Powertrain, generic (extended)

P21B7 Cold Start Cylinder 2 Injection Timing

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

P21B7 flags cylinder 2 during the cold-start phase only: the injection timing commanded for catalyst heating did not arrive as intended on that cylinder. The heating phase is brief and ends as coolant and catalyst warm up, so the monitor gets one chance per cold start and may need several cold starts before the code is confirmed. GM's 2025 diesel calibrations show the principle at system level: requested against scheduled start of injection, one sample per revolution, 88 failures out of 176 set P05EC with the lamp in the first trip. A cylinder-specific code like P21B7 appears in no public manufacturer document. If P020B (cylinder 2, warm) or P2B09 (cold-start pulse) are stored as well, the injector or its circuit is more likely than software.

Likely causes

Injector of cylinder 2 worn or coked, opening late at low temperature

Chafed or high-resistance wiring to injector 2

Weak battery lowering injector supply voltage during and after cranking

Symptoms

Check engine light that appears only after cold starts
Uneven running or slight smoke shortly after starting a cold engine
Code may take several cold starts to return after clearing

Repair estimate

DifficultyHardWorkshop job

Parts and labour€80–1,200

Time0.5–4 h

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

Parts involved

fuel injectorinjector wiringengine control module

References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk.