Symptom index: where to start

You have a symptom, not a diagnosis. Find the row that matches what you see, run the first checks, and follow the link to the detailed material. Every row assumes the diagnostic ladder as the baseline: levels → clean → swap → OTDR.

Link state

Symptom	First checks	Likely causes	Read
Port down, module shows unsupported / invalid / Mod_Inv	host log, identity on the bench	vendor lock; identity or compliance codes the host rejects	Vendor lock, No link checklist
Port down, Rx −40 dBm / LOS on both ends	is Tx enabled? fibre continuity?	fibre cut, Tx disabled by host, Tx/Rx crossed, wrong BiDi pair, wrong wavelength for the mux	No link checklist, Physical mismatches
Port down, Rx and Tx levels fine	FEC and speed on both ends; AN on DAC	FEC mismatch, rate/breakout config, SGMII vs 1000BASE-X, CDR at wrong rate	Speed & rate, FEC & link training
Port down, Rx very high (> −3 dBm) on a short link	long-reach optic on a short span?	receiver overload	Physical mismatches
Works in one direction / LOS on one end only	compare Rx on both ends	dirty connector or weak laser on the failing side; polarity	DDM levels, Physical mismatches
Link flaps up/down	levels, CRC counters, connectors	contamination, marginal budget, thermal throttling	Link flapping
400G/800G module recognised but never ready, stuck LowPwr	CMIS module state, power class vs port	handshake / firmware / power class	CMIS issues, Power & thermal
DAC does not link, optic in the same port does	AN/link training, cable length, identity	AN disabled, unsupported cable, too long for the host	Copper & DAC, Speed & rate

Link up, but…

Symptom	First checks	Likely causes	Read
CRC / input errors climbing	Rx level vs baseline; FEC corrected counters; clean	dirt, marginal budget, dispersion at 10G+ on long CWDM	Link flapping, DDM levels
Slow throughput on a multi-lane link, no alarms	per-lane Rx/bias	one degraded lane hidden by LACP	Failures, Per-lane diagnostics
Works at 25G, dead at 10G (or 100G ok, 4×25G breakout not)	rate codes, CDR bypass, host breakout config	CDR range, module not breakout-capable, missing extended compliance code	Speed & rate, CDR
Absurd DDM values (0 °C, +10 dBm, 0.00 V)	byte 92 calibration bits	external calibration not applied, zeroed constants	Calibration
Tx power identical across a batch	compare modules	constant Tx written to memory	DDM levels
Module hot, port disables it, "power exceeds" log	power class byte vs port rating; airflow	class mismatch, thermal shutdown	Power & thermal
Everything fine for a year, then sudden failures	Tx bias trend, batch	laser ageing, bad batch, heat	Failures
Modules died after a switch firmware upgrade	host release notes; identity	stricter identity checks; clones failing authentication	Vendor lock, Failures

On the bench (programmer / CodingBox)

Symptom	First checks	Likely causes	Read
Module not detected on the programmer	seating, form-factor adapter, power-up time	not seated, wrong cradle, module in LowPwr	EEPROM read/write errors
Reads return FFh / 00h or garbage	address (A0h/A2h/page), bus speed	I²C NACK, wrong page, bus too fast	EEPROM read/write errors
WRITE FAIL	password, WP type, clock speed	protected module, hardware WP, timing	Write-protection types, EEPROM recoding
Write "succeeds" but reverts after power cycle	module type	temporary storage needs a save command	Write-protection types
Host rejects module after edit	checksums	stale CC_BASE / CC_EXT / CC_DMI	Memory map

Access networks

Symptom	First checks	Likely causes	Read
ONU does not register on the OLT	serial / LOID, PON standard, ONU Rx	whitelist, wrong wavelength plan (GPON vs XGS-PON), Rx out of class	PON problems
ONU shows LOS	SC/APC cleanliness, splitter, polish	dirt, UPC in an APC plant, fibre	PON problems

If nothing matches

Work the no-link checklist top to bottom, then read the module on the bench with CodingBox: the Check transceiver screen tells you what the module claims to be, and DDM what it is actually doing.