Code P0136: lambda sensor circuit (bank 1, sensor 2)

Original definition (SAE J2012): O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)^[1][2]

• P0136Oxygen sensor (lambda sensor) circuit malfunction, bank 1, sensor 2.
• P0137Oxygen sensor (lambda sensor) circuit, bank 1, sensor 2: low voltage.^{[7][9][12][13]}
• P0138Oxygen sensor (lambda sensor) circuit, bank 1, sensor 2: high voltage.^{[8][10][14][15]}
• P0140Oxygen sensor circuit (bank 1, sensor 2): no activity detected.^[1]

1. 1Faulty or aged rear lambda sensor (bank 1, sensor 2).^[1][2][6]
2. 2Wiring or connectors in poor condition between the sensor and the ECU: damaged or shorted wires, loose or corroded connectors or damaged terminals.^[1][2][6]
3. 3Dirty or contaminated sensor: HELLA lists silicone sealant, coolant, burnt oil and fuel additives among the causes of sensor contamination.^[1][4]
4. 4Exhaust leaks before the rear sensor, which let air in and distort the oxygen reading.^[6][7]
5. 5Faulty sensor heater or blown heater fuse.^[1]
6. 6Damaged catalytic converter: Carista notes that, when the catalytic converter lets rich gases through, P0138 often appears together with P0420.^[1][14]
7. 7A genuinely lean (P0137) or rich (P0138) mixture due to another engine problem: Carista mentions it as a possible cause of a persistently low or high rear sensor voltage.^[8][12][14]
8. 8Faulty engine ECU: an uncommon cause.^[1]

• Engine warning light on in the instrument cluster.^[1][12][16]Engine warning light: faults that cause it
• Increased fuel consumption.^[1][6]Uses more fuel: faults that cause it
• Worse emissions values and a possible failure in an emissions test until the ECU completes the sensor check.^[1][12]
• Rough idle or jerking at constant speed, according to Autodoc Club; Carista and Mechanic Base, on the other hand, say the car usually runs normally.^[1][12][16]Vibrations: faults that cause it
• In many cases nothing is noticeable other than the warning light being on.^[12][16]

1. 1Read the code with a diagnostic tool and see whether there are other stored codes (front sensor, heater or mixture): the Saab 9-2X workshop manual starts by ruling out other codes. Clear it and do a road test to see whether it comes back.^[13]
2. 2Inspect the rear sensor, its connector and the wiring: contamination on the sensor, melted or chafed wires, corrosion and poor terminals.^[1][6][16]
3. 3Look for exhaust leaks before the rear sensor: listen for a hiss and check the joints, cracks and holes. The Saab 9-2X manual asks for the exhaust between the two sensors to be checked.^[6][13][16]
4. 4With the engine warm, look at the rear sensor voltage in the live data: it should be steadier and of smaller amplitude than the front one, because the catalytic converter stores oxygen. The sources give different values for a healthy sensor (0.6 to 0.8 V according to Carista, 0.1 to 0.95 V according to Mechanic Base, around 350 to 500 mV on the Saab 9-5 and 0.2 to 0.4 V on the Saab 9-2X): compare it with your car's manual.^{[3][4][12][13][16]}
5. 5Check the sensor heater fuse and the resistance of its heater with a multimeter, against the manufacturer's specifications.^[1][6]
6. 6If the wiring is fine, the 2011 Saab 9-5 manual suggests measuring, with the sensor connector unplugged, the resistance of the low reference wire (less than 5 Ω) and checking with the ignition on that the sensor parameter on the diagnostic tool is around 450 mV, to locate a short to earth, a short to voltage or an open circuit before replacing anything.^[3]
7. 7Replace the rear sensor only when the fuse, wiring, exhaust leaks and mixture have been ruled out: in the Saab 9-5 manual it is the last step, when all the circuits are fine.^[1][3]
8. 8If P0420 or other mixture codes also appear, diagnose them separately: Carista says rear sensor codes often come up together with P0420.^[12][14]