P0604

P0604 means the PCM's internal RAM (working memory used during runtime, not the flash that stores calibration) has failed its diagnostic self-test. Unlike P0603 KAM faults, P0604 is rarely a soft fault — RAM either works or it doesn't, and there is no relearn path. Still, start cheapest-first by confirming battery voltage above 12.4V at rest and steady during cranking, because severe voltage sags can momentarily corrupt RAM contents and latch P0604. Inspect the PCM for water intrusion, heat damage, swollen capacitors, or signs of a prior shorted-sensor event (a 5V reference short can take out internal regulators that feed the RAM). Try clearing the code and monitoring whether it returns within one drive cycle; a hard-fail P0604 returns almost immediately. Check that the PCM grounds are tight and clean (under 0.1V drop) and that no aftermarket device is back-feeding voltage onto a sensor signal wire. The expensive misdiagnosis is replacing the PCM without first confirming that no external 5V reference short is present — install a new PCM into a vehicle with a shorted MAP or TPS sensor and you can damage the replacement module within minutes, repeating the failure.

Vehicle-specific patterns: 2003-2007 GM trucks and SUVs with the E40 PCM occasionally throw P0604 from internal failure, almost always requiring module replacement and SPS programming per GM bulletin 04-06-04-039. 2004-2008 Ford 6.0L Power Stroke and 2008-2010 6.4L Power Stroke set P0604 from FICM/PCM failures aggravated by chronic low battery voltage; Ford TSB 07-14-1 emphasizes battery health before condemning the module. 2005-2010 Chrysler/Dodge with the NGC module and 2011+ vehicles with the powertrain control module behind the right kick panel show P0604 after coolant leaks from a failed heater core soak the module. 2002-2006 BMW E46 and E39 with the MS43/MS45 DME show equivalent internal RAM faults from solder fatigue. Estimated repair: $400 to $1,800.