P1240

P1240 on a Saab is Sensor Power Supply Voltage Low — the ECM's internal 5V VREF supply that powers TPS, MAP, MAF, and other analog sensors has dropped below the healthy 4.9-5.1V window, typically to 4.5V or lower. A failing 5V VREF rail upstream poisons every analog sensor downstream simultaneously, which means P1240 often shows up alongside seemingly random sensor-range codes (TPS, MAP, IAT) and chasing those individual codes is a waste of time until the VREF supply itself is fixed. Cheapest-first ladder: (1) Backprobe the 5V reference at any convenient sensor connector with the key on and engine off — TPS or MAP are easiest — and confirm the actual voltage. Healthy is 4.9-5.1V; anything below 4.8V is the real problem regardless of what other codes are also present. (2) If VREF is low at one sensor, check it at another sensor on a different harness branch — if VREF is low everywhere, the supply itself (inside the ECM) is failing or is being pulled down by a shorted sensor; if VREF is low at one branch only, suspect a chafed harness shorting VREF to ground partway down that branch. (3) Disconnect sensors one at a time while monitoring VREF — if disconnecting a specific sensor restores VREF to 5.0V, that sensor's internal resistance has collapsed and is pulling the rail down. (4) Check ECM B+ and ground points before condemning the ECM — Saab ECMs are sensitive to ground-point corrosion at the chassis stud near the strut tower on 9-3 cars and behind the battery tray on 9-5 cars. Do NOT throw a $700 ECM at P1240 without disconnecting sensors one at a time to find the downstream short — a single shorted MAP or TPS pulling 5V to ground will mimic a failed ECM rail and the ECM itself is almost never the actual problem.

Vehicle-specific patterns: 1997-2010 Saab 9-5 with the B205/B235 2.3L turbo is the most common P1240 platform — the MAP sensor in the intake manifold and the boost-pressure sensor downstream of the intercooler both have a known internal-short failure mode at 100K+ mi that pulls 5V VREF down. 2003-2011 Saab 9-3 with the B207 2.0L turbo throws P1240 from MAP sensor failure and from harness chafe at the engine-cover anchor points where the wiring rubs the cover edge. 1999-2003 Saab 9-3 (first generation) with the B205 throws P1240 from the same MAP/TPS pattern as later cars. 2005-2009 Saab 9-7X (5.3L Vortec V8) inherits GM code definitions and does not throw Saab-specific P1240. Estimated repair: $40 (single shorted sensor replacement + harness inspection DIY) to $400 (multiple sensor replacement + ECM ground-point refresh + harness repair at a Saab specialist).