PACCAR SPN 8064 FMI 12: SCR Catalyst Temperature Device Fault
SPN 8064 FMI 12 (FMI 12) — SCR Catalyst Temperature - Bad Intelligent Device or Component
Quick Answer
SPN 8064 FMI 12 means the SCR catalyst temperature sensor or its module is flagged as a bad intelligent device. Expect unreliable aftertreatment control and a possible torque derate until wiring or the sensor is verified.
Overview: Paccar SPN 8064 FMI 12
When an MX-powered Kenworth or Peterbilt throws SPN 8064 FMI 12, the truck loses trustworthy SCR temperature feedback exactly when the aftertreatment system needs it most. ProfessionalDieselRepair.com sees this code after heat-soaked harness failures at the SCR flange and after moisture intrusion in the sensor connector.
Repair Snapshot
Time
2 - 4 Hours
Difficulty
Advanced
Parts Cost
$180 - $650
Dealer Labor
$850+
Tools Required
- PACCAR Davie4
- Noregon JPRO
- Multimeter
- Heat-Shrink Repair Kit
What SPN 8064 FMI 12 Means
SPN 8064 FMI 12 tells the aftertreatment controller the SCR catalyst temperature sensor or its module is reporting a bad intelligent device. PACCAR MX engines may limit DEF dosing and escalate to a torque derate or shutdown if catalyst temperature feedback stays invalid during loaded operation.
Symptoms Drivers Notice
- Amber check engine lamp with SCR or aftertreatment warning text on the dash
- DEF dosing may suspend and SCR efficiency warnings appear in PACCAR Davie4
- Reduced engine torque or an active emissions derate after repeated drive cycles
- Erratic or frozen SCR catalyst temperature readings in live data
- Possible MIL flash during regen attempts when temperature feedback drops out
Most Likely Root Causes
Heat-damaged harness at the SCR catalyst flange
CommonRepeated thermal cycles crack insulation and open the signal circuit, which the ECM interprets as a bad intelligent device rather than a simple voltage fault.
Failed SCR catalyst temperature sensor element
OccasionalInternal sensor failure sends implausible or missing temperature data even when supply voltage and grounds are correct.
Corroded Deutsch connector or poor ground
CommonRoad salt and DEF mist corrode pins, raising circuit resistance until the aftertreatment module stops trusting the sensor entirely.
Common Misdiagnoses (Don't Waste Parts)
Replacing the SCR brick because temperature reads low
Why it happens: A frozen low reading looks like a cold or dead catalyst.
Do this instead: That is a parts-cannon reaction—compare both SCR temperature sensors and verify harness integrity before condemning the catalyst.
Clearing the code without a regen verification drive
Why it happens: Shops want a quick turnaround after sensor replacement.
Do this instead: Run a stationary regen in Davie4 and confirm temperature rises smoothly; otherwise the derate will return on the next loaded pull.
Master Tech Commentary
Mechanic's Notes
ProfessionalDieselRepair.com shop note: FMI 12 on SCR temperature is often a harness fault at the brick flange, not a dead sensor. Avoid the parts-cannon swap until you ohm the signal circuit and watch live temperature during a regen attempt.
Step-by-Step Diagnostic Procedure
- Step 1: Connect PACCAR Davie4 and read active/inactive faults plus freeze-frame SCR catalyst temperature at the time of failure.
- Step 2: Inspect the SCR temperature sensor harness near the DOC/SCR brick for heat damage, melted insulation, or loose Deutsch connectors.
- Step 3: Back-probe the sensor supply at the connector with key on: verify 5.0 V ±0.5 V reference and less than 0.5 V on signal ground.
- Step 4: Compare upstream and downstream SCR temperature sensors in live data during idle and loaded snap throttle; a stuck reading more than 50 °F from expected range points to a failed sensor.
- Step 5: Repair wiring or replace the SCR catalyst temperature sensor per PACCAR MX spec, clear faults, and confirm temperature tracks within ±30 °F of DOC outlet during a stationary regen.
