SPN 46 FMI 1
Out of servicePneumatic Supply Pressure — Below Normal, Most Severe
SAE J1939 · heavy duty · Freightliner, Volvo Trucks, PACCAR (Kenworth / Peterbilt), International, Mack, Western Star, kenworth, peterbilt
Stop driving
Air system pressure is below the level the brakes need. This is a brake system fault, not an engine fault. Do not move the truck on the theory that it will build back up.
DrivingNo. Low air pressure on a commercial vehicle is a brake defect. If pressure drops far enough the spring brakes will apply on their own, and where that happens is not your choice. This is out-of-service territory at a roadside inspection and it is genuinely dangerous. Get it repaired where it sits.
Typical repair cost
- Parts
- $910–$3,680
- Labor
- 1–8 hr × $125–$210/hr = $125–$1,680
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 46 FMI 1 means
SPN 46
SPN 46 is pneumatic supply pressure — the air pressure available to the vehicle's pneumatic systems, which on a commercial vehicle means the service brakes, the spring brakes, and everything downstream of the supply reservoir.
FMI 1
FMI 1 means data valid but below normal operating range, most severe level. Pressure is genuinely low, not intermittent and not a sensor circuit fault.
Read together, SPN 46 FMI 1 is the ECM saying: pneumatic supply pressure — below normal, most severe. 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
- Air leak in a line, fitting, chamber, or gladhandThe most common cause by a wide margin in service literature, and the one a driver can often find with soapy water.
- Air compressor not building — worn rings, failed unloader, or broken drive
- Air dryer purge valve stuck open, venting supply continuously
- Governor out of adjustment or failed, so the compressor never loads
- Frozen moisture in lines or valves in cold weather
- Failed pressure sensor or wiring reporting low pressure falsely
- Reservoir drain valve left open or leaking
No frequency data yet. We have fewer than 5 verified repair receipts for SPN 46 FMI 1, 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 46 FMI 1 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
Confirm the reading against the dash gauges and a mechanical test gauge at the service port. Two independent readings settle sensor-versus-real in a minute.
Tool: Mechanical air pressure gauge
- 2
Build the system to governor cut-out with the engine running and note the cut-out pressure and the time taken.
Spec: TODO(verify): cut-out pressure and build-time criteria — confirm against 49 CFR 393.52 and OEM specification
- 3
Shut down and perform a static leakage test, then an applied leakage test, and compare against the federal limits.
Spec: TODO(verify): applied and static leakage limits per 49 CFR 393.45 / 393.52
- 4
Walk the truck and trailer with soapy water: gladhands, chamber diaphragms, hose ends, tank fittings, and the dryer purge.
- 5
Check the air dryer. A dryer that hisses continuously after cut-out is venting your supply and is a common, cheap fix.
- 6
Drain the wet tank and look at what comes out. Oil in the tanks points at a compressor passing oil and is a different, larger repair.
- 7
Verify governor cut-in and cut-out settings before condemning the compressor.
Tool: Mechanical air pressure gauge
Parts and labour
| Part | Part number | Parts cost |
|---|---|---|
| Air dryer cartridge | TODO(verify) | $60–$220provisional |
| Air dryer assembly | TODO(verify) | $250–$800provisional |
| Air governor | TODO(verify) | $40–$160provisional |
| Air compressor (reman to new) | TODO(verify) | $500–$2,200provisional |
| Brake chamber | TODO(verify) | $60–$300provisional |
| Labour | 1–8 hr | $125–$1,680 |
Labour priced at provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these numbers.
Can I keep driving?
No. Low air pressure on a commercial vehicle is a brake defect. If pressure drops far enough the spring brakes will apply on their own, and where that happens is not your choice. This is out-of-service territory at a roadside inspection and it is genuinely dangerous. Get it repaired where it sits.
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
Had this repaired? Tell us what you paid.
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