Mazda 3 Check Engine Light P0126 & P0128

Ingrid, take a look at this information from the service manual. The actual installation is not a horribly complicated job and definitely doable:

DTC P0126 DTC P0128

Coolant thermostat stuck open

DETECTION CONDITION

DTC P0126

• If the ECT signal never exceeds 71°C {160 OF} after engine start for specified period, PCM determines that the coolant thermostat is stuck open.

MONITORING CONDITIONS -IAT: above -10°C {14 OF} -Vehicle speed: over 6.0 km/h {3.7 mph} DTC P0128 • The PCM monitors MAF, IAT, VSS and EAT signals and calculate radiator's heat radiation ratio while following monitoring conditions are met. If calculated value exceeds threshold, PCM determines that the coolant thermostat is stuck open.

MONITORING CONDITIONS -IAT: above -10°C {14 OF} -ECT at engine start: Below 36°C {97 OF} -Difference between ECT at engine start and minimum IAT: Below 6 °C {10.8 OF} -Vehicle speed: over 30 km/h {18.6 mph} Diagnostic support note

• This is an intermittent monitor (Engine cooling system).

• The MIL illuminates if PCM detects above malfunction condition in two consecutive drive cycles or in one drive cycle while DTC for the same malfunction has been stored in PCM.

• PENDING CODE is available if PCM detects the above malfunction condition during first drive cycle.

• FREEZE FRAME DATA is available.

• DTC is stored in PCM memory.

• DIAGNOSTIC MONITORING TEST RESULT is available.

POSSIBLE CAUSE

• ECT sensor malfunction

• Cooling system malfunction

• Coolant thermostat malfunction

• PCM malfunction

THERMOSTAT REMOVAL/INSTALLATION [ZJ, Z6]

Warning

• Remove and install all parts when the engine is cold, otherwise they can cause severe burns or serious injury.

1. Remove the battery cover. (See BATTERY REMOVAL/INSTALLATION [ZJ, Z6].)

2. Disconnect the negative battery cable.

3. Remove the under cover.

4. Drain the engine coolant. (See ENGINE COOLANT REPLACEMENT.)

5. Disconnect the lower radiator hose from the thermostat.

6. Remove in the order indicated in the table.

THERMOSTAT REMOVAL/INSTALLATION [ZJ, Z6]

7. Install in the reverse order of removal.

8. Add engine coolant. (See ENGINE COOLANT REPLACEMENT.)

9. Inspect the system for engine coolant leakage. (See ENGINE COOLANT LEAKAGE INSPECTION.)

Thermostat Installation Note

1. Install the thermostat into the thermostat case with the jiggle pin at the top.

THERMOSTAT REMOVAL/INSTALLATION [LF]

Warning

• Remove and install all parts when the engine is cold, otherwise they can cause severe burns or serious injury.

1. Remove the battery cover. (See BATTERY REMOVAL/INSTALLATION [LF].)

2. Disconnect the negative battery cable.

3. Remove the under cover and splash shield as a single unit.

4. Drain the engine coolant. (See ENGINE COOLANT REPLACEMENT.)

5. Set the coolant reserve tank out of the way.

6. Remove the plug hole plate. (See PLUG HOLE PLATE REMOVAL/INSTALLATION [LF].)

7. Set the drive belt out of the way. (See DRIVE BELT REPLACEMENT [LF].)

8. Remove the drive belt tensioner. (See TIMING CHAIN REMOVAL/INSTALLATION [LF].)

9. Remove in the order indicated in the table.

10. Install in the reverse order of removal.

11. Add engine coolant. (See ENGINE COOLANT REPLACEMENT.)

12. Inspect for the engine coolant leakage. (See ENGINE COOLANT LEAKAGE INSPECTION.)

The important part to look for with this answer, is the engine type ZJ, Z6, and LF.

Then there is this message: Model Years affected 2004, 2005, 2008

Some vehicles may experience Check Engine Light illumination with a trouble code indicating the thermostat is stuck open. The thermostat will need to be replaced with a modified one and there is a software update for the Powertrain Control Module.

I would think this is a 2-hour job and an average repair cost of approximately $260 to replace a thermostat on a Mazda 3. Personally, I would opt to purchase a 2004-2011 Mazda 3 Thermostat—Beck Arnley 143-0838 from places like this, spend $30 and plan on a whole day to do it ;-)

I do hope I did not confuse you and that this response wasn't too lengthy. Hope this helps, good luck.