Powertrain, generic (extended)

P2688 Fuel Supply Heater Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

With P2688 the heater control line reads too low while the driver is off, which points to a grounded wire: the relay would then be energized all the time, and the filter heater runs even in summer and drains the battery after shutdown. GM's 2020 diesel table requires 0.5 ohm or less to ground and counts 10 failures in 20 samples taken every 100 ms; the code is Type C but, unlike its siblings, triggers a service-vehicle-soon message after one trip (the 2018 table matches). In 2014 GM still rated the same fault Type B with MIL. On the 2015 and 2016 Ford 3.2 L Transit diesel the code belonged to the vaporizer glow plug of the exhaust fuel system and was set after 5 s inside a 1.95–2.175 V window; later model years report that as P269D.

Likely causes

Control wire chafed against the engine or frame and grounded
Heater relay coil shorted internally
Water or fuel in the filter-housing connector

Symptoms

  • Service message or MIL, depending on model year
  • Battery drained after the vehicle stands
  • Fuel heater running constantly

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–300
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel supply heaterwiring harness

Related codes

P2687P2689P269D

References: gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.