P0842 on 2014-2021 Mazda CX-5: Transmission Pressure Switch 'B' Fault Explained

What's Unique About the 2014-2021 Mazda CX-5

For the Mazda CX-5 and other models with the SKYACTIV 6-speed automatic transmission, P0842 is rarely an isolated code. It is part of a well-documented pattern of faults described in multiple Mazda TSBs. The root cause is often contamination within the transmission's hydraulic passages, causing one or more pressure switches on the valve body to stick. Mazda's official repair procedure, outlined in a long series of superseded TSBs, prioritizes a software update to the TCM before any parts are replaced, as updated logic can filter out erroneous signals from a temporarily sticking switch.

Diagnostic Flowchart

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

Generation note: The 2014-2021 year range covers the late first generation (KE, through 2016) and the second generation (KF, 2017-2021) of the CX-5. Both generations use a SKYACTIV-Drive 6-speed automatic transmission (FW6A-EL) and are covered under the same Technical Service Bulletins (TSBs) for this issue, indicating the cause and repair are consistent across these model years.

Symptoms You May Notice

• Check Engine Light illuminated
• Automatic Transaxle (AT) warning light illuminated.
• Harsh or erratic gear shifting.
• Transmission may feel like it's slipping, hesitating, or banging into gear.
• Vehicle may enter a "limp mode," getting stuck in a single gear (often 3rd gear).
• In some cases, the gear indicator on the dashboard may disappear.

Most Likely Causes

1. Sticking Internal Transmission Oil Pressure Switch 🔴 High Probability → Shop Engine Oil Pressure Switch Mazda TSBs state this is caused by contamination entering the hydraulic passages of the control valve body, causing the switch to stick. This has been a persistent issue, leading Mazda to release numerous TSB updates over the years.
   How to confirm: A professional technician will scan the TCM for P0842 and other related codes (P0847, P0872, P0877, P0780, P1738). The first step per the TSB is a TCM software update. If the code returns after the update, the switches are the confirmed cause.
   Typical fix: First, reprogram the Transmission Control Module (TCM) with updated software. If the problem persists or the software is already current, replace oil pressure switches 'A' and 'B' located on the transmission's internal valve body.
   Est. part cost: $100-$200 for the switches
2. Outdated Transmission Control Module (TCM) Software 🟡 Medium Probability → Shop Transmission Assembly Mazda released software updates specifically to make the diagnostic logic less sensitive to minor, temporary fluctuations from the pressure switches, effectively preventing the code from being set unnecessarily.
   How to confirm: A Mazda dealership or a qualified shop with a J2534 pass-through device can check the current software calibration number against the latest version listed in TSBs.
   Typical fix: Reprogram the TCM to the latest calibration. This is often the first and only step needed, especially if the code was intermittent.
   Est. part cost: $0 (parts), but labor for programming applies
3. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While the TSB points to contamination as the root cause of the switches sticking, simply having low or dirty fluid can also cause a variety of pressure-related issues and codes. The switches themselves are very sensitive to foreign particles in the oil.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level and appear clean and red, not dark, burnt, or full of debris.
   Typical fix: Perform a transmission fluid drain and fill. If significant debris is found in the pan, the oil strainer should also be replaced as recommended by Mazda's TSB.
   Est. part cost: $70-$150 for fluid and filter

Rare But Worth Checking

• Wiring Harness Issue: Damage to the wiring between the TCM and the transmission's internal harness could cause a low voltage signal. This is less common than the switch issue but possible.
• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly In very rare cases, the TCM itself may fail. This should only be considered after all other possibilities, especially those outlined in the TSB, have been exhausted.

Diagnosis Steps

1. Verify the Check Engine Light and AT warning light are on.
2. Use an OBD-II scanner to read codes from the Transmission Control Module (TCM). Confirm if P0842 is present, along with any of the common companion codes (P0847, P0872, P0877, P0780, P1738).
3. Check the transmission fluid level and condition. If low, top it off. If dark or burnt, a fluid service may be warranted.
4. Consult a Mazda dealer or qualified independent shop to check the current TCM software version. If it is not the latest version, perform the TCM reprogramming as specified in TSB #05-002-23.
5. Clear the codes and road test the vehicle. If the code does not return, the repair is complete.
6. If the code returns after the software update, the internal oil pressure switches are the cause.
7. To replace the switches, drain the transmission fluid and remove the transmission oil pan.
8. Disconnect and remove oil pressure switches 'A' and 'B' from the valve body and install new ones. Tighten the new switch bolts to the specified torque: 9-10 N·m (80-88 in·lbf).
9. If a large amount of debris was found in the pan, replace the oil strainer (P/N: FZ01-21-500).
10. Clean the mating surfaces and reinstall the oil pan. Mazda specifies applying a light coat of silicone sealant.
11. Refill the transmission with the correct type and amount of Mazda SKYACTIV-Drive ATF.
12. Clear codes and perform the Initial Learning Procedure as specified in the workshop manual, followed by a final road test to verify the repair.

