What does it mean?
OBD II fault code P0443 is defined as “Evaporative Emission Control System Purge Control Valve Circuit Malfunction”, and is set when the PCM (Powertrain Control Module) detects a malfunction in the circuit that controls the purge valve in the EVAP (Evaporative Emission Control) system, or sometimes a malfunction of the purge valve itself.
Typical EVAP systems consist of the fuel tank, fuel filer cap, fuel lines, vacuum lines, a charcoal canister, various pressure and flow sensors, control solenoids, and electrical wiring/connectors. The function of the EVAP system is to capture fuel vapors before they can escape into the atmosphere. Once captured, fuel vapors are routed through a charcoal canister via vacuum lines to the engine to be burned as part of the air/fuel mixture.
Note that fuel vapors will typically not be fed into the system during idling, or when the load on the engine, and therefore fuel demand, is steady. As a general rule, fuel vapors will only be fed to the engine when the demand for fuel increases, or when the vapor pressure in the EVAP system exceeds predefined levels.
Fluctuations in the flow or pressure in the EVAP system are converted into varying signal voltages by the EVAP pressure sensor, which signals the PCM uses to make adjustments to fuel trim to ensure the optimum air/fuel mixture (14.7 parts of air to 1 part of fuel) at all times.
For instance, when the purge valve is commanded open to allow fuel vapors to enter the engine (thus enriching the air/fuel mixture), the change in the exhaust gas composition is immediately “noticed” by the #1 oxygen sensor.
Changes in the exhaust gas composition (increased oxygen levels in this case) are transmitted to the PCM in the form of changing signal voltages, which the PCM will use to either reduce the fuel vapor flow, or close the purge valve altogether to maintain, or restore the optimum air/fuel ratio. Thus, when a malfunction occurs in the purge valve control circuit and the PCM cannot control the purge valve, code P0443 will be set as a result.
Note that on some applications a single failure will set the code, while on others, several fault cycles may be required before a code is set and a warning light is illuminated. Also note that malfunctions of the EVAP purge valve control circuit can prevent the PCM from completing self-diagnostic tests of the system, which tests include measuring the rate at which an induced vacuum or pressure decays. Failure by the PCM to complete self-tests will also set code P0443.
The image below shows a simplified schematic drawing of a typical EVAP system. Note the location of the purge valve in relation to other major components of the system.