P2D21 Turbocharger Interstage Pressure Sensor A Circuit

Engines with two turbochargers in series can measure the pressure between the low-pressure and the high-pressure compressor. GM's patent on two-stage boost control uses this mid-stage value to split the pressure ratio between both stages and to steer the low-pressure wastegate; Cummins describes the same sensor between the stages and the intercooler. P2D21 is the general circuit code: the module rejects the signal but does not assign it to the low, high or erratic codes. No manufacturer documents P2D21 publicly. California requires input sensors to be checked for circuit faults and boost systems for under- and overboost. Single-turbo engines never set it. Expect reduced boost and power.

Likely causes

Symptoms

• Check engine light
• Reduced power, boost held to a safe value
• Sluggish response while accelerating

Repair estimate

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

Parts involved

Related codes

References: patents.google.com, patents.google.com, law.cornell.edu.