P1738 on 2014-2017 Mazda MAZDA6: Automatic Transaxle Internal Malfunction Guide

What's Unique About the 2014-2017 Mazda MAZDA6

The 2014-2017 Mazda6 uses the Skyactiv-Drive 6-speed automatic transmission (model FW6A-EL). 🎬 Watch: A detailed technical overview of the Skyactiv-Drive transmission. For this specific transmission, code P1738 is very frequently linked to a known issue documented in multiple Technical Service Bulletins (TSBs). The problem is that internal oil pressure switches, located on the control valve body, can get stuck due to contamination in the transmission fluid. This prevents the TCM from correctly verifying hydraulic pressures during gear changes, leading it to set the general P1738 code as a catch-all when specific pressure switch codes are also present.

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 jerky shifting.
• Transmission may feel like it's slipping or hesitating between shifts
• Vehicle may enter a 'limp mode' with limited gear availability (e.g., stuck in 3rd gear)
• Manual shift mode may be disabled.
🎬 See a demonstration of these common Mazda 6 transmission symptoms.
• Traction control light may also illuminate.

Most Likely Causes

1. Faulty Transmission Control Valve Body 🔴 High Probability → Shop Transmission Valve Body Multiple Mazda TSBs identify that oil pressure switches within the valve body can stick due to contamination, a known issue on the FW6A-EL transmission. Mazda later issued updated valve body assemblies and, in some cases, updated TCM software to correct the problem.
   How to confirm: Diagnosis typically involves confirming the presence of P1738, often alongside other pressure switch codes (like P0842, P0877, etc.). A technician will follow the diagnostic procedure outlined in the relevant TSB, which may start with a TCM software update but often leads to valve body replacement.
   Typical fix: Replace the entire control valve body assembly with the updated part. This must be followed by an 'Initial Learning Procedure' using a Mazda-specific scan tool. Some TSBs suggest replacing only the two affected pressure switches (A and B) as a possible, less expensive repair.
   Est. part cost: $800-$1500
2. Low or Contaminated Automatic Transmission Fluid (ATF) ⚪ Low Probability → Shop Transmission Assembly While not the root cause as often as the valve body itself, low or dirty fluid can cause hydraulic pressure issues and is the source of the contamination that makes the pressure switches stick. By the time P1738 is set, the damage to the switches is usually done.
   How to confirm: Check the level and condition of the ATF according to the manufacturer's procedure. The fluid should be bright red (Mazda ATF-FZ), not dark, burnt-smelling, or low.
   Typical fix: Perform a transmission fluid drain-and-fill or flush. However, if the P1738 code is already present, a fluid change alone is highly unlikely to fix the stuck internal components and the code will return.
   Est. part cost: $100-$250

Rare But Worth Checking

• Faulty Transmission Control Module (TCM) Software: → Shop Transmission Assembly

Diagnosis Steps

1. Scan the Transmission Control Module (TCM) for all stored trouble codes. Note if P1738 is present with other codes like P0780 or any pressure switch codes (P08xx).
2. Check the automatic transmission fluid (ATF) level and condition. The level should be correct, and the fluid should be bright red (Mazda ATF-FZ), not dark or burnt.
3. Consult Mazda Technical Service Bulletins (TSBs) related to P1738 for this vehicle (e.g., 05-002-23, 05-005-14). The TSBs provide the most direct diagnostic path.
4. Per the TSBs, a dealer may first attempt to reprogram the TCM with the latest calibration file.
5. If reprogramming does not work or is not applicable, the next step is internal inspection. This involves removing the transmission oil pan.
6. Based on the TSBs, the most likely cause is a faulty control valve body or the individual pressure switches on it. Some TSBs allow for the replacement of just the pressure switches (A and B). However, the most common and surefire repair is the replacement of the entire valve body assembly.
7. After replacing the valve body or switches, the transmission oil pan gasket and bolts should also be replaced. Apply sealant as specified.
8. Refill the transmission with the correct type of Mazda ATF (Type FZ).
9. Perform the mandatory 'Initial Learning Procedure' using a compatible diagnostic tool like the Mazda M-MDS to ensure the new components communicate correctly with the TCM.
10. Clear all codes and perform a road test to verify the repair.

