SPN 3226 FMI 16
CautionSCR Outlet NOx Sensor — Reading Above Normal Range
SAE J1939 · heavy duty · Freightliner, Volvo Trucks, PACCAR (Kenworth / Peterbilt), International, Mack, Cummins, Western Star
Get it looked at
Too much NOx is getting past the SCR catalyst. Either the system is not dosing enough DEF, the catalyst has lost efficiency, or the outlet sensor is lying. All three are worth catching now.
DrivingYes. This is a caution-level fault. Left alone it typically progresses to an SCR conversion efficiency fault and then to inducement, so the useful question is not whether you can finish this run but whether you can get it diagnosed before it starts limiting your speed.
Typical repair cost
- Parts
- $3,100–$9,150
- Labor
- 1–6 hr × $125–$210/hr = $125–$1,260
Provisional figures — pending verification against our receipt dataset.
Labor rate: provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these ranges.
Need it fixed
Phone-verified shops, with the date we last called each one.
What SPN 3226 FMI 16 means
SPN 3226
SPN 3226 is aftertreatment 1 outlet NOx — the sensor downstream of the SCR catalyst. The controller compares it against the inlet sensor (SPN 3216) to calculate conversion efficiency, which is the number the emissions system is actually regulated on.
FMI 16
FMI 16 means data valid but above normal operating range, moderately severe level. Tailpipe NOx is measurably higher than the calibration expects for the current operating point.
Read together, SPN 3226 FMI 16 is the ECM saying: scr outlet nox sensor — reading above normal range. The SPN identifies which parameter, the FMI identifies how it failed. Two codes sharing an SPN but carrying different FMIs are different faults with different repairs, which is why the SPN alone is never enough.
Common causes
- Insufficient DEF dosing — clogged injector, low pressure, or a failing pump
- SCR catalyst degraded, contaminated, or thermally damagedSulphur and oil contamination both poison the catalyst. TODO(verify): typical service life before efficiency loss.
- Outlet NOx sensor drift or failure
- Exhaust leak between the catalyst and the sensor
- Poor DEF quality reducing conversion
- Low exhaust temperature from extended idle or light duty cycle keeping the catalyst below its working window
No frequency data yet. We have fewer than 5 verified repair receipts for SPN 3226 FMI 16, so we are not going to print percentages. Causes above are listed in no particular order. Had this fixed? Tell us what it turned out to be.
Technician note pending
A credentialed technician annotation for SPN 3226 FMI 16 has not been commissioned yet. Until it is, this page stays a draft and is excluded from our sitemap. We do not publish invented first-hand experience. How we verify content.
How a technician diagnoses it
- 1
Key-on, engine-off: compare inlet and outlet sensor readings in ambient air. They should agree. A split at rest identifies the drifting sensor before you spend anything.
Tool: OEM service tool
- 2
Test the DEF with a refractometer. A dosing problem and a fluid problem look identical from the tailpipe.
Tool: DEF refractometer · Spec: 32.5% urea
- 3
Run the DEF dosing system self-test and watch actual versus commanded dosing quantity and rail pressure.
Tool: OEM service tool
- 4
Inspect the DEF injector tip for crystallisation. White urea crystal build-up at the nozzle is common and blocks spray pattern long before it blocks flow entirely.
- 5
Check exhaust temperature history. A truck that idles most of its hours may simply never get the catalyst hot enough — that is a duty cycle conversation, not a parts conversation.
- 6
Pressure or smoke test the exhaust between catalyst and sensor for leaks.
- 7
Replace the outlet sensor only after dosing and catalyst have been cleared. Catalyst replacement is the expensive last resort and should never be the first guess.
Parts and labour
| Part | Part number | Parts cost |
|---|---|---|
| SCR outlet NOx sensor and harness assembly | TODO(verify) | $350–$900provisional |
| DEF dosing injector | TODO(verify) | $250–$750provisional |
| SCR catalyst assembly | TODO(verify) | $2,500–$7,500provisional |
| Labour | 1–6 hr | $125–$1,260 |
Labour priced at provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these numbers.
Can I keep driving?
Yes. This is a caution-level fault. Left alone it typically progresses to an SCR conversion efficiency fault and then to inducement, so the useful question is not whether you can finish this run but whether you can get it diagnosed before it starts limiting your speed.
Find a shop that handles this
We call every shop before it appears here, and publish the date.
Sample records only — seeded to demonstrate the directory. None of these numbers has been called.
Memphis, TN · 24/7 · mobile service
Indianapolis, IN · Mon-Fri 6am-6pm · mobile service
Laredo, TX · Mon-Sat 6am-9pm · mobile service
Phoenix, AZ · 24/7 · mobile service
Codes that usually show up with this one
- SPN 3216 FMI 16SCR Inlet NOx Sensor — Reading Above Normal RangeCaution
- SPN 4364 FMI 18SCR Conversion Efficiency — Below Normal, Moderately SevereDerate risk
- SPN 5246 FMI 0SCR Operator Inducement — Most Severe Level ActiveOut of service
- SPN 3364 FMI 2DEF Quality / Concentration — Data Erratic or IncorrectDerate risk
Had this repaired? Tell us what you paid.
Every price range on this site comes from real invoices. Two minutes of your time makes the next driver's estimate better — and you get to see the aggregate for SPN 3226 FMI 16 once it publishes.
No account. No email required. We publish aggregates only, never your invoice.
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