P0842 on 2016-2021 Mazda CX-3: Transmission Pressure Switch Failure Causes and Fixes

What's Unique About the 2016-2021 Mazda CX-3

The Mazda CX-3 uses a SKYACTIV-Drive 6-speed automatic transmission (model FW6A-EL) which has a well-documented history of this specific failure. Mazda has issued multiple TSBs acknowledging that internal contamination can cause various oil pressure switches to stick, triggering P0842 and a cluster of related codes. Unlike a simple sensor failure on other cars, this code on a CX-3 almost always points directly to a fault within the transmission's control valve body assembly.

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 erratic shifting
• Transmission getting stuck in a single gear (limp mode)
• Delayed engagement when shifting from Park to Drive or Reverse
• Poor acceleration

Most Likely Causes

1. Faulty Transmission Control Valve Body 🔴 High Probability → Shop Transmission Valve Body Mazda TSBs #05-002-23 and #0500215 explicitly state this code is caused by an oil pressure switch within the valve body getting stuck due to contamination. This is a known weakness in the FW6A-EL transmission used in the CX-3.
   How to confirm: A professional technician will confirm by verifying the code, checking for the presence of other related codes (like P0847, P0872, etc.), and following the diagnostic procedure 🎬 See: Expert tips for diagnosing common Mazda transmission issues. outlined in the TSB. The ultimate confirmation is the replacement of the part resolving the issue.
   Typical fix: Replace the entire transmission valve body assembly. This part is often a mechatronic unit that includes the solenoids and sometimes the Transmission Control Module (TCM), which must be programmed to the vehicle.
   Est. part cost: $1100-$1700

Rare But Worth Checking

• Wiring Harness Damage: While the valve body is the most common culprit, damage to the wiring between the TCM and the transmission valve body could theoretically cause a similar circuit fault. This should be inspected before condemning the expensive valve body.
• Contaminated or Incorrect Transmission Fluid: → Shop Transmission Assembly Using fluid other than the specified Mazda ATF-FZ (a unique blue fluid) can cause shifting problems and potentially contribute to component failure over time. While a fluid flush alone is unlikely to fix a stuck switch, ensuring the correct fluid is used is critical.
• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly In some cases, the TCM itself can fail. However, because the TCM is often integrated into the valve body assembly for this transmission, they are typically replaced as a single unit.

Diagnosis Steps

1. Scan the TCM for all stored Diagnostic Trouble Codes (DTCs). Note if P0842 is present with any of the related codes (P0847, P0872, P0877, P0780, P1738).
2. Check for any applicable Technical Service Bulletins (TSBs) for the vehicle's VIN. TSBs 05-002-23 and 0500215 are highly relevant.
3. Inspect the transmission fluid level and condition. The fluid should be the correct Mazda ATF-FZ (blue in color) and not burnt or full of debris.
4. Visually inspect the transmission wiring harness and connectors for any signs of damage, corrosion, or loose connections.
5. If multiple related pressure switch codes are present, the diagnostic path per Mazda's TSBs points directly to replacing the control valve body.
6. If only P0842 is present, a technician may perform advanced circuit testing with a multimeter to rule out a wiring issue before replacing the valve body.

Parts You'll Likely Need

• Automatic Transmission Valve Body (OEM #FZ21-21-100J) — This is the component identified by Mazda as the root cause of the problem due to internal contamination causing pressure switches to stick. It is typically replaced as a complete assembly.
  Trusted brands: Mazda (OEM)
  OEM price range: $1100-$1700
  Aftermarket price range: N/A
• Mazda ATF-FZ Automatic Transmission Fluid (OEM #0000-FZ-113E-01) — The transmission must be refilled with this specific low-viscosity blue fluid after the valve body replacement.
  Trusted brands: Mazda (OEM)
  OEM price range: $20-$30 per quart
  Aftermarket price range: $15-$25 per quart

Related Codes That Often Appear With This One

• P0847 — Relates to Oil Pressure Switch No. 2. Both are in the same valve body and fail for the same reason.
• P0872 — Relates to Oil Pressure Switch No. 3. Both are in the same valve body and fail for the same reason.
• P0877 — Relates to Oil Pressure Switch No. 4. Both are in the same valve body and fail for the same reason.
• P0780 — A general 'Gear Shifting Malfunction' code that is triggered by the underlying pressure switch failures.
• P1738 — An 'Automatic Transaxle Internal Malfunction' code, also triggered as a result of the valve body issue.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: CHECK ENGINE LIGHT AND A/T WARNING LIGHT ON WITH DTCs (P0842, P0847, P0872, P0877, P0780, P1738)
• 0500215: Check Engine light and A/T Warning light on with one or more of these DTS (P0842, P0847, P0872, P0877, P0780, P1738) Stored in TCM Memory

Platform-Specific Known Issues

• A known issue, documented in Mazda TSB #05-002-23, states: 'Some vehicles may experience the check engine light and automatic transaxle warning light ON with one or more of these DTCs stored in the TCM memory (P0842, P0847, P0872, P0877, P0780, P1738).' The cause is identified as contamination causing oil pressure switches in the control valve body to stick.