Parts You'll Likely Need

• Automatic Transmission Control Valve Body (OEM #FZ2121100J (example, verify by VIN)) — This is the component identified by Mazda in numerous TSBs as the root cause of code P1738 due to internal oil pressure switches sticking. A Reddit user was quoted ~$1300 for this part alone.
  Trusted brands: Mazda (OEM)
  OEM price range: $1000-$1500
  Aftermarket price range: $700-$1200
• Oil Pressure Switch 'A' & 'B' (OEM #FZ01212C0 (Switch A), FZ01212J0 (Switch B)) — Some TSBs allow for the replacement of just the two faulty switches instead of the entire valve body. This is a less common but more affordable repair path.
  Trusted brands: Mazda (OEM)
  OEM price range: $50-$100 each
  Aftermarket price range: $30-$70 each
• Automatic Transmission Fluid (ATF) (OEM #0000-FZ-113E-01) — New fluid is required when replacing the valve body. The system uses Mazda's specific low-viscosity ATF-FZ.
  Trusted brands: Mazda ATF-FZ
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart
• Transmission Oil Pan Gasket — The oil pan must be removed to access the valve body, and the gasket (or liquid sealant, depending on the specific procedure) should be replaced to prevent leaks.
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0780 — Stands for 'Gear Shifting Malfunction'. It is frequently stored with P1738 because the underlying valve body issue directly causes improper gear shifts.
• P0842, P0847, P0872, P0877 — These are specific codes for 'Oil Pressure Switch Stuck On'. P1738 is often set as a general fault when one or more of these specific switch faults are detected by the TCM.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: Notes P1738 is stored with other transmission codes (P0842, P0847, P0872, P0877, P0780) when the check engine and AT lights are on.
• 05-005-14-2902D: Defines P1738 as 'AUTOMATIC TRANSAXLE INTERNAL MALFUNCTION' and links it to oil pressure switches A and/or B getting stuck due to contamination.
• 0500215: Mentions P1738 as one of several codes that appear with the Check Engine and A/T warning lights.
• 05-005/13: Recommends replacing the valve body assembly with a new pr

Platform-Specific Known Issues

• A series of Technical Service Bulletins (TSBs), including #05-002-23, #05-005-14-2902D, and #05-004-18, explicitly state that P1738 on these vehicles is caused by oil pressure switches in the control valve body getting stuck from contamination. The official fix is to replace the valve body with an improved part and sometimes reprogram the TCM. In some cases, the TSB allows for replacing only the two pressure switches ('A' and 'B') on the valve body.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity — expected: The switch should change state (from open to closed circuit, or vice-versa) when hydraulic pressure is applied. A normally closed switch will show continuity (near 0 ohms) with no pressure and open circuit (infinite resistance) with pressure.. Failure: A switch that is stuck will not change its electrical state. However, the failure mode documented in TSBs is mechanical (sticking) due to contamination, so electrical testing is often skipped in favor of direct replacement of the switches or the entire valve body.

Hidden / Shadow Codes Worth Checking

• Pxxxx:00: The ':00' suffix seen on P1738 and related codes in Mazda TSBs is a component identifier within the OBD-II standard, but does not represent a separate 'shadow code'. Its consistent presence in dealer documentation confirms the specific fault variant being addressed. (see via Mazda M-MDS or advanced scan tools like FORScan.)

Scan Tool Commands That Help

• Mazda M-MDS (Mazda Modular Diagnostic System): Initial learning of AT — This procedure is mandatory after replacing the control valve body or the individual oil pressure switches. It allows the TCM to learn the hydraulic characteristics of the new components. Failure to perform it can cause harsh shifts or engine racing.
• Mazda M-MDS (Mazda Modular Diagnostic System): Mechanical System Test — This test is performed after the 'Initial Learning' procedure to verify the mechanical operation of the transmission and confirm the repair was successful.

Real Owner Repair Stories

• AliExpress Product Page Technical Description (2014 Mazda CX-5 with 120,000 miles (Uses the same FW6A-EL transmission)) — Delayed shifts and occasional hesitation during acceleration.
  ❌ Tried (didn't work) Not specified, but symptoms are classic for this failure.
  ✅ What actually fixed it The valve body was replaced with a new FW6A-EL unit, which resolved the issues, resulting in smoother gear changes and reduced shift shock.

OEM Part Supersession History

• FZ2121100H → FZ2121100J — The part was updated as part of mass production changes to reduce contamination and improve the diagnostic logic, as stated in TSBs.

Model Year Variations Within This Range

• 2014-2017: While the transmission hardware (FW6A-EL) is consistent, the official repair procedure varies based on the vehicle's VIN and the installed TCM software version. Some TSBs instruct technicians to first attempt a TCM software update for certain VIN ranges, whereas later vehicles may require immediate parts replacement, indicating a rolling software and/or hardware update during production.