Professional service / OEM-level scan tools are recommended before clearing this fault for good.
Live Data & Freeze-Frame Tips
In PACCAR Davie4, graph SCR catalyst temperature against DOC outlet temperature and engine load. Both sensors should rise together during regen. A flat line or a reading stuck near ambient while DOC temperature climbs above 400 °F confirms a sensor or module fault.
Safety Warnings
- Allow the exhaust aftertreatment system to cool below 200 °F before touching sensors or harness near the SCR brick.
- Wear heat-resistant gloves and eye protection when working around hot DOC/SCR housings.
- Support exhaust components properly; do not rely on flex pipe to hold sensor weight during connector service.
When to Limp In vs Tow
Tow if the truck enters a hard torque derate, DEF dosing is suspended, or regen cannot start because catalyst temperature feedback is invalid. Continuing under load without valid SCR temperature risks incomplete regen and further aftertreatment faults.
Prevention Tips for Fleets & Owner-Operators
- Inspect SCR sensor harness routing annually for rubbing against frame brackets.
- Apply dielectric grease to connectors after cleaning corrosion from DEF and road salt exposure.
- Address exhaust leaks upstream of the SCR that can overheat local wiring bundles.
- Keep aftertreatment ECM software current with PACCAR service bulletins for sensor module updates.
Related Fault Codes
SCR intake gas temperature faults often appear alongside catalyst temperature communication errors.
Low SCR conversion efficiency may follow invalid catalyst temperature feedback that disrupts dosing strategy.
Frequently Asked Questions
What does PACCAR SPN 8064 FMI 12 mean?
It indicates the SCR catalyst temperature sensor or its intelligent device is reporting a failure to the aftertreatment controller, not just a simple out-of-range reading.
Will SPN 8064 FMI 12 cause a derate?
Yes. Many MX calibrations limit torque or suspend DEF dosing when SCR temperature feedback is unreliable, especially during regen or heavy load.
Can I drive with this code active?
Short distances to a safe stop may be possible, but continued loaded driving risks aftertreatment damage and escalating derate timers.
Is the sensor always the failed part?
No. Heat-damaged wiring at the SCR housing is at least as common as a failed sensor element.
Which scan tool reads this code best?
PACCAR Davie4 provides the most complete SCR temperature and regen data; JPRO can read the fault but may show fewer aftertreatment parameters.
How much does repair usually cost?
Harness repair or sensor replacement typically runs $250-$900 in parts and labor; catalyst replacement is only needed if separate efficiency codes confirm brick failure.
More PACCAR codes on this system
- Heavy-Duty SPN 4360 FMI 2 — Exhaust Temperature Sensor
- PACCAR SPN 4364 FMI 18 — SCR NOx Efficiency
- PACCAR SPN 5246 FMI 16 — SCR System
- Paccar SPN 4094 FMI 16 — DPF Soot Load
- PACCAR SPN 3251 FMI 18 — DPF Differential Pressure Sensor
- PACCAR SPN 3936 FMI 0 — SCR Conversion Efficiency (NOx Sensors)
- Paccar SPN 4358 FMI 16 — SCR Differential Pressure Sensor
- PACCAR SPN 3242 FMI 16 — DPF Intake Gas Temperature Sensor
Guides that go deeper
- Detroit DDDL Basics: Finding the Root Code Behind a Derate
Use Detroit DDDL the right way: separate root SPN/FMI faults from inducement result codes, read derate stage data, and stop replacing parts on guesses.
- SPN 4364 SCR Efficiency Low: Fix Order That Saves Money
A money-saving fix order for SPN 4364 SCR efficiency low—DEF quality, dosing, NOx sensor trust, then catalyst—so you stop buying the brick first.
- Mack MP8 Aftertreatment Faults: First Checks That Save Shop Time
Mack MP8 aftertreatment stacks look noisy until you separate DPF, DEF dosing, and NOx sensor rationality. Start with these high-yield checks.