---
title: FMVSS 121 Air Brake Systems Inspection Procedure
description: "FMVSS 121 air brake inspection guide for ICE and EV chassis. Covers leakage, low-air warning, and spring brake pop-out tests. Includes specific EV protocols using the \"Air System Test\" switch to simulate \"engine off\" state for safety compliance."
---

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# FMVSS 121 Air Brake Systems Inspection Procedure

### Tools Required

- Wheel Chocks
- Stopwatch or Timer
- Calibrated Air Pressure Gauges (Vehicle-mounted)

### Vehicle Preparation

1. **Positioning**: Park the vehicle on a level surface.
2. **Security**: Chock the wheels and ensure the parking brake is applied.
3. **System Charge**: Start the engine (ICE) or turn the key to the ON position (EV). Allow the compressor to build pressure until the governor cut-out is reached (typically **125 PSI**).
4. **Initial Shutdown**:
   
     - **ICE Units**: Shut off the engine. Turn the key back to the **ON** position.
     - **EV Units**: Locate the **Momentary Air System Test Switch** near the ignition. Press and release this switch to disable the High-Voltage (HV) system and the air compressor.

### Procedure

#### Part 1: Air Leakage Test

1. **Set State**: Ensure the key is ON but the compressor/engine is OFF.
2. **Release**: Disengage the parking brake.
3. **Application**: Fully apply and hold the service (foot) brake pedal.
4. **Observation**: Monitor the gauges for 60 seconds.
   
     - **Pass/Fail (Single Unit)**: Loss must not exceed **3 PSI** per minute.
     - **Pