P0255

P0255 sets when the ECM detects an intermittent fault on the secondary fuel metering control circuit of the injection pump — most commonly an electronic rotary diesel pump like the Bosch VP44 or VP30, where a Pump Control Module (PCM/FPCM) on top of the pump modulates fuel quantity via a high-current solenoid. "Intermittent" is the key word: the fault drops in and out, often with heat soak, which means you cannot bench-test the pump cold and trust the result. Cheapest-first ladder: scan-tool live data for fuel delivery commanded vs actual and watch for momentary dropouts during a 20-minute drive cycle (commanded 30-50 mm3/stroke at cruise should track actual within +/- 2 mm3; any time actual collapses to 0 while commanded stays steady, the metering loop is failing). Wiggle-test the PCM connector and the harness on top of the pump with the engine idling — the connector pins corrode and the ribbon wires inside the pump module crack from underhood heat cycling. Then check pump-to-ECM communication wiring (continuity end-to-end, no chafing against the valve cover or intake horn) and verify clean 12V supply + ground at the PCM (drop test: less than 0.2V on the ground side under load). Expensive-misdiagnosis caveat: shops routinely sell a $1,200-$2,500 injection pump when the actual failure is the $400-$700 PCM module bolted to the top of that pump, or a $30 corroded connector — verify the module before condemning the pump body.

Vehicle-specific patterns: 1998-2002 Dodge RAM 2500/3500 with the Cummins 5.9L 24-valve and the Bosch VP44 injection pump is the textbook P0255 case — the PSG (Pump Solenoid Group / electronic control module) on top of the VP44 cooks from underhood heat sitting directly above the exhaust manifold, and starts dropping intermittent metering signals around 100k-150k miles; the famous fix is relocating the PCM off the pump with an extension harness kit. 1994-2003 Ford 7.3L Powerstroke uses an Injector Driver Module (IDM) that can throw similar metering codes when the IDM connector corrodes or the IDM itself fails — the IDM lives on the driver fender and is easier to access than a VP44 module. 1994-2000 GM 6.5L Detroit Diesel with the DS4 electronic injection pump has the well-documented PMD (Pump Mounted Driver) failure that throws metering and timing codes intermittently — the standard shop fix is relocating the PMD to a finned heatsink at the front bumper. 2003-2007 Dodge RAM with the CP3 common-rail pump (5.9L and 6.7L Cummins) is a different architecture and rarely throws P0255 — if you see it on a CP3 truck, suspect wiring or ECM, not the pump itself. Estimated repair: $30 (connector clean) to $2,500 (full pump + PCM replacement on a VP44).