An error code stored in the engine control unit “P0037 – Heated lambda sensor bank 1 sensor 2 heating circuit, low voltage/short circuit to ground” or “P0031 – Heated lambda sensor bank 1 sensor 1 heating circuit, low voltage/short circuit to ground” indicates an electrical fault in the lambda sensor heating system.
As a result, monitoring of the catalyst is deactivated and the function of mixture control is restricted.
Heat-activated lambda sensors are components of the lambda control circuit in modern mixture formation systems for combustion engines. They are located upstream of the catalytic converter as sensor 1 (control sensor) and are mainly used for mixture control. Downstream of the catalytic converter, they are located as sensor 2 (monitoring sensor) and are also used for catalytic converter monitoring.
System description:
Lambda sensors are sensors connected to the engine control unit, whose signals serve as input information for the engine control system regarding the mixture state and the efficiency of the catalytic converter. The aim is to quickly achieve operational readiness of the sensor after start-up and to maintain a constant temperature of the lambda sensor during operation. For this purpose, the component is electrically heated by an internal heating element with a PTC characteristic curve. The systems use one lambda sensor upstream and another downstream of the catalytic converter per cylinder bank. Sensor 1 is located downstream of the catalytic converter and is referred to as the control sensor. Sensor 2 is located downstream of the catalytic converter and is also referred to as the monitor sensor.
Fault code P0037 – Heated lambda sensor bank 1 sensor 2 heating circuit, low voltage/short circuit to ground (VW error code 16421)
Error code P0031 – Heated lambda sensor bank 1 sensor 1 heating circuit, low voltage/short circuit to ground
Fault description:
The probe heater is located in a control loop with the engine control unit and is controlled by pulse width modulation. This allows the component temperature to be adjusted to the fluctuating temperatures of the exhaust gas volume flow and the variable external air flow in a control loop. The engine control unit monitors the voltage in the heating element circuit during the low level phase of the pulse width modulated voltage signal.
If the monitoring circuit measures a voltage below the threshold value for a defined period of time, the error is set.
Consequences of the error:
To prevent further damage, the output stage of the lambda sensor heater in the engine control unit can be deactivated in the event of a static fault. If this is not done or if the fault is ignored for too long, internal damage to the engine control unit may occur.
Possible emergency operation strategy for the engine control system:
- Blocking of lambda adaptation
- Power reduction
- Control of catalytic converter temperature
- OBD error, readiness test incomplete
Diagnosis/measurements/repair:
The previously mentioned tests and repairs must be carried out before installing new parts or a reconditioned engine control unit.
- Check the resistance of the lambda sensor heater and the insulation against the housing/ground according to the manufacturer’s specifications
- Check the component’s cable set for short circuits between each other and against ground
- Power supply on the positive side faulty (main relay, fuse defective)
- The lambda sensor must be replaced even if no fault is found, as prolonged operation without heating often leads to damage to the sensor element
- In the event of repeated defects in the sensor heating, check that the sensor is correctly assigned to the system in accordance with the manufacturer’s specifications