Parts You'll Likely Need

• Transmission Oil Pressure Switch 'B' (OEM #FZ01-21-2J0) — This is the specific component identified by Mazda's TSB for code P0842. It gets stuck due to contamination. It is highly recommended to replace both switch 'A' and 'B' at the same time.
  Trusted brands: Mazda OE
  OEM price range: $50-$100 per switch
  Aftermarket price range: $30-$60 per switch
• Mazda SKYACTIV-Drive ATF FZ (OEM #0000-FZ-113E-01) — The correct fluid must be used when refilling the transmission after replacing the switches. Using non-specified fluid can cause further damage.
  Trusted brands: Mazda OE
  OEM price range: $18-$25 per quart
• Transmission Oil Strainer (Filter) (OEM #FZ01-21-500) — The TSB recommends 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: $40-$60
  Aftermarket price range: $20-$40
• Transmission Oil Pan Gasket / Sealant — The oil pan must be re-sealed upon reinstallation to prevent leaks. Mazda specifies using silicone sealant rather than a pre-formed gasket.
  Trusted brands: Permatex The Right Stuff, Honda Bond
  OEM price range: $20-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0847 — Relates to Oil Pressure Switch No. 2 ('A') sticking on. Both switches A and B are often replaced together as a set.
• P0872 — Relates to Oil Pressure Switch No. 3 ('A') sticking on.
• P0877 — Relates to Oil Pressure Switch No. 4 ('B') sticking on.
• P0780 — A general code for a gear shifting malfunction, often triggered as a result of the pressure switch failures.
• P1738 — Indicates an automatic transaxle internal malfunction, which is a companion code to the specific switch faults.

Technical Service Bulletins (TSBs) & Recalls

• 05-002-23: 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).
• 05-002-15 / 05-005-14: Earlier versions of the TSB also describe the same cause: oil pressure switch A and/or B getting stuck due to contamination. These bulletins confirm the issue is shared across Mazda3, Mazda6, and CX-5 models.

Platform-Specific Known Issues

• Mazda has issued multiple superseding Technical Service Bulletins, with the most recent being #05-002-23, addressing P0842 and related codes across many models with the SKYACTIV 6-speed automatic transmission.

Mechanic-Grade Diagnostic Values

• Oil Pressure Switch Continuity to Ground — expected: In Park or Neutral with engine running, resistance should be infinite (no continuity). In Drive with engine running, resistance should be 0 ohms (continuity to ground).. Failure: Continuity to ground while in Park or Neutral indicates a stuck-on switch, which is the direct cause of code P0842.

Scan Tool Commands That Help

• Mazda IDS (Integrated Diagnostic System) or equivalent J2534 device: Module Reprogramming — This is the first step in the official TSB repair procedure to update the TCM with improved diagnostic logic before replacing any parts.
• Mazda IDS (Integrated Diagnostic System) or equivalent J2534 device: Initial Learning [FW6A-EL] — This procedure must be performed after replacing the oil pressure switches or the control valve body to allow the TCM to learn the characteristics of the new components.

Wiring & Ground Locations

• G01 / J/C G01 — Under the left (driver's side) headlight assembly in the engine compartment.. This is a primary ground distribution point in the engine bay for multiple components, including the TCM. A poor connection here could potentially cause erratic behavior or low voltage signals.
• G03 — Located on the left front of the engine compartment.. This ground point serves the Powertrain Control Module (PCM) and other critical engine components. While the TCM is separate, ensuring all main powertrain grounds are clean and secure is a crucial diagnostic step for any electronic fault.
• Transmission Case Ground — A ground cable from the negative battery terminal bolts to a bracket on top of the transmission housing.. This is the main ground for the entire transmission assembly. A loose or corroded connection here can cause a wide range of electrical issues, including sensor faults.