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Downstream Ho2S

DTC P1137, P1138 DETECTION CONDITION AND POSSIBLE CAUSE

DTC P1137, P1138 DOWNSTREAM HO2S
DETECTION CONDITION 
  • P1137 indicates the downstream HO2S is forced lean by the PCM. The test fails if the PCM does not detect the output of the HO2S during KOER Self Test in a calibrated amount of time.
  • P1138 indicates the downstream HO2S is forced rich by the PCM. The test fails if the PCM does not detect the output of the HO2S during KOER Self Test in a calibrated amount of time.
POSSIBLE CAUSE 
  • Wiring concerns:
    • Pinched, shorted, and corroded wiring and pins.
    • Crossed sensor wires.
  • Other concerns:
    • Exhaust leaks.
    • Contaminated or damaged sensor.
    • HO2S torque.
    • Puncture and cracks in catalyst and pipes.
HINT  HO2S Sensor Description 
  • Detects the presence of oxygen in the exhaust and produces a variable voltage (0.0 -1.1 V) according to the amount of oxygen detected.
    • A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a low voltage signal (less than 0.4 V).
    • A low concentration of oxygen (rich air/fuel ratio) in the exhaust produces a high voltage signal (greater than 0.6 V).
  • Provides feedback to the PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation.
G03988152Courtesy of MAZDA MOTORS CORP.
DTC P1137, P1138 INSPECTION

STEP INSPECTION ACTION
1
  • Was PCM Quick Test performed? 
 Yes Go to next step.
No Perform PCM Quick Test.
2 DTCS P1137 AND/OR P1138 INDICATE LACK OF HO2S SWITCHING
NOTE: Any exhaust leaks between the engine and the end of the catalyst can also cause DTCs P0136 and P0156.
  • Inspect for pinched wires or corrosion.
  • Inspect for exhaust leaks.
  • Are any of the possible causes present? 
 Yes Repair as necessary.
Go to step 9.
No Go to next step.
3 CHECK HO2S HARNESS CIRCUIT FOR SHORT TO VPWR AND GROUND
  • Disconnect NGS Tester from DLC.
  • Disconnect PCM.
  • Install Breakout Box, leave PCM disconnected.
  • Disconnect rear HO2S.
  • Measure resistance between PCM test pin 35 (HO2S SIG) and the following PCM test pins:
    • 91 (SIG RTN).
    • 71 (VPWR).
    • 103 (PWR GND).
  • Is each resistance greater than 10K ohms? 
 Yes Go to next step.
No Repair short in harness.
Go to step 9.
4 CHECK RESISTANCE OF HO2S AND HO2S GROUND CIRCUITS
  • Measure resistance of HO2S SIG circuit between PCM test pin 35 and the HO2S vehicle harness connector.
  • Measure resistance of SIG RTN circuit between PCM test pin 91 and HO2S SIG RTN vehicle harness connector.
  • Is the resistance reading less than 5.0 ohms? 
 Yes Go to next step.
No Repair open circuit.
Go to step 9.
5 CHECK HO2S CIRCUIT VOLTAGE
  • Connect PCM to Breakout Box.
  • Reconnect HO2S connector.
  • Reconnect NGS Tester.
  • Key ON, Engine OFF (KOEO).
  • Access O2S12 PID using NGS Tester.
  • Key OFF.
  • Was the voltage greater than 1.5 V? 
 Yes Go to step 8.
No Go to next step.
6 CHECK RESISTANCE OF HO2S GROUND CIRCUIT IN THE PCM
  • Disconnect vehicle harness from Breakout Box.
  • Measure resistance between PCM test pin 91 (SIG RTN) and PCM test pin 103 (PWR GND).
  • Is the resistance less than 5.0 ohms? 
 Yes Go to step 7.
No Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to step 9.
7 APPLY 12 VOLTS TO SUSPECT HO2S CIRCUIT
  • Remove Breakout Box.
  • Reconnect PCM.
  • Disconnect rear HO2S.
  • Jumper VPWR to HO2S SIG circuit at the HO2S harness connector.
  • Key ON, Engine OFF (KOEO).
  • Access O2S12 PID with NGS Tester and monitor.
  • Is the PID value greater than 1.5 V? 
 Yes Replace HO2S.
See HEATED OXYGEN SENSOR (HO2S) in REMOVAL, OVERHAUL AND INSTALLATION article in Engine Performance .
Go to step 9.
No Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to step 9.
8 CHECK FOR OVER VOLTAGE ON THE HO2S CIRCUIT IN THE PCM
  • Key ON, Engine OFF (KOEO).
  • Disconnected rear HO2S.
  • Measure voltage between SIG RTN circuit at the vehicle harness connector and battery negative post.
  • Measure voltage of HO2S SIG circuit between HO2S vehicle harness connector and battery negative post.
  • Are the voltage readings greater than 1.5 V? 
 Yes Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to step 9.
No Replace HO2S.
See HEATED OXYGEN SENSOR (HO2S) in REMOVAL, OVERHAUL AND INSTALLATION article in Engine Performance .
Go to step 9.
9 VERIFY TROUBLESHOOTING OF DTCs COMPLETED
  • Make sure to reconnect all disconnected connectors.
  • Clear DTCs from PCM memory using NGS tester generic OBD-II function.
  • Perform PCM Quick Test.
  • Is same DTC present? 
 Yes Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Repeat PCM QUICK TEST .
No If any other DTC is present, go to applicable DTC inspection.
If no DTC is present, troubleshooting complete.
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