P0843 on 2012-2013 Mazda MAZDA3: Transmission Pressure Sensor Fault Causes and Fixes

What's Unique About the 2012-2013 Mazda MAZDA3

The 2012-2013 Mazda3 was one of the first models to feature the new Skyactiv-G engine and the FW6A-EL Skyactiv-Drive automatic transmission. 🎬 Watch: A detailed technical overview of the Skyactiv-Drive transmission. This P0843 code is a well-documented issue on these early production units. Unlike many other vehicles where this code might point to a simple external sensor or wiring, on this specific Mazda, it is strongly associated with an internal failure of the transmission's valve body assembly. This fact is confirmed by manufacturer Technical Service Bulletin #05-003/14, which explicitly calls for replacing the entire valve body to resolve the issue for P0843 and a host of other codes.

Diagnostic Flowchart

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

Symptoms You May Notice

• Harsh, jerky, or delayed gear shifts.
• AT (Automatic Transmission) warning light is on.
• Check Engine Light is on.
• Vehicle enters "limp mode," often getting stuck in 3rd gear.
• Hesitation or sudden jerk when shifting from Park into Drive.
• Abrupt or banging gear engagement when pulling away from a stop.

Most Likely Causes

1. Faulty Transmission Valve Body Assembly 🔴 High Probability → Shop Transmission Valve Body Mazda issued TSB #05-003/14 specifically for this issue on 2012-2013 models, identifying a faulty valve body as the root cause for P0843 and other related codes. The pressure sensor is an integral part of this mechatronic assembly, and early production units were prone to internal failures.
   How to confirm: After ruling out fluid and external wiring issues, the diagnosis points to the valve body per the TSB, especially if the vehicle's VIN is within the affected range (lower than JM1BL**733728). The presence of this code alongside P0780 or P1738 is a very strong indicator.
   Typical fix: Replace the entire valve body assembly with the updated part from Mazda and perform the 'Initial Learning Procedure' with a compatible scan tool. The new valve body comes as a programmed unit including the TCM and solenoids.
   Est. part cost: $1200-$1800
2. Low or Dirty Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While possible and always worth checking, it is a very infrequent cause for a 'Circuit High' code on this platform. The TSB and widespread owner experiences point to a hardware failure, not a fluid issue.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level and appear clean and reddish. The correct fluid is Mazda ATF-FZ, which is a low-viscosity blue fluid.
   Typical fix: Perform a transmission fluid drain-and-fill using only Mazda ATF-FZ fluid. 🎬 See this step-by-step guide for a transmission fluid dump and fill. This is unlikely to resolve a P0843 code but is good preventative maintenance.
   Est. part cost: $70-$180
3. Wiring or Connector Issue ⚪ Low Probability
   How to confirm: Visually inspect the transmission wiring harness for any signs of damage, corrosion, or loose connections where the harness connects to the transmission case. A multimeter can be used to check for a short to power on the sensor's signal wire. However, since the sensor and its immediate wiring are internal to the transmission, this is less likely to be the cause than the valve body itself.
   Typical fix: Repair the damaged section of the external wiring harness or clean the main connector.
   Est. part cost: $20-$150

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly A failing TCM can theoretically cause this code. However, on this specific vehicle, the TCM is integrated into the valve body assembly. Therefore, a faulty TCM necessitates the replacement of the entire valve body, which is already the primary fix identified in the TSB.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner. Note all present codes, paying close attention to P0780 and P1738.
2. Check the transmission fluid level and condition. Ensure it is full and clean. The correct fluid is Mazda Type ATF-FZ. If the fluid is low, top it off and re-evaluate, but do not expect this to fix a P0843 code.
3. Review Mazda Technical Service Bulletin #05-003/14. Check if the vehicle's VIN is lower than JM1BL**733728. If the VIN matches and codes P0843, P0780, or P1738 are present, the cause is confirmed to be the valve body.
4. If the TSB applies, the definitive next step is to replace the transmission valve body assembly.
5. During replacement, also replace the transmission oil pan bolts and the internal oil filter/strainer (Part # FZ01-21-500).
6. After replacement, a Mazda-specific scan tool must be used to perform the 'Initial Learning Procedure' as specified by Mazda to ensure the new valve body is calibrated to the vehicle.
🎬 Watch: How to perform a simple transmission reset for better shifting.
7. Clear all codes and perform a thorough road test to verify the repair is complete and shifting is smooth.

