Powertrain, generic (extended)

P21B9 Cold Start Cylinder 4 Injection Timing

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

Four-cylinder petrol direct injection is the most widespread engine type this code can apply to, and cylinder 4 is its last cylinder. Ford's 2011 OBD summary for its first turbo direct-injection engine explains the background: during catalyst heating the injection timing may be specific to that mode, possibly as two injections with one during the compression stroke, which in turn needs a rail pressure window (checked as P053F). P21B9 reports that this cold-start timing went wrong on cylinder 4 alone. No manufacturer lists the code publicly, so the description rests on CARB's cold-start rules and the documented system codes. Warm-engine counterpart: P020D; electrical injector fault: P0204; cold-start pulse: P2B0B.

Likely causes

Deposits on the tip of injector 4 changing its opening behaviour

Harness fault on the injector 4 circuit

Rail pressure barely inside its window during cold start, affecting the last cylinder first

Symptoms

Check engine light, usually without drivability complaints
Slightly longer catalyst warm-up and possibly higher cold-start emissions
Occasional hesitation right after a cold start

Repair estimate

DifficultyHardWorkshop job

Parts and labour€80–1,000

Time0.5–3.5 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, newtonnet.co.uk, gsi.ext.gm.com.