A P0455 – EVAP Large Leak Detected fault on the 2017 MG ZS indicates that the Engine Control Module (ECM) has detected a significant leak or an inability to maintain vacuum within the Evaporative Emission Control (EVAP) system. The EVAP system prevents fuel vapours from escaping into the atmosphere by storing them in a charcoal canister before routing them to the engine for combustion.

During its self-diagnostic routine, the ECM seals the EVAP system and monitors pressure changes using the fuel tank pressure sensor. If the system detects a leak exceeding the calibrated threshold or cannot achieve the required vacuum, it stores DTC P0455 and illuminates the Check Engine Light. Although a loose or damaged fuel filler cap is one of the most common causes, the fault may also result from damaged hoses, valves, the charcoal canister, or other EVAP components.

Symptoms

• Check Engine Light illuminated
• Diagnostic Trouble Code (DTC) P0455 stored
• Fuel odour around the vehicle
• Failed emissions inspection
• Loose fuel cap warning may be displayed
• No noticeable change in engine performance in most cases
• Intermittent EVAP monitor failure
• Additional EVAP-related Diagnostic Trouble Codes may also be present

Possible Causes

The fault may be caused by a loose, missing, or damaged fuel filler cap, cracked or disconnected EVAP hoses, leaking fuel vapour lines, faulty EVAP purge valve, defective EVAP vent valve, damaged charcoal canister, leaking fuel tank, damaged filler neck, faulty fuel tank pressure sensor, deteriorated seals or O-rings, wiring harness faults, loose or corroded electrical connectors, Engine Control Module (ECM) faults, or CAN Bus communication issues affecting EVAP system operation. A P0455 fault indicates a large leak and often points to a substantial loss of system integrity rather than a minor vapour leak.

Diagnosis

Diagnosis begins by confirming DTC P0455 using a professional diagnostic scan tool and reviewing freeze-frame data to determine the operating conditions when the fault occurred. Live data should then be analysed to evaluate EVAP purge command, vent valve operation, fuel tank pressure sensor readings, and any related EVAP Diagnostic Trouble Codes. Any accompanying fuel system or emission-related faults should be diagnosed simultaneously, as they may contribute to the EVAP failure.

A P0455 fault is not always caused by a faulty fuel filler cap. Technicians typically inspect the fuel cap seal, filler neck, EVAP hoses, vapour lines, charcoal canister, purge valve, vent valve, and electrical connectors before replacing any components. A low-pressure smoke test is commonly performed to identify large leaks in hoses, canisters, seals, or the fuel tank. The fuel tank pressure sensor, wiring harnesses, battery voltage, and ground circuits should also be tested to verify correct system operation.

If the EVAP components and associated circuits operate within specification, further diagnosis should include evaluation of the Engine Control Module, fuel tank pressure sensor, purge and vent valve control circuits, and the CAN Bus communication network. Access to OEM wiring diagrams, connector pinouts, EVAP circuit descriptions, smoke test procedures, component locations, and manufacturer diagnostic flowcharts is essential for accurate fault isolation. Following the OEM troubleshooting workflow helps identify the true source of the leak while avoiding unnecessary replacement of EVAP components.

Recommended Resources

OEM Service and Repair Manual

The OEM Service and Repair Manual is the most reliable resource for diagnosing and repairing a P0455 EVAP Large Leak Detected fault. It typically includes:

• Diagnostic procedures specific to DTC P0455
• EVAP system wiring diagrams and circuit descriptions
• Fuel tank pressure sensor testing procedures
• EVAP purge valve and vent valve diagnostic procedures
• Smoke leak testing procedures
• Charcoal canister inspection procedures
• Fuel filler neck and fuel tank inspection procedures
• Connector pinout information
• Component locations and removal procedures
• OEM troubleshooting workflows
• Repair specifications and electrical testing procedures

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