Parts You'll Likely Need

• Transmission Control Valve Body Assembly (OEM #PEY0-21-C00) — This is the specific part identified by Mazda in TSB #05-003/14 to fix P0843 and related shifting problems on these vehicles. It is a mechatronic unit that includes the TCM and solenoids.
  Trusted brands: Mazda (OEM)
  OEM price range: $1200-$1800
  Aftermarket price range: Not recommended due to programming requirements.
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — Should be replaced any time the valve body is replaced, as the oil pan must be removed.
  Trusted brands: Mazda (OEM)
  OEM price range: $45-$60
  Aftermarket price range: $20-$40
• Mazda Automatic Transmission Fluid FZ (OEM #0000-FZ-113E-01) — This is the required fluid for the Skyactiv-Drive transmission. Approximately 4-5 quarts are needed for a drain and fill associated with the valve body replacement.
  Trusted brands: Mazda (OEM)
  OEM price range: $18-$25 per quart
  Aftermarket price range: Not recommended

Related Codes That Often Appear With This One

• P0780 — Stands for 'Gear Shifting Malfunction'. It often appears with P0843 because the incorrect pressure reading directly causes shifting errors. The Mazda TSB lists this as a primary companion code.
• P1738 — This is a manufacturer-specific code for 'Automatic Transaxle Internal Malfunction'. It's a general indicator of an internal transmission problem, which is consistent with a faulty valve body. The Mazda TSB also lists this as a primary companion code.
• P0842 — This code is for 'Transmission Fluid Pressure Sensor/Switch 'A' Circuit Low'. It's the opposite of P0843 but is also listed in the same family of TSBs, indicating a general failure of the pressure sensor circuit within the valve body.
• P0847, P0872, P0877 — These codes relate to other pressure switches (B, C, D) within the transmission. Their presence alongside P0843 reinforces that the fault is systemic within the valve body's hydraulic control and sensing circuits.

Technical Service Bulletins (TSBs) & Recalls

• 05-003/14: MIL ON WITH DTC P0780 AND/OR P1738 OR OTHER DTCS. This is the primary TSB for the 2012-2013 Mazda3 that directly calls for valve body replacement to fix P0843 and its companion codes.
• 05-005/13: The original TSB number, later superseded by 05-003/14.
• 05-002/15 / 05-004/18 / 05-002/23: Later TSBs for other Skyactiv models that address similar codes (P0842, P0847, etc.) caused by contamination or stuck pressure switches.

Platform-Specific Known Issues

• Mazda issued Technical Service Bulletin #05-003/14, which superseded several previous versions, for 2012-2013 Mazda3 vehicles with the 2.0L Skyactiv engine and a VIN lower than JM1BL**733728. It states that if the MIL is on with DTC P0780, P1738, or a host of other pressure-related codes including P0843, the fix is to replace the valve body with a 'new programmed valve body'. The repair is covered under the Federal Emission Warranty (96 months / 80,000 miles).

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch 'A' Continuity On-Vehicle Test — expected: With the engine running and selector in D, there should be continuity between the switch terminal and ground.. Failure: No continuity when in Drive, or continuity when in Park/Neutral, indicates a stuck or failed switch.

Hidden / Shadow Codes Worth Checking

• P06B8:00: Non-volatile memory (NVM) error, specifically displayed if the mandatory 'Initial Learning' procedure is not performed or fails after replacing the valve body assembly. (see via This code is retrieved using a Mazda-compatible scanner (like M-MDS) from the TCM after a valve body replacement.)

Scan Tool Commands That Help

• Mazda M-MDS (or equivalent): Initial Learning Procedure — This procedure is mandatory after replacing the transmission valve body. It calibrates the new, pre-programmed TCM to the transmission's specific hydraulic and mechanical characteristics.
• Mazda M-MDS (or equivalent): Data Logger -> Module -> TCM -> PID: TFT (Transmission Fluid Temperature) — Used to monitor the transmission fluid temperature before starting the Initial Learning Procedure. The temperature must be between 40—110 °C {72—198 °F} for the procedure to run correctly.

Wiring & Ground Locations

• Transmission Case Grounds — There are multiple ground points directly on the transmission case; one on the top of the transmission harness and two on the front of the case (one large, one small).. A poor ground connection for the transmission assembly could potentially cause erratic electrical behavior in internal sensors, although the TSB points to an internal component failure rather than a grounding issue.
• Main TCM Connector — The TCM is internal and integrated with the valve body. The main external electrical connection is the large harness connector on the outside of the transmission case.. While unlikely to be the cause, damage or corrosion at this main external connector could disrupt communication between the vehicle and the internal TCM, causing various codes. It is the primary point for external wiring checks.

Real Owner Repair Stories

• Reddit user r/MechanicAdvice (2018 Mazda6 (different model, but same FW6A-EL transmission family)) — Rough and early shifting, particularly into 3rd and 5th gear.
  ❌ Tried (didn't work) A temporary relearn procedure would make symptoms disappear for a few days before returning.
  ✅ What actually fixed it The user replaced the valve body themselves. The car initially threw a speed sensor code after the repair. The final fix was reprogramming the new TCM with a programming interface, after which all shifting issues were resolved and no codes returned.

OEM Part Supersession History

• PEY0-21-C00B → PEY0-21-C00C — Revision and improvement of the valve body assembly.
  Heads up: The TSB specifies part PEY0-21-C00 for the repair. Later revisions like C00C are generally acceptable, but using parts from different engine/model configurations, even if they physically fit, can cause issues due to different TCM programming.

Model Year Variations Within This Range

• 2012-2013: The TSB-identified fault is specific to vehicles produced before September 28, 2012 (VINs lower than JM1BL**733728). Vehicles produced after this date have factory-installed improvements to mitigate the issue, making the valve body failure less common.