Fault Family System: Powertrain (P-Codes) 16 SAE Codes

Fuel Control System B Lean Rich

Diagnostic & Technical Explanation

P21F4 (Fuel Control System B Too Lean Off Idle Bank 1) applies to engines with a second, independently trimmed fuel control system — the B side of a dual-bank/dual-fuel arrangement — whose closed-loop correction has hit its lean or rich limit at the specific operating point named in the code (idle, off-idle, or higher load). The same physics as conventional fuel-trim codes apply to the B system: unmetered air or low fuel pressure drives it lean, while a leaking injector, high fuel pressure, or a biased oxygen sensor on that side drives it rich.

These 16 diagnostic trouble codes share one detection mechanism and one repair path, which is why MechanicDB groups them into a single authored fault family rather than repeating the same explanation 16 times.

Codes in this family (16)

Code	Definition	Repair coverage
P21F4	Fuel Control System B Too Lean Off Idle Bank 1	4 ranked fixes
P21F5	Fuel Control System B Too Rich Off Idle Bank 1	4 ranked fixes
P21F6	Fuel Control System B Too Lean Off Idle Bank 2	4 ranked fixes
P21F7	Fuel Control System B Too Rich Off Idle Bank 2	4 ranked fixes
P21F8	Fuel Control System B Too Lean at Idle Bank 1	4 ranked fixes
P21F9	Fuel Control System B Too Rich at Idle Bank 1	4 ranked fixes
P21FA	Fuel Control System B Too Lean at Idle Bank 2	4 ranked fixes
P21FB	Fuel Control System B Too Rich at Idle Bank 2	4 ranked fixes
P21FC	Fuel Control System B Too Lean at Higher Load Bank 1	4 ranked fixes
P21FD	Fuel Control System B Too Rich at Higher Load Bank 1	4 ranked fixes
P21FE	Fuel Control System B Too Lean at Higher Load Bank 2	4 ranked fixes
P21FF	Fuel Control System B Too Rich at Higher Load Bank 2	4 ranked fixes
P2BF0	Fuel Control System B Too Lean Bank 1	4 ranked fixes
P2BF1	Fuel Control System B Too Rich Bank 1	4 ranked fixes
P2BF2	Fuel Control System B Too Lean Bank 2	4 ranked fixes
P2BF3	Fuel Control System B Too Rich Bank 2	4 ranked fixes

What this page does not show

This page publishes the fault mechanism and the code index. The licensed dataset adds, for every one of the 15,886 codes, the full authored repair set (56,729 procedures in total) with the fields deliberately withheld here:

Procedures 2..n after the first-listed procedure
Aftermarket parts cost ranges in USD (est_parts_cost_min/max_usd)
Professional labor-hour estimates (est_labor_hours)
Step-by-step diagnostic and repair instructions
Replacement-part mappings and catalog lookups
6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)

Need All 15,886 Codes as Data?

Pipe-delimited CSV, Apache Parquet and a relational SQLite database, with commercial application rights and instant download. Built for scan tools, shop software and research - not assembled one web page at a time.

License OEM Complete · $149 Standard (SAE only) · $49

$49 Standard: 9,249 SAE codes · $149 OEM Complete: all 15,886 codes, including 6,637 OEM codes across 32 makes · $149 Heavy Duty: 4,212 fault pairs (J1939) · one-time · compare tiers