P1738 on 2016-2020 Mazda CX-9: Automatic Transaxle Malfunction Causes and Fixes

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

The SKYACTIV-Drive 6-speed automatic transmission (model FW6A-EL) used in the second-generation CX-9 and other Mazda models of this era is known for this specific issue. Mazda has acknowledged the problem through multiple Technical Service Bulletins (TSBs), which state that internal contamination leads to sticking oil pressure switches within the valve body. This triggers a cascade of fault codes including the general P1738. The contamination is often fine metallic debris from normal wear that finds its way into the sensitive switch passages.

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.
• Vehicle may enter a 'fail-safe' or 'limp' mode, inhibiting manual shifting and other features like i-stop.
• Harsh or jerky shifting may be noticeable.
• In some cases, the transmission may seem to shift normally despite the warning lights.

Most Likely Causes

1. Stuck Internal Oil Pressure Switches 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSBs 05-004/19 and 05-002-23 explicitly state this is caused by contamination entering the fluid passages for oil pressure switches A and/or B on the control valve body, causing them to stick.
   How to confirm: A professional technician will scan for associated codes (P0842, P0847, P0872, P0877, P0780) which directly point to pressure switch failures. The presence of these codes alongside P1738 is a strong confirmation.
   Typical fix: Replace both oil pressure switches (A and B) on the transmission valve body, replace the transmission filter/strainer, and perform a drain and fill with new ATF FZ fluid.
   Est. part cost: $150-$300
2. Low Automatic Transmission Fluid (ATF) Level 🟡 Medium Probability → Shop Transmission Assembly
   How to confirm: Check the transmission fluid level according to the manufacturer's procedure. This is often the first diagnostic step listed in the factory service manual.
   Typical fix: Top off the fluid to the correct level and inspect for leaks. If the fluid is old or dirty, a fluid and filter change is recommended.
   Est. part cost: $50-$150
3. Outdated Transmission Control Module (TCM) Software ⚪ Low Probability → Shop Transmission Assembly Early versions of the TCM software were less tolerant of minor pressure fluctuations. Mazda issued updates to improve the diagnostic logic, as noted in TSB 05-004/19.
   How to confirm: A Mazda dealership or qualified shop can check the current software version against the latest available version mentioned in TSBs.
   Typical fix: Reprogram the TCM to the latest calibration. This is often done first, and if the code returns, the switches are then replaced.
   Est. part cost: $0 (if part of another repair or under warranty)

Rare But Worth Checking

• Failed Control Valve Body: → Shop Transmission Valve Body If the passages in the valve body are severely blocked, warped, or scored by debris, replacing just the switches may not solve the problem. In these cases, the entire valve body assembly may need to be replaced.
• Internal Transmission Failure: → Shop Transmission Assembly In the worst-case scenario, if P1738 returns after other repairs or is present without any accompanying pressure switch codes, Mazda's own diagnostic tree points to replacing the entire automatic transaxle assembly. This is very unlikely to be the cause if other switch codes are present.

Diagnosis Steps

1. Verify the presence of P1738 and any accompanying codes using an OBD-II scanner.
2. Check the automatic transmission fluid (ATF) level and condition. Top off or replace if necessary.
3. Inspect for any visible transmission fluid leaks.
4. If codes P0842, P0847, P0872, or P0877 are present, the primary suspects are the internal oil pressure switches.
5. Consult Mazda TSB 05-004/19 or 05-002-23 to see if a TCM reprogram is recommended for your specific vehicle's VIN and production date before parts replacement.
6. Gain access to the transmission valve body by removing the oil pan.
7. Inspect the oil pan for excessive metal shavings or debris, which would indicate a more severe internal failure.
8. Replace both oil pressure switches (A and B) on the valve body.
9. Install a new transmission filter/strainer and O-rings.
10. Reinstall the oil pan with a new gasket and fill with the correct type and amount of Mazda ATF FZ fluid.
11. Clear all DTCs, perform the 'Initial Learning' procedure as specified by Mazda, and perform a test drive to ensure the issue is resolved.

Parts You'll Likely Need

• Transmission Oil Pressure Switch Kit (A & B) (OEM #FZ01-21-2C0 (Switch A), FZ01-21-2J0 (Switch B)) — These switches are the most common cause of failure according to multiple Mazda TSBs. They are replaced as a pair.
  Trusted brands: Mazda OE
  OEM price range: $120-$200 for the pair
  Aftermarket price range: $80-$150
• Automatic Transmission Fluid (ATF) (OEM #0000-FZ-113E-01) — New fluid is required when dropping the pan to replace components. Only Mazda ATF FZ should be used.
  Trusted brands: Mazda ATF FZ
  OEM price range: $15-$25 per quart
  Aftermarket price range: N/A (Use OEM)
• Transmission Oil Pan Gasket (OEM #FW6A-21-51Y) — A new gasket is required to prevent leaks when reinstalling the oil pan.
  Trusted brands: Mazda OE, Fel-Pro
  OEM price range: $30-$50
  Aftermarket price range: $15-$30
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — It is best practice to replace the filter when the pan is removed, as it may be clogged with the same contaminants that caused the switches to fail.
  Trusted brands: Mazda OE
  OEM price range: $40-$60
  Aftermarket price range: $20-$40

Related Codes That Often Appear With This One

• P0780 — Stands for 'Gear Shifting Malfunction' and is triggered by the same underlying pressure control issue. It is listed in the same TSBs as P1738.
• P0842 / P0847 / P0872 / P0877 — These are specific codes for the oil pressure switches being stuck on (P0842/P0847) or off. They directly identify the root cause that also triggers the general P1738 code.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: The latest bulletin confirming the list of DTCs (including P1738) related to the automatic transaxle warning light and stuck pressure switches.
• 05-004-19 / 05-004-18: A series of superseded bulletins that detail the cause (stuck pressure switches due to contamination) and the full repair procedure, which may include TCM reprogramming or parts replacement for the 2016-2019 CX-9. This TSB explicitly lists the affected models, including the CX-9 and its platform mates.

Platform-Specific Known Issues

• A known issue across multiple Mazda models using the SKYACTIV-Drive 6-speed automatic transmission is contamination causing oil pressure switches to stick, as documented in TSB #05-002-23 and others.
• This issue can occur at relatively low mileage. One owner on the Mazdas247 forum reported the failure on a 2019 CX-9 with only 14,000 miles, which was repaired under warranty by replacing the switches per TSB 05-004-19.

Mechanic-Grade Diagnostic Values

• Shift Solenoid D/E Resistance — expected: 10.9—26.2 ohms. Failure: A reading outside this range indicates a faulty solenoid.
• Shift Solenoid A/B/C Resistance — expected: 1.0—4.2 ohms. Failure: A reading outside this range indicates a faulty solenoid.
• Pressure Control Solenoid A Resistance — expected: 2.4—7.3 ohms. Failure: A reading outside this range indicates a faulty solenoid.
• ATF Temperature for Initial Learning — expected: 140-230°F (60-110°C). Failure: The learning procedure will abort if the fluid temperature is outside this range.

Scan Tool Commands That Help

• Mazda IDS (Integrated Diagnostic System): Module Reprogramming — Used to update the TCM to the latest calibration as specified by TSBs. This is often the first step before replacing parts, as software updates can change diagnostic logic and tolerance for minor pressure fluctuations.
• Mazda IDS (Integrated Diagnostic System): Initial Learning — This procedure must be performed after replacing the oil pressure switches or the valve body to make the TCM learn the characteristics of the new components. Failure to do so can result in continued poor shifting.
• Manual Procedure (No Scan Tool): Transmission Initial Learning / Adaptive Memory Reset — This procedure can be attempted by owners to resolve minor shifting issues or after a fluid change. It forces the TCM to relearn clutch engagement points. It involves a specific sequence of starting the engine, shifting between Neutral and Manual mode, and holding the shifter in M+ and M- positions until the idle speed changes. Note: This will not fix a hardware failure like a stuck pressure switch.

Wiring & Ground Locations

• Transmission Control Module (TCM) — On the 2016-2020 CX-9, the TCM is mounted directly on top of the transmission case, under the battery, battery tray, and air filter housing.. While the P1738 code is almost always an internal transmission issue, in very rare cases of communication errors or multiple seemingly unrelated codes, the TCM's connection or ground could be suspect. Access requires removing components in the engine bay.
• Pressure Switch A & B Connectors — Inside the transmission, connected to the valve body. Each switch has a two-pin connector with red and white wires.. These are the primary components that fail. A technician must access them by removing the transmission oil pan and valve body components.

OEM Part Supersession History

• FZA1-21-2C0 → FZ01-21-2C0 — Part revision or manufacturer change.
  Heads up: FZ01-21-2C0 is the current, correct part number for Pressure Switch A. Using the old part number may result in sourcing incorrect or old stock.

Model Year Variations Within This Range

• 2016-2020: The transmission model designation changes based on the drivetrain. FWD models use the FW6A-EL, while AWD models use the FW6AX-EL. However, the internal components, fluid type (ATF FZ), and the P1738 failure mode related to the pressure switches are the same for both.