P1233

P1233 on a Mercedes is a Fuel Pump Stage 1 circuit fault — Mercedes runs a two-stage fuel pump strategy on the M271/M272/M273 platforms (some via in-tank pump module, some via PWM-controlled relay/EKP module), and stage 1 is the low-pressure circuit that primes the rail at idle and light throttle. The ECU watches commanded duty cycle versus measured current draw on the stage 1 circuit; when commanded duty sits at 30-60% at idle but current draw drops below ~3A (typical healthy stage 1 pulls 5-8A at idle, 8-12A under load, and a healthy stage 2 add-on brings total system draw to 10-15A at WOT), it sets P1233. Diagnostic order on the shop floor: (1) Scan live data with XENTRY or a capable bidirectional scanner — look at "Fuel Pump Stage 1 Commanded Duty" (0-100%) and "Fuel Pump Stage 1 Current Draw" (Amps). If duty is pegged at 80-100% and current is below 2A, the pump or its wiring is the problem; if duty matches expected (30-50% at idle) and current is normal but the code still sets, the EKP control module itself is failing. (2) Check the fuel pump relay/EKP module under the rear seat or in the trunk — corrosion on the connector is shockingly common on W211/W221 cars and clears the code for $0. (3) Drop the rear seat and ohm the pump motor (healthy is 0.5-2 ohms across the motor terminals; open-circuit means the brushes have worn out). Expensive-misdiagnosis caveat: don't replace the in-tank pump assembly before verifying the EKP control module is actually commanding the pump — the EKP module on W211/W221 cars fails internally and reports phantom low-current faults, and dropping a brand-new $400 pump into a tank only to find the $300 EKP module was the real culprit is a Saturday-morning special at every Mercedes independent shop.

Vehicle-specific patterns: 2005-2011 Mercedes C-Class/E-Class/S-Class with the M272 3.0L/3.5L V6 or M273 4.6L/5.5L V8 (W203/W204/W211/W212/W221) throws P1233 most often from a corroded EKP relay connector or from the in-tank pump module wearing out around 100-130K miles — the cars also famously suffer balance-shaft gear wear that's covered by an extended warranty on affected VINs, but that's a P0016/P0017 issue not a fuel pump issue. 2003-2014 Mercedes C-Class/SLK with the M271 1.8L Kompressor (C230/C250/SLK200) throws P1233 from the same EKP corrosion pattern plus a known intake-manifold tumble-flap failure that loads the pump harder than spec. 2007-2010 Mercedes E320/E350 BlueTec and ML320 BlueTec with the M642 3.0 diesel use a different high-pressure pump architecture, but the low-pressure lift pump still sets P1233 when its wiring corrodes at the tank-top bulkhead connector. 2007-2014 Sprinter vans with the OM642 are notorious for fuel-pump-circuit codes from chassis-flex fatigue at the wiring loom. Estimated repair: $80 (corroded relay/connector clean-up) to $1,400 (full in-tank pump module plus EKP control module replacement on a W221 S550).