What 393.43(c) means in plain language
The relay emergency valve is a critical component in your truck's air brake system. This valve manages air pressure distribution between your service brakes and emergency brakes, ensuring both systems work in coordination. When an inspector cites you for 393.43(c), they've found that this valve is either defective or not functioning as designed.
A malfunctioning relay valve can compromise your ability to brake safely. It may fail to deliver proper air pressure to your service brakes, delay emergency brake engagement, or leak air from the system. Any of these conditions creates a safety risk and triggers a citation. The regulation requires that this valve remain in proper working order at all times.
This is not an out-of-service violation. Even though the valve is critical, inspectors are not required to immediately remove your vehicle from service for this defect alone. However, you should treat it as urgent—the longer you drive with a faulty valve, the greater the risk of brake failure.
What our enforcement data actually shows
Our inspection records show 393.43(c) is extremely rare in roadside enforcement. Across 13 million inspections in our database, we have recorded only 23 all-time citations for this code. In the last 12 months, we documented zero citations, and in the last 90 days, we recorded zero citations as well.
None of the 23 all-time cases resulted in an out-of-service placement—all 23 trucks were allowed to continue operating. This gives 393.43(c) a 0.0% out-of-service rate, which is dramatically lower than the all-FMCSR average of 31.4%. The code ranks #1881 out of 3,036 FMCSR codes by citation volume, placing it in the bottom tier of enforcement frequency.
The rarity of this citation reflects either the reliability of modern relay emergency valves or the challenge inspectors face in diagnosing their malfunction during a roadside inspection. Either way, if you've been cited, you're in an uncommon situation.
Who gets cited most
Our data shows enforcement for 393.43(c) is so sparse that no single state dominates the citation pattern. Among the 10 carriers in our all-time database with citations for this code, each has recorded exactly 1 citation. This includes fleets such as Eiker Incorporated, Bunger Steel Inc, and Deer Creek Special Order Inc, but the single-citation count means no pattern of systemic non-compliance is evident.
Looking at vehicle makes, our records show Ford trucks account for 7 of the 23 citations, followed by Dodge with 5 citations. Chevrolet and unpublished makes each appear 4 times. This distribution likely reflects the prevalence of these makes in the commercial trucking fleet rather than any inherent reliability issue.
How severe is this compared to similar codes
To understand where 393.43(c) sits in the enforcement landscape, compare it to related brake and maintenance violations. The code 393.47E (Slack adjuster defective) has 180,363 all-time citations with a 0.0% OOS rate—far more frequent but equally unlikely to result in immediate removal. By contrast, 396.3(a)(1) (Inspection/repair/maintenance general) shows 236,919 citations with a 45.3% OOS rate, meaning inspectors are much more likely to ground trucks for general maintenance failures.
Another peer code, 393.9(a) (Inoperable required lamps), accounts for 660,737 citations with a 15.4% OOS rate. The massive citation gap between lamp defects and relay valve defects underscores how uncommon relay valve citations are. In practical terms, you're far more likely to encounter enforcement for lighting or slack adjusters than for this specific valve defect.
How to avoid it
Because relay emergency valve defects are rare but serious, prevention requires focused attention to your air brake system as a whole:
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Inspect your air brake system weekly during your pre-trip walk-around. Listen for hissing sounds near the relay valve location (typically mounted near the rear axle), which indicates air leaks. Check all air brake connections for tightness and corrosion.
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Test your service and emergency brakes separately on every trip. Apply service brakes gently to confirm firm, immediate response. Engage the emergency brake to confirm it engages smoothly without delay. Any sluggishness or soft feel warrants immediate roadside diagnosis or a trip to the shop.
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Maintain your entire air brake system proactively. The relay valve doesn't fail in isolation—cracked hoses, loose fittings, and water in the air tank often precede valve failure. Have a certified mechanic inspect your air system at every PM interval.
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Keep detailed brake maintenance records. Document every inspection, repair, and air system service. If you're cited, these records provide evidence of diligent maintenance and help you dispute the citation if the valve was actually functional.
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Pay extra attention if you operate Ford or Dodge trucks. Our data shows these makes appear more frequently in relay valve citations. While the sample size is small, proactive brake inspection on these vehicles is warranted.
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Don't ignore subtle brake symptoms. If you notice your truck brakes gradually becoming less responsive, or if the emergency brake feels slow to engage, have the relay valve inspected before a roadside inspector finds the problem.