NHTSA ID Number: 11030598
Manufacturer Communication Number: 01-001-26b
TSB/Document Date: 2026-04-02
Summary
Some customers may experience a check engine light ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: Engine takes longer to warm up. Engine temperature gauge fluctuates. Low coolant temperature indicator (blue) light stays on longer or turns on while driving. Poor heater performance, especially at idle. Vehicles with SKYACTIV-G 2.5T are not applicable to this service information. DTC P0126:00 is not required to be present to perform this repair. If the vehicle has not previously had the repair specified in this TSB and is exhibiting any of the listed symptoms, proceed with the repair procedure outlined in this bulletin. If DTC P2B61:00 (Coolant Control Valve Circuit Range/Performance) is present along with DTC P0126:00, this TSB does not apply. Diagnose the symptom and/or stored DTCs according to the procedures in MGSS. If only DTC P0126:00 is present and this TSB has already been completed previously, this bulletin does not apply, diagnose the symptom and/or stored DTCs according to the procedures in MGSS. This concern may be caused if the coolant control valve fail-safe thermostat opens at a temperature lower than the normal temperature due to a deformation of the thermostat diaphragm. To eliminate the concern, the water flow control valve specification has been modified. Affected vehicles will require PCM reprogramming with the Mazda
Modular Diagnostic System (M-MDS), using either a) Mazda Diagnostic and Repair Software (MDARS) or b) Mazda Integrated Diagnostic System (IDS) 140.04 or later software.
BULLETIN NOTES
This bulletin supersedes the previously issued bulletin(s) listed below. The changes are noted in Red text.
Previous TSBs:
Date(s) Issued:
01-001/26
01/07/2026 and 01/30/2026
01-003/25
03/13/25,12/03/25 and 12/23/25
01-005/24
06/10/24, 06/27/24, 08/29/24 and 11/04/24
01-005/23
02/16/23 and 12/20/23
01-013/22
10/19/22
01-011/21
09/27/21 and 04/22/21
APPLICABLE MODEL(S)/VINS
2019-2022 CX-3 2.0/2.5L with VINs lower than JM1DK******605934 (produced before July 8, 2022)
2020-2023 CX-30 2.0/2.5L (Mexico built) with VINs lower than 3MVDM******589428 (produced before Sep. 1, 2023)
2018-2025 CX-5 2.0/2.5L with VINs lower than JM3KF******617579 (produced before Dec. 18, 2024)
2019-2023 Mazda3 2.0/2.5L (Japan built) with VINs lower than JM1BP******623951 (produced before May 23, 2023)
2019-2023 Mazda3 2.0/2.5L (Mexico built) with VINs lower than 3MZBP******384076 (produced before Sep. 5, 2023)
2018-2021 Mazda6 2.0/2.5L with VINs lower than JM1GL******620105 (produced before Nov. 12, 2021)
NOTE: Vehicles with SKYACTIV-G 2.5T (turbo) engine are not applicable to this service information.
DESCRIPTION
Some customers may experience a check engine light ON with DTC P0126:00 (Thermostat stuck open) and one or more
of the following symptoms:
•
•
•
•
Engine takes longer to warm up.
Engine temperature gauge fluctuates.
Low coolant temperature indicator (blue) light stays on longer or turns on while driving.
Poor heater performance, especially at idle.
NOTE:
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• Vehicles with SKYACTIV-G 2.5T are not applicable to this service information.
• DTC P0126:00 is not required to be present to perform this repair. If the vehicle has not previously had the repair
specified in this TSB and is exhibiting any of the listed symptoms, proceed with the repair procedure outlined in
this bulletin.
• If DTC P2B61:00 (Coolant Control Valve Circuit Range/Performance) is present along with DTC P0126:00, this
TSB does not apply. Diagnose the symptom and/or stored DTCs according to the procedures in MGSS.
• If only DTC P0126:00 is present and this TSB has already been completed previously, this bulletin does not
apply, diagnose the symptom and/or stored DTCs according to the procedures in MGSS.
This concern may be caused if the coolant control valve fail-safe thermostat opens at a temperature lower than the
normal temperature due to a deformation of the thermostat diaphragm. To eliminate the concern, the water flow control
valve specification has been modified.
Affected vehicles will require PCM reprogramming with the Mazda Modular Diagnostic System (M-MDS), using either a)
Mazda Diagnostic and Repair Software (MDARS) or b) Mazda Integrated Diagnostic System (IDS) 140.04 or later
software.
Refer to the table below for MDARS or IDS reprogramming requirement by model / model year:
Model:
MDARS:
IDS
CX-3
2019 - Present
---
CX-30
2020 - Present
---
CX-5
20201 - Present
2013-20201
Mazda3
2019 - Present
2004-2018
Mazda6
---
2003-2021
NOTE: 2020 CX-5 with Type B Mazda Connect (7th Gen Infotainment) use MDARS and models with Type A Mazda
Connect (6th Gen Infotainment) use IDS.
