HighMX-13

SPN 8064 FMI 12 Paccar MX-13 — SCR Efficiency Low (2026)

SPN 8064 FMI 12 on Paccar MX-13 means scr catalyst temperature - bad intelligent device or component. 60–80% of field cases trace to heat-damaged harness at the scr catalyst flange. Shop repair with parts runs $180 - $650; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.

What This Code Means

SPN 8064 FMI 12 on Paccar MX-13 tells the ECM that scr catalyst temperature sensor is outside calibrated limits. On J1939, FMI 12 specifies the failure mode: SCR Catalyst Temperature - Bad Intelligent Device or Component. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.

Common Symptoms

• Amber check engine lamp with SCR or aftertreatment warning text on the dash (100% of cases)
• DEF dosing may suspend and SCR efficiency warnings appear in PACCAR Davie4 (80% of cases)
• Reduced engine torque or an active emissions derate after repeated drive cycles (60% of cases)
• Erratic or frozen SCR catalyst temperature readings in live data (40% of cases)
• Possible MIL flash during regen attempts when temperature feedback drops out (25% of cases)

Likely Causes (Ranked by Probability)

1. Heat-damaged harness at the SCR catalyst flange (45% of cases) — Repeated thermal cycles crack insulation and open the signal circuit, which the ECM interprets as a bad intelligent device rather than a simple voltage fault.

2. Failed SCR catalyst temperature sensor element (25% of cases) — Internal sensor failure sends implausible or missing temperature data even when supply voltage and grounds are correct.

3. Corroded Deutsch connector or poor ground (15% of cases) — Road salt and DEF mist corrode pins, raising circuit resistance until the aftertreatment module stops trusting the sensor entirely.

Step-by-Step Diagnostic Guide

1. Connect PACCAR Davie4 and read active/inactive faults plus freeze-frame SCR catalyst temperature at the time of failure. — Tool: scan tool or multimeter. Document readings on the work order.

2. Inspect the SCR temperature sensor harness near the DOC/SCR brick for heat damage, melted insulation, or loose Deutsch connectors. — Tool: scan tool or multimeter. Document readings on the work order.

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. — Tool: scan tool or multimeter. Document readings on the work order.

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. — Tool: scan tool or multimeter. Document readings on the work order.

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. — Tool: scan tool or multimeter. Document readings on the work order.

Repair Options (with Costs)

Repair	DIY	Independent Shop	Dealer
SCR Catalyst Temperature Sensor wiring repair	$25–$150	$150–$350	$250–$500
SCR Catalyst Temperature Sensor sensor/part	$180 - $650	$$180 - $650 + labor	$850+
Full module replacement	N/A	$600–$1,500	$850+

Platform-Specific Notes

On Paccar MX-13, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.

When This Code Sets

SPN 8064 FMI 12 sets when scr catalyst temperature sensor fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Paccar captures RPM, torque, exhaust temperature, and DEF level at fault set.

Scan Tool Parameters to Log

• Inlet vs outlet NOx (ppm)
• SCR efficiency %
• DEF level and quality/concentration
• DEF tank temperature

Common Misdiagnoses

Replacing the SCR brick because temperature reads low — A frozen low reading looks like a cold or dead catalyst.. 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 — Shops want a quick turnaround after sensor replacement.. Instead: Run a stationary regen in Davie4 and confirm temperature rises smoothly; otherwise the derate will return on the next loaded pull.

Shop Notes from the Field

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.

Safety Warning

• 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.

Prevention Tips

• 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.

Paccar Field Notes for SPN 8064 FMI 12

• Document ambient temperature and key-cycle count at fault set on Paccar MX-13 — cold-soak faults on scr catalyst temperature sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 8064 FMI 12 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If scr catalyst temperature sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Paccar.
• Capture a 20-minute data log of scr catalyst temperature sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
• Check for OEM technical service bulletins covering SPN 8064 FMI 12 on MX-13 — many bulletin fixes address harness routing rather than sensor replacement.
• Torque scr catalyst temperature sensor mounting fasteners per Paccar OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
• Document ambient temperature and key-cycle count at fault set on Paccar MX-13 — cold-soak faults on scr catalyst temperature sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 8064 FMI 12 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If scr catalyst temperature sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Paccar.
• Capture a 20-minute data log of scr catalyst temperature sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.

Related Codes

• SPN 4360 FMI 2 — often stacks with SPN 8064 FMI 12
• SPN 4364 FMI 18 — often stacks with SPN 8064 FMI 12

FAQ

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.

What does SPN 8064 FMI 12 mean on Paccar MX-13?

SPN 8064 FMI 12 means scr catalyst temperature - bad intelligent device or component. The ECM detected scr catalyst temperature sensor outside calibrated limits. Verify with freeze-frame data before clearing.

Can I drive with SPN 8064 FMI 12 active?

Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.

How much does it cost to fix SPN 8064 FMI 12?

DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $180 - $650. Dealer pricing runs $850+.

What is the most common cause of SPN 8064 FMI 12?

Heat-damaged harness at the SCR catalyst flange — confirm with visual inspection and live data before ordering parts.

Sources

• SAE J1939-71 — SPN 8064 parameter definition
• Paccar OEM service information — verify by engine serial
• Mitchell1 ProDemand — 2026 labor and parts cost surveys

Related Reading

• Paccar fault codes hub
• Diesel repair cost estimator
• SCR Catalyst Temperature Sensor diagnostic guide
• SPN 4360 FMI 2
• SPN 4364 FMI 18