What does it mean?
The upstream oxygen sensor (O2S) is located ahead of the catalytic converter. It is used to determine the concentration of oxygen in the exhaust gas. This information is used by the PCM to control the engine air/fuel mixture. The sensor compares amount of oxygen in the exhaust to the surrounding air (there is an opening in the sensor that is exposed to the atmosphere). It generates a corresponding voltage which is transmitted to the PCM. The PCM then controls injector pulse based on this value.
Modern vehicles use a heated oxygen sensor (HO2S). These sensors contain a heating element that brings the sensor to operating temperature faster. This allows the PCM to use the signals input sooner, for more precise fuel control and reduced emissions. The heater circuit is energized through a relay that closes when the engine is cranked. The PCM monitors the heater circuit and will turn on the check engine light if it finds a problem.
Code P0030 stands for Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1). This indicates the PCM has detected a problem with the bank 1 oxygen sensor. Bank 1 refers to the side of the engine that has the #1 cylinder. Bank 2 is the opposite side of the engine. If you’ve got a four-cylinder, there will only be one bank.
A typical heated oxygen sensor Note: “Circuit Malfunction” indicates that there is a malfunction in the control circuit, as opposed to a fault in a sensor or other component. With “Circuit Malfunction” codes, replacement of sensors and components in the affected circuit will almost never resolve the problem, since as the code suggests, the trouble is in the circuit.
This distinction between “circuit” and “sensor/component” is a great help to anyone trying to diagnose a circuit malfunction code, since it narrows the list of possible causes down considerably.
The causes of “Circuit Malfunction” codes are much the same as those for “Open Circuits” i.e., broken wiring, poor connections across electrical connectors or previously repaired wiring, loss of ground that prevents current flow, blown fuses, defective relays, faulty switches, or any of a host of other issues and problems that prevents a flow of current through wiring. Issues like high/low/intermittent voltages can set a “Circuit Malfunction” code on some applications.
Moreover, “Circuit Malfunction” codes could also indicate a problem with negative current control / flow, as well as issues with failed or failing PCM’s (Powertrain Control Modules), although control module failure is a rare event.