Type A:
Type B:
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Customers having this concern should have their vehicle repaired using the following repair procedure.
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REPAIR PROCEDURE
PCM Reprogramming with MDARS:
1. Verify the customer concern.
2. Reprogram the PCM with the Mazda Modular Diagnostic System (M-MDS), using Mazda Diagnostic and Repair
Software (MDARS). Refer to MDARS ECU Reprogramming Procedure.
NOTE: Be sure to submit Diagnostic Information (ODR) data as it is a required item and will be checked during
warranty claim submission. Instructions can be found under M-MDS -> MDARS (Uploading Diagnostic
Information (ODR) Data).
3. Clear any other DTCs.
4. Verify the repair by starting the engine (READY ON) and confirming there is no CHECK ENGINE light ON or abnormal
warning lights present.
PCM Reprogramming with IDS:
1. Reprogram the PCM with Mazda Modular Diagnostic System (M-MDS) using Mazda Integrated Diagnostic System
(IDS) 140.04 or later software. Refer to IDS Reprogramming Procedure.
CAUTION: IDS calibration files are now locally installed on user's PC. IDS 140.00 and 140.04 must be installed to
properly complete this calibration.
Click Here to download IDS 140.00
Click here to download IDS patch update 140.04 (All IDS Applicable models contain calibration files in patch 140.04.)
NOTE:
• Upon launching IDS 140.00, you must select your scan tool (VCM-II or DST-010B) before scanning a
vehicle. Click here for a brief training video
• After CCV replacement and PCM reprogramming, download and print the log viewer session that shows the
updated PCM software file and VIN and attach the data when submitting a warranty claim.
• With IDS there is no automatic collection function.
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2. Clear any other DTCs.
3. Verify the repair by starting the engine and confirming there is no CHECK ENGINE light ON or abnormal warning
lights present.
NOTE: If any DTCs should remain after deleting DTC, diagnose the DTCs according to the appropriate Troubleshooting
section of the Workshop Manual.
Explanation to Customer Regarding Avg. Fuel Economy:
Any ECU reprogramming resets the BCM's average fuel economy (remaining mileage).Therefore, the remaining mileage
may change when the customer drives the vehicle after reprogramming. After learning the customer's average fuel
consumption (calculated over several drives), the remaining mileage will be correctly displayed.
Parts Replacement:
NOTE: If DTC P2B61:00 (Coolant control valve circuit range/performance) registered, this Service Information is not
applicable. Diagnose according to the instruction on MGSS online.
1. Verify customer concerns and check the coolant temperature in the freeze frame data.
- If it is at around 40-80 °C {104-176 °F} proceed to step 2.
- If it is not at around 40-80 °C {104-176 °F}, this Service Information is not applicable. Diagnose according to the
instruction on MGSS online.
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2. Remove the coolant control valve according to the instruction on MGSS:
•
•
•
•
•
Mazda3 (COOLANT CONTROL VALVE REMOVAL/INSTALLATION)
Mazda6 (COOLANT CONTROL VALVE REMOVAL/INSTALLATION)
CX-3 (COOLANT CONTROL VALVE REMOVAL/INSTALLATION)
CX-5 (COOLANT CONTROL VALVE REMOVAL/INSTALLATION)
CX-30 (COOLANT CONTROL VALVE REMOVAL/INSTALLATION)
3. Remove thermostat cover (3) and thermostat cover gasket (2) from coolant control valve.
1. Thermostat
2. Thermostat cover gasket
3. Thermostat cover
4. Water outlet gasket
a) Mark the four (4) screws and the respective holes before loosening them to ensure that the screws are placed in
their original holes when reinstalling.
NOTE: If the screws are not reinstalled in their original holes, the holes may become deformed.
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Marking Example 1: Draw marking lines over the
screws and the thermostat cover as shown
above.
Marking Example 2: Number each screw and their
respective hole as shown above.
b) Loosen the four (4) tapping screws fastening the thermostat cover.
CAUTION: Use hand tools only, do not use power tools.
4. Replace the thermostat with the dummy plug, which is included in the Dummy Plug for CCV set.
NOTE: Do not use lubricants, spray lube, silicone sealer or petroleum grease on any of the gaskets.
IMPORTANT:
• Pay close attention to the orientation of the dummy plug when installing it into the coolant control
valve (CCV).
• DO NOT force the dummy plug in the wrong direction.
• If the dummy plug is installed in the wrong direction, it may cause DTC P0126:00 to return and slow
heating of the HVAC system.
• Be aware that Fig. 1 and Fig. 2 (below) are both correct, but show the dummy plug orientation from
opposite ends.
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Fig. 1: Shows correct orientation with the flat side (or short end) of the dummy plug (with o-ring) going into the upper hole
(see arrow) of the CCV.
NOTE: Be sure to install the dummy plug into the correct CCV hole.
Dummy Plug for CCV set includes:
1. Thermostat cover gasket*
2. Thermostat cover
3. Water outlet gasket
4. Dummy Plug
*Thermostat cover gasket is already assembled to the ther
mostat cover.
Fig. 2: Correct dummy plug (4) orientation to the thermostat cover (2).
a) Before installing the dummy plug into the CCV, check the mounting area for dirt and clean the area.
b) When installing the dummy plug into the CCV, push it firmly until it makes a clicking noise to ensure it has been
installed properly.
NOTE:
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• Insert the dummy plug so that its notch matches the protuding area in the CCV hole.
• Add 1-2 drops of coolant as necessary to the O-ring to help slide it into the thermostat cover.
5. Install the new thermostat cover and the thermostat cover gasket (included in the Dummy Plug for CCV set) to the
coolant control valve body.
a) Ensure that the thermostat cover gasket is inserted correctly in thermostat cover slot.
b) Place each of the marked four (4) screws in their respective holes and tighten them by hand. If resistance is felt,
unscrew once, and repeat the procedure.
NOTE:
• If screws are not inserted in their original holes, holes may become deformed.
• Do not use lubricants, spray lube, silicone sealer or petroleum grease on any of the gaskets.
c) Tighten the screws in a criss-cross pattern.
Tightening torque: Tighten screws to 26 in. lbs (3 Nm), then repeat tightening to 44 in. lbs (5 Nm).
CAUTION: Use hand tools only, do not use power tools.
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6. Reinstall the coolant control valve and the new water outlet gasket (included in the Dummy Plug for CCV set) to the
vehicle according to the instruction on MGSS.
7. Verify the repair by inspecting for engine coolant leakage.
NOTE: If the subject vehicle comes back again with DTC P0126:00, contact the Technical Hotline.
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CALIBRATION
Click on a model to view its calibration file details:
MAZDA3
File Name (OLD)
File Name (NEW)
PAH9-188K2-N
PAH9-188K2-P
PAJ2-188K2-L
PAJ2-188K2-M
PXJ7-188K2-H
PXJ7-188K2-J
PX05-188K2-S
PX05-188K2-T
PX07-188K2-U
PX07-188K2-V
PX58-188K2-R
PX58-188K2-S
PAJ1-188K2-P
PAJ1-188K2-R
PAJ3-188K2-M
PAJ3-188K2-N
PXJ8-188K2-J
PXJ8-188K2-K
PX06-188K2-S
PX06-188K2-T
PX08-188K2-U
PX08-188K2-V
PX59-188K2-R
PX59-188K2-S
PAJ8-188K2-J
PAJ8-188K2-K
PAK1-188K2-G
PAK1-188K2-H
PXK5-188K2-G
PXK5-188K2-H
PX13-188K2-L
PX13-188K2-M
PX15-188K2-L
PX15-188K2-M
PAJ9-188K2-K
PAJ9-188K2-L
PAK2-188K2-H
PAK2-188K2-J
PXK6-188K2-H
PXK6-188K2-J
PX14-188K2-L
PX14-188K2-M
PX16-188K2-L
PX16-188K2-M
PX4V-188K2-E
PX4V-188K2-F
PX4X-188K2-D
PX4X-188K2-E
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PX4W-188K2-E
PX4W-188K2-F
PX5A-188K2-D
PX5A-188K2-E
PX6R-188K2-E
PX6R-188K2-F
PX6S-188K2-E
PX6S-188K2-F
PXGX-188K2-B
PXGX-188K2-C
PXHD-188K2-C
PXHD-188K2-D
PXHB-188K2-C
PXHB-188K2-D
PGR1-188K2-C
PGR1-188K2-D
PGP8-188K2-B
PGP8-188K2-C
PXJF-188K2-B
PXJF-188K2-C
PXJD-188K2-B
PXJD-188K2-C
PXJG-188K2-B
PXJG-188K2-C
PXJE-188K2-B
PXJE-188K2-C
PG1N-188K2-A
PG1N-188K2-B
PG1R-188K2-B
PG1R-188K2-C
PG1P-188K2-A
PG1P-188K2-B
PG1S-188K2-B
PG1S-188K2-C
PXHA-188K2-B
PXHA-188K2-C
PXHE-188K2-C
PXHE-188K2-D
PXHC-188K2-C
PXHC-188K2-D
PGR2-188K2-C
PGR2-188K2-D
PGP9-188K2-B
PGP9-188K2-C
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MAZDA6
File Name (OLD)
File Name (NEW)
PYH3-188K2-M
PYH3-188K2-N
PXD8-188K2-D
PXD8-188K2-E
PXD6-188K2-D
PXD6-188K2-E
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TSB/Document ID: 01-001-26b
Replacement Service Bulletin Number:
MFR Communication Date: 2026-03-04
MFR Internal Campaign ID/Software Version:
Communication Type: Service Bulletin/Repair Instructions
NHTSA Components: ENGINE
MFR Component System:
MFR Component Subsystem: Cooling System
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