P0384

P0384 means the glow-plug control circuit is reading HIGH — the bus voltage during the wait-to-start interval is above the expected battery-rail value (typically more than 0.5 V above battery voltage on a 12V system) or the current draw is below the floor while the module's output transistor is commanded on (an open-circuit condition reads as 'high voltage at the load'). Cheapest-first ladder: (1) backprobe the glow-plug bus at the first plug and measure voltage during pre-heat — voltage above 13.8 V on a 12V system with the engine off and the module commanded on means the load isn't pulling the bus down (open bus or open plug bank). (2) Clamp current at the GPCM output — under 5 A during pre-heat at coolant below 50 F is a confirmed open-circuit fault. (3) Disconnect and ohm-check each plug individually — an open plug (resistance reads infinity, healthy is 0.4-2.0 ohms) breaks current draw and reports as 'high circuit voltage' even though the wiring is fine. (4) Inspect the bus-bar continuity — a broken solder joint at the bus eyelet looks identical electrically to an open plug. (5) Verify the wait-to-start timer is enabled — coolant temp sensor faults that report falsely high (warm engine when actually cold) bypass glow operation and can throw P0384 in a corner case. Expensive-misdiagnosis caveat: before replacing the glow-plug control module on a P0384, individually resistance-check each plug — an open plug pulls bus voltage high during commanded-on and trips the module's fault detection, but the module is fine; replacing the module without fixing the open plug sets P0384 again on the next cold start.

Vehicle-specific patterns: 2003-2007 Ford 6.0L Powerstroke commonly throws P0384 when a single glow plug fails open (very common at 80k+ miles) — the GPCM detects the bank's open circuit and codes high; the proper fix is replacing the failed plug AND inspecting the remaining seven since they age together. 2008-2010 Ford 6.4L Powerstroke throws P0384 from open plugs on the rear bank specifically. 2001-2010 GM Duramax LB7-L5P throws P0384 from a known failure where one of the eight plugs corrodes at its bus terminal and breaks continuity electrically while looking fine visually. 2007-2014 RAM Cummins 6.7L throws P0384 from individual in-head plug failures — the head heat-soaks the plug tips and they oxidize internally. 2010-2015 VW 2.0 TDI throws P0384 from open-plug failures on the 4-plug bus; Mercedes Sprinter OM642 throws P0384 from a failed PDC-mounted module not driving the bus at all. Estimated repair: $100 to $800.