P21B8 Cold Start Cylinder 3 Injection Timing

Cylinder 3 is the last cylinder of three-cylinder engines such as Ford's 1.0-litre EcoBoost, a turbo direct-injection unit. There a single cylinder accounts for a third of all combustion events, so a cold-start fault on it tends to be felt more than on larger engines. P21B8 reports that while the cold-start heating strategy was active, the injection timing of cylinder 3 left its tolerance. The code belongs to a per-cylinder series made necessary by CARB's demand to keep cold-start faults apart from warm-running faults; no manufacturer documents its use publicly. On engines with four or more cylinders P21B8 simply marks cylinder 3. Neighbouring checks: P020C for the warm engine, P2B0A for the cold-start injection pulse, P0303 for misfire.

Likely causes

Symptoms

• Noticeable shaking at cold idle, especially on three-cylinder engines
• Check engine light after cold starts
• Normal running once warm

Repair estimate

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

Parts involved

Related codes

References: law.cornell.edu, en.wikipedia.org, patents.google.com.