Powertrain, generic (extended)

P21B6 Cold Start Cylinder 1 Injection Timing

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

After a cold start, petrol direct-injection and diesel engines deliberately move the moment of injection – for example with an extra late pulse – so that more heat reaches the catalyst. CARB 13 CCR 1968.2 (e)(11) wants every element of this strategy checked while it runs and reported apart from warm-engine faults. P21B6 pins such a deviation on cylinder 1. No manufacturer documents this code publicly; GM's 2025 diesel tables compare requested and scheduled start of injection on every cylinder but store only the system code P05EC. If cylinder 1 alone is flagged, a shared cause such as crank/cam synchronisation is unlikely; the injector, its wiring or its driver stage come first. Warm counterpart: P020A.

Likely causes

Injector of cylinder 1 responds late or inconsistently when cold
Corroded or loose connector or harness at injector 1
Injector driver stage in the engine control module faulty
Engine module software with an overly strict cold-start check

Symptoms

  • Check engine light after one or more cold starts
  • Slightly rough idle in the first minute after a cold start
  • No noticeable fault once the engine is warm

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

Related codes

P21B7P020AP2B08P0201P0301P05EC

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