Mechanic-Grade Diagnostic Values

• Pressure Control Solenoid Resistance — expected: 2.4 - 7.3 ohms. Failure: A reading outside this range indicates a faulty solenoid coil.
• On-Vehicle Oil Pressure Switch Continuity Test — expected: In Park/Neutral with engine running, there should be no continuity to ground. In Drive with brake applied, there should be continuity to ground.. Failure: Continuity in Park/Neutral (stuck on) or no continuity in Drive (stuck off) indicates a failed switch.
• Diagnostic Trigger Condition — expected: Code sets when ATF temperature is 20°C {68°F} or more and the TCM detects the switch is stuck on.. Failure: This is a condition for the code to set, not a direct test value.

Hidden / Shadow Codes Worth Checking

• P06B8:00: Internal Control Module Non-Volatile Random Access Memory (NVRAM) Error. This code is temporarily stored after a new valve body is installed. (see via This DTC will be present when scanning the TCM with a tool like the Mazda M-MDS. It indicates that the 'Initial Learning' procedure has not yet been completed. It should clear after the procedure is successful.)

Scan Tool Commands That Help

• Mazda M-MDS (Mazda Modular Diagnostic System): Initial Learning Procedure — This procedure is mandatory after replacing the control valve body or the individual oil pressure switches. Failure to perform it can result in harsh shifting or other transmission issues. The process involves warming the ATF to 40-110°C, then using the M-MDS to initiate the learning while cycling the gear selector.
• Mazda M-MDS (IDS): Module Reprogramming (TCM) — For some earlier model years sharing this transmission, TSBs recommended reprogramming the TCM with updated logic as the first step. For the 2016-2021 CX-3, TSBs direct the technician to replace the hardware, but checking for the latest software is still a valid diagnostic step.

Wiring & Ground Locations

• Engine Block Ground Point — A designated metal tab or bolt on the engine block, often used for jump-starting procedures.. When testing wiring for continuity or shorts, a known-good ground reference is essential. The official ground point shown in the owner's manual for jump-starting serves as a reliable and accessible ground for multimeter testing of the transmission harness.
• Transmission Case Ground — A ground wire from the negative battery terminal is typically bolted to a bracket on top of the transmission case.. A loose or corroded main ground for the transmission can cause a host of erratic electrical issues, including faulty sensor readings. Verifying this ground is clean and tight is a fundamental step before diagnosing internal components.

Real Owner Repair Stories

• Reddit user post in r/mazda (2015 Mazda 3 iSport (uses related FW6A-EL transmission)) — No communication between the vehicle and the internal TCM.
  ❌ Tried (didn't work) Replacing the main Powertrain Control Module (PCM) did not solve the communication issue with the transmission.
  ✅ What actually fixed it The dealer diagnosed a faulty valve body, as the TCM is integrated within it. The final recommendation was to replace the valve body assembly (Part No. FZ21-21-100J). The post also noted the individual part numbers for the pressure switches as 'A' (FZ01-21-2C0) and 'B' (FZ01-21-2J0) for reference.

OEM Part Supersession History

• FZ21-21-100 (blank) → Various pre-programmed part numbers (e.g., PEY0-21-C00) — Around 2013, Mazda began supplying pre-programmed valve bodies for specific VIN ranges to simplify replacement, eliminating the need for TCM configuration.
  Heads up: The original blank valve body is no longer available as a service part.
• FZ21-21-100B → FZ21-21-100E — Standard part revision and improvement.
• FZ21-21-100J → Current — This is the most commonly cited part number for modern replacements covering the 2016-2021 CX-3.
  Heads up: This part is for non-turbo models. Turbo or diesel models use different valve bodies, such as FZ31-21-100C.

Model Year Variations Within This Range

• 2016-2021: According to Mazda TSB 05-002-23, vehicles in this range (including the CX-3) that present with P0842 should proceed directly to 'Oil Pressure Switch Replacement' (i.e., valve body replacement). Unlike some earlier models, a TCM reprogram is not the primary recommended fix.