What 393.43(d) means in plain language
This regulation requires that every relay valve and emergency valve on your commercial motor vehicle be in proper working condition. If either component is defective or not functioning as intended, you're in violation — full stop.
These valves are critical pieces of your air brake system. The relay valve speeds up brake application and release on vehicles with long air lines, and the emergency valve is the failsafe that applies the brakes automatically if air pressure drops to a dangerous level. When either one fails, your ability to stop the vehicle safely is compromised in ways that aren't always obvious from the driver's seat.
The regulation doesn't require a catastrophic failure to trigger a citation. A sluggish response, a slow leak, or a valve that doesn't fully cycle can be enough for an inspector to write you up. That's why understanding the rule before you hit the scale matters.
What our enforcement data actually shows
Across our 13 million+ inspection records, 393.43(d) has generated 13,151 all-time citations, placing it at #186 out of 3,036 FMCSR codes by citation volume. That's a meaningful enforcement footprint — this is not an obscure technical code that inspectors rarely pursue.
What makes this code stand out is what happens after the citation is written. Our inspection records show an all-time out-of-service rate of 96.7% — meaning that of the 13,151 vehicles cited, 12,720 were placed out of service. Only 431 citations did not result in an OOS order. To put that in perspective, the all-FMCSR average OOS rate across all codes is 31.4%. At 96.7%, this code runs more than three times the system-wide average.
That gap is significant. It tells you that when an inspector finds a defective relay or emergency valve, they almost always consider the vehicle too dangerous to move. The code is technically marked OOS-ineligible in the current regulatory framework, yet our historical data shows the overwhelming majority of cited vehicles were nonetheless placed out of service — a reflection of how seriously inspectors treat brake-system integrity.
One notable pattern in our database: both the last 12 months and last 90 days show 0 new citations logged. This suggests either a shift in how inspectors classify these violations under updated coding structures, or a period of lower enforcement activity recorded under this specific code number.
Who gets cited most
The statistics block for this code does not include a state-level breakdown, so we cannot identify specific top states by citation count. What the data does show clearly is a strong vehicle-make pattern. Among the 13,151 all-time citations, Ford vehicles account for 3,174 citations, making them the most frequently cited make by a wide margin. Dodge follows with 1,384 citations, and Chevrolet accounts for 911 citations. RAM (639) and GMC (550) round out the top five identified makes.
This skew toward light-duty and medium-duty domestic trucks — Ford, Dodge/RAM, Chevrolet, GMC — is striking. It suggests that a significant portion of 393.43(d) citations involve vocational or work trucks (think service bodies, utility fleets, rental equipment haulers) rather than Class 8 over-the-road tractors. Trailers tagged as BIG TEX (441 citations) reinforce the idea that equipment-hauling and contractor-type operations are heavily represented.
Looking at carriers, our data shows fleets such as LBS LOGISTICS INC (USDOT 3621068) with 23 citations and BENNETT TRUCK TRANSPORT LLC (USDOT 600382) with 11 citations appearing at the top of the citation list. These numbers reflect long-term accumulation across large fleets operating significant equipment inventories.
How severe is this compared to similar codes
Compared to other codes in the Vehicle Maintenance category, 393.43(d) is in a class of its own on the OOS dimension.
Take 393.9(a) — Inoperable required lamps, the highest-volume code in this category with 660,737 all-time citations. Its OOS rate is just 15.4%. Drivers get cited constantly, but they rarely get parked. The relay/emergency valve code has roughly one-fiftieth the citation volume, yet its OOS rate is more than six times higher.
396.3(a)(1) — Inspection, repair, and maintenance (general) sits at 236,919 citations with a 45.3% OOS rate — well above the all-FMCSR average, but still less than half the rate seen with 393.43(d).
393.47E — Slack adjuster defective shows 180,363 citations with a 0.0% OOS rate in our records, which illustrates how differently inspectors treat various brake-related codes. A faulty slack adjuster gets documented; a faulty relay or emergency valve gets the vehicle parked.
The takeaway for fleet managers: 393.43(d) is a low-frequency, high-consequence code. You won't see it on every inspection report, but when you do, the vehicle is almost certainly going nowhere until it's repaired.
How to avoid it
Given that Ford, Dodge, Chevrolet, RAM, and GMC vehicles dominate the citation list — vehicle types often operated by smaller fleets or contractors with less structured maintenance programs — the prevention approach has to start with pre-trip discipline. Here's what you can do before you leave the yard:
- Check your brake system build-up time during pre-trip. With the engine running, time how long it takes the air system to build from 50 to 90 PSI. A relay valve that's partially failed can slow this process. If it takes longer than your vehicle's spec allows, don't move the truck.
- Fan the brakes and listen for recovery. After building full air pressure, apply and release the brakes several times. A healthy relay valve responds crisply. A sluggish or sticky pedal response, or pressure that doesn't recover quickly, is a red flag.
- Perform a static leak-down test. With the engine off and full air pressure built, apply the brakes and watch the gauge for 60 seconds. More than a 3 PSI drop in a single-vehicle system (or 4 PSI for a combination vehicle) indicates a leak that could involve the relay or emergency valve circuit.
- Inspect trailer glad hands and emergency lines. Because the emergency valve activates through the trailer supply line, a damaged or improperly connected emergency (red) glad hand can mask an underlying valve problem until an inspector tests it properly.
- Flag any air dryer or compressor issues immediately. Contaminated air — wet or oily — degrades valve seals faster than normal wear. If your air dryer is overdue for service, relay and emergency valves are downstream victims. Don't let a $50 dryer cartridge become a roadside OOS order.
- For Ford, Dodge/RAM, Chevrolet, and GMC work trucks specifically: these platforms are often used in vocational roles where the vehicles sit for extended periods between uses. Valves that aren't cycled regularly can develop internal corrosion or seized components. If your truck has been idle for more than a few days, cycle the brakes thoroughly before heading to a weigh station or inspection corridor.