Diagnosing Fuel Delivery and Emission Concerns

Check fuel pressure with key on engine off. If pressure is low or zero — check pump operation, relay, fuse, and wiring. If pressure builds but drops quickly after key off — the fuel is leaking back. A pressure drop test isolates the cause: pinch the return line — if pressure holds, the regulator is leaking. If pressure still drops — a leaking injector or failed pump check valve.

Pull fuel trims on the scan tool at idle and at 2,500 RPM cruise. High positive trims at idle that drop to normal at cruise — vacuum leak. The leak is a large percentage of total airflow at idle but small at cruise. High positive trims at all RPM — fuel delivery problem, weak pump, or restricted filter. High negative trims — rich condition from leaking injector, high fuel pressure, or contaminated MAF sensor.

P0440 through P0457. Check the gas cap first — tighten or replace. Clear and drive. If it returns, smoke test the EVAP system. Introduce smoke into the system and visually inspect every line, connection, the canister, the purge valve, and the vent valve for smoke escaping. The leak point glows with smoke. Most common locations: cracked EVAP hose near the canister, loose canister connection, failed purge valve seal.

Before replacing a converter that costs $500 to $2,000 — verify the engine is running correctly. Check for misfires, check fuel trims, verify no oil consumption or coolant loss. Compare upstream and downstream O2 waveforms on the scan tool — the downstream should be nearly flat while the upstream switches rapidly. If the downstream mirrors the upstream, the converter is confirmed inefficient. Fix any engine running problem FIRST — a new converter on an engine that is misfiring or burning oil will fail again.