to be running on older, more sensitive software. The TSB repair procedure prioritizes checking and updating this software before replacing any hardware.