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DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.

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HOMESERVICE MANUALSMAZDA1999626 ES, 2.5 D, STANDARDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 432 (ON-BOARD DIAGNOSTIC (ENGINE CONTROL SYSTEM 13BMSP])DTC P0139DIAGNOSTIC PROCEDURE

1999 Mazda 626 ES, 2.5 D, Standard

Diagnostic procedure

1999 Mazda 626 ES, 2.5 D, StandardSECTION Diagnostic procedure

WARNING: This page is about a different car, the 2005 Mazda RX-8. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0139 DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	VERIFY FREEZE FRAME DATA AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS (HO2S related) been recorded?	Yes	Go to the next step.
1		No	Record the FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS on the repair order, then go to the next step.
2	VERIFY RELATED REPAIR INFORMATION AVAILABILITY Verify related Service Bulletins and/or on-line repair information availability. Is any related repair information available?	Yes	Perform repair or diagnosis according to the available repair information. If the vehicle is not repaired, go to the next step.
2		No	Go to the next step.
3	VERIFY RELATED PENDING CODE OR STORED DTC Turn the ignition switch off, then to the ON position (Engine off). Verify the related PENDING CODE or stored DTCs. Is the DTC P0443 also present?	Yes	Go to the DTC P0443 inspection. (See DTC P0443 .)
3		No	Go to the next step.
4	IDENTIFY TRIGGER DTC FOR FREEZE FRAME DATA Is DTC P0139 on FREEZE FRAME DATA?	Yes	Go to the next step.
4		No	Go to the FREEZE FRAME DATA DTC inspection. (See DTC TABLE .)
5	VERIFY CURRENT INPUT SIGNAL STATUS OF REAR HO2S Connect the WDS or equivalent to the DLC-2. Start the engine and warm it up completely. Access the O2S12 PID. Read the O2S12 PID under following accelerator pedal conditions (in PARK or NEUTRAL). More than 0.55 V when accelerator pedal is suddenly depressed (rich condition). Less than 0.55 V just after release of accelerator pedal (lean condition). Is the PID normal?	Yes	Go to Step 8.
5		No	Go to the next step.
6	INSPECT INSTALLATION OF REAR HO2S Inspect the rear HO2S for looseness. Is it normal?	Yes	Go to the next step.
6		No	Tighten the rear HO2S, then go to Step 14. (See REAR HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION .)
7	INSPECT EXHAUST SYSTEM FOR EXHAUST GAS LEAKAGE Visually inspect exhaust gas leakage in the exhaust system. Is there exhaust gas leakage?	Yes	Repair or replace the malfunctioning part, then go to Step 14.
7		No	Replace the rear HO2S, then go to Step 14. (See REAR HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION .)
8	INSPECT LONG TERM FUEL TRIM Connect the WDS or equivalent to the DLC-2. Access the LONGFT1 PID. Compare the LONGFT1 PID with recorded FREEZE FRAME DATA at Step 1. Is the LONGFT1 PID below FREEZE FRAME DATA?	Yes	Engine is driven under rich condition. Go to the next step.
8		No	Engine is driven under lean condition. Go to Step 10.
9	INSPECT FUEL LINE PRESSURE (EXCESSIVE FUEL LINE PRESSURE) Perform the "FUEL LINE PRESSURE INSPECTION". (See FUEL LINE PRESSURE INSPECTION .) Is there any malfunction?	Yes	Repair or replace the malfunctioning part according to the inspection results. Then go to the Step 14.
9		No	Go to the next step.
10	INSPECT FUEL LINE PRESSURE (LOW FUEL LINE PRESSURE) Perform the "FUEL LINE PRESSURE INSPECTION". (See FUEL LINE PRESSURE INSPECTION .) Is there any malfunction?	Yes	Go to the next step.
10		No	Go to step 12.
11	INSPECT FUEL SYSTEM FOR FUEL LEAKAGE Visually inspect fuel leakage in the fuel system. Is there fuel leakage?	Yes	Repair or replace the malfunctioning part, then go to Step 14.
11		No	Replace the fuel pump unit, then go to Step 14. (See FUEL PUMP UNIT REMOVAL/INSTALLATION .)
12	INSPECT AIR SYSTEM OPERATION Perform the "AIR System Inspection". (See SECONDARY AIR INJECTION (AIR) SYSTEM INSPECTION .) Is there any malfunction?	Yes	Repair or replace the malfunctioning part, according to inspection results. Then go to Step 14.
12		No	Go to the next step.
13	INSPECT ENGINE COOLANT PASSAGE FOR ENGINE COOLANT LEAKAGE Perform the "ENGINE COOLANT LEAKAGE INSPECTION". (See ENGINE COOLANT LEAKAGE INSPECTION .) Is there any malfunction?	Yes	Repair or replace the malfunctioning part according to the inspection results. Then go to the next step.
13		No	Go to the next step.
14	VERIFY TROUBLESHOOTING OF DTC P0139 COMPLETED Make sure to reconnect all disconnected connectors. Clear the DTC from the PCM memory using the WDS or equivalent. Perform the DRIVE MODE 1 and 3. (See OBD-II DRIVE MODE .) Is the PENDING CODE same as the DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION .)
14		No	Go to the next step.
15	VERIFY AFTER REPAIR PROCEDURE Perform the "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE .) Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE .)
15		No	DTC troubleshooting completed.

RENDER: 1.0x

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.
• Brakes feel soft, pull hard to one side, or make grinding noises.
• The engine overheats, stalls repeatedly, or misfires under load.
• You are missing required tools, torque specs, or safe lifting equipment.
• You are not confident in the next step or safety outcome.