P0877 on 2012-2013 Mazda MAZDA3: Transmission Pressure Switch Fault Causes and Fixes

What's Unique About the 2012-2013 Mazda MAZDA3

The 2012-2013 Mazda3, particularly models equipped with the SKYACTIV-Drive 6-speed automatic transmission, has a well-documented history with this specific family of codes (including P0842, P0847, P0872, and P0877). Mazda acknowledged this issue through a long series of evolving Technical Service Bulletins. The root cause is contamination within the transmission fluid causing the oil pressure switches (A and/or B) on the control valve body to stick. The manufacturer's prescribed fix evolved over time; early TSBs for certain VINs suggested only a TCM software update, but later bulletins emphasized the replacement of the physical switches as the primary repair, confirming this is a known hardware failure point.

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 delayed gear shifts
• Transmission slipping between gears
• Vehicle may get stuck in one gear (limp mode)
• Erratic RPM behavior

Most Likely Causes

1. Faulty Transmission Oil Pressure Switch 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSBs explicitly state that contamination in the transmission fluid can cause these switches to stick, leading to failure. This is a widely acknowledged weak point in the SKYACTIV 6AT transmission from this era.
   How to confirm: A technician will need to access the valve body inside the transmission to test the switches' electrical circuits and function. This is the most direct way to confirm a failed switch.
   Typical fix: Replace the faulty oil pressure switch (or switches, as they are often replaced in pairs 'A' and 'B'). The transmission pan must be removed for access. Per TSBs, both switches A and B should be replaced together.
   Est. part cost: $50-$150 for the switches.
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Early versions of the TCM software were overly sensitive to momentary pressure fluctuations. Mazda released updates to improve the diagnostic logic and prevent false codes from being set.
   How to confirm: A Mazda dealership or a shop with a compatible scan tool (like the Mazda IDS) can check the current software version against the latest available calibration file mentioned in the TSBs. The TSBs provide specific VIN ranges that were eligible for a software-first approach.
   Typical fix: Reprogram the TCM to the latest software version. Per TSBs, this was the first recommended step for certain VIN ranges. However, if the issue re-occurs after the update, parts replacement is necessary.
   Est. part cost: $0 (parts), but labor for reprogramming will apply.
3. Contaminated Transmission Fluid or Faulty Valve Body ⚪ Low Probability → Shop Transmission Assembly The TSBs mention contamination as the root cause of the switches sticking. In severe cases, or if the fluid has never been changed, the entire valve body, which houses the switches and solenoids, may be compromised by debris.
   How to confirm: Inspecting the transmission fluid and pan for excessive metal shavings or debris when the pan is removed. If the fluid is very dirty or burnt, it points to a larger issue. The TSBs note that if a large amount of foreign material is found, the oil strainer should also be replaced.
   Typical fix: In less severe cases, a transmission fluid and filter change may help. In more severe cases, the entire valve body assembly needs to be replaced.
   Est. part cost: $100-$200 for fluid/filter, $800-$1300 for a new valve body.

Rare But Worth Checking

• Wiring Harness or Connector Issue: While less common than switch failure, the wiring leading to the pressure switches inside the transmission can become damaged or corroded, causing a 'Circuit Low' fault. This should be inspected when the transmission pan is off.
• TCM Failure: In very rare instances, the Transmission Control Module itself can fail. However, this is unlikely and all other causes, especially the pressure switches and wiring, should be ruled out first. Some owners on forums have reported needing a TCM replacement to finally resolve shifting issues.

Diagnosis Steps

1. Scan the TCM for all stored trouble codes. Note if P0877 appears with other pressure switch codes (P0842, P0847, P0872).
2. Check for any applicable Technical Service Bulletins (TSBs) for your vehicle's VIN. TSBs like 05-002/15 and 05-003/20 are highly relevant. Some vehicles may only require a TCM reprogram as the first step.
3. Check the transmission fluid level and condition. A low or burnt fluid level indicates other potential problems. The correct fluid is Mazda ATF-FZ.
4. If required by the TSB or if reprogramming doesn't work, a technician will need to remove the transmission oil pan.
5. Visually inspect the wiring and connectors for the pressure switches on the valve body for any signs of damage or corrosion.
6. Test the resistance and continuity of Oil Pressure Switch No. 4 ('B') to confirm electrical failure.
7. If the switch is faulty, replace it. It is highly recommended to replace both 'A' and 'B' switches at the same time, per Mazda's TSBs.
8. Inspect the transmission pan magnet for excessive metal debris. If significant material is found, replace the transmission oil strainer (filter) P/N FZ01-21-500.
9. Reassemble the transmission with a new pan gasket (or sealant as specified) and refill with the correct type of Mazda automatic transmission fluid (ATF-FZ).
10. Clear all DTCs and perform an 'Initial Learning' procedure for the transmission as specified by Mazda. This is a critical step to ensure smooth shifting after the repair and involves a specific sequence of shifter movements and key cycles.

Parts You'll Likely Need

• Transmission Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — This is the specific switch ('No. 4') identified by Mazda TSBs as the primary cause for code P0877. It monitors pressure for the High Clutch (gears 4-5-6) and Low Clutch (gears 1-2-3-4).
  Trusted brands: Mazda OE
  OEM price range: $40-$70
  Aftermarket price range: $30-$60
• Transmission Oil Pressure Switch 'A' (OEM #FZ01-21-2C0) — TSBs recommend replacing both switches ('A' and 'B') at the same time, as they are prone to the same failure mode.
  Trusted brands: Mazda OE
  OEM price range: $40-$70
  Aftermarket price range: $30-$60
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — TSBs recommend replacing the strainer if a large amount of foreign material is found in the oil pan, which is common with this issue.
  Trusted brands: Mazda OE
  OEM price range: $45-$60
  Aftermarket price range: $25-$45
• Automatic Transmission Fluid (Type FZ) (OEM #0000-FZ-113E-01) — The transmission must be refilled with the correct fluid after dropping the pan. Using the wrong fluid type can cause shifting problems. A drain and fill requires 4-5 quarts.
  Trusted brands: Mazda OE

Related Codes That Often Appear With This One

• P0842 — Relates to Oil Pressure Switch No. 1 (also part of switch 'B'). Both are part of the same system and often fail due to the same root cause (contamination).
• P0847 — Relates to Oil Pressure Switch No. 2 (part of switch 'A'). Often flagged along with P0877 in Mazda TSBs as a group of related codes.
• P0872 — Relates to Oil Pressure Switch No. 3 (part of switch 'A'). Also commonly seen with P0877 as part of the same known issue.
• P0780 — Indicates a 'Gear Shifting Malfunction'. This is a general code that can be triggered as a result of the pressure switch failure causing improper shifting. It is listed in the same TSBs.
• P1738 — A Mazda-specific code for 'Automatic Transaxle Internal Malfunction'. This may be set alongside P0877 when the TCM detects a serious internal fault stemming from the pressure switch issue.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23
• 05-004-18
• 05-002/15
• 05-005/14

Platform-Specific Known Issues

• Multiple Mazda TSBs have been issued for the 2012-2013 Mazda3 regarding P0877 and related codes. The most relevant are 05-005/14, 05-002/15, 05-004/18, and 05-002-23. These bulletins confirm the cause is oil pressure switches on the valve body sticking due to contamination and outline a repair that evolved from TCM reprogramming to mandatory parts replacement.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity (On-Vehicle) — expected: In Park or Neutral, there should be infinite resistance (no continuity) between the switch terminal and ground. In Drive, there should be 0 ohms (continuity) to ground.. Failure: If the switch shows continuity in Park/Neutral or no continuity in Drive, it is stuck and has failed the test.

Scan Tool Commands That Help

• Mazda Modular Diagnostic System (M-MDS) or equivalent: Initial Learning Procedure — This procedure is mandatory after replacing the valve body or oil pressure switches. It allows the TCM to relearn the hydraulic and mechanical characteristics of the new components to ensure smooth shifting.
• Mazda Modular Diagnostic System (M-MDS) or equivalent: TCM Reprogramming — For specific VIN ranges outlined in TSBs, reprogramming the TCM with updated software is the first step to correct overly sensitive diagnostic logic before replacing parts.

Wiring & Ground Locations

• TCM (Transmission Control Module) — For 2012-2013 Mazda3 with the SKYACTIV-Drive 6-speed automatic transmission, the transmission control functions are integrated into the Powertrain Control Module (PCM), which is located attached to the battery box in the engine compartment. For non-SKYACTIV 5-speed models, the TCM is a separate module located on top of the transmission, under the battery and battery box.. Knowing the correct module location is critical for diagnosis. Technicians may need to test the wiring harness between the PCM/TCM and the transmission's main connector for continuity or shorts before dropping the transmission pan.
• G07 (Ground) — Located on the top rear of the engine.. A poor ground connection at this point can cause a variety of communication and sensor-related issues for the powertrain, potentially leading to erratic behavior and fault codes.

Real Owner Repair Stories

• YouTube user comment on a TCM replacement video (BL Mazda 3 (2009-2013)) — Hard shifting, AT light on, transmission going into limp mode.
  ❌ Tried (didn't work) Replacing the TCM and mounting it in the original location on top of the transmission.
  ✅ What actually fixed it After a replacement TCM also failed, the owner was advised to mount the new module away from the transmission case to prevent heat damage. The final fix involved zip-tying the replacement TCM to the brake lines on the firewall, away from the direct heat of the transmission, which is a common preventative modification suggested by rebuilders.

OEM Part Supersession History

• fza1212c0 → FZ01-21-2C0 — Part number format update and potential minor revision by the manufacturer.

Model Year Variations Within This Range

• 2013: For the 2013 model year, Mazda introduced a new touchscreen infotainment system with navigation, which was not available on the 2012 model. Additionally, more trim levels, like the 'i Grand Touring', became available with a manual transmission option. These changes do not directly affect the P0877 fault but are key differentiators between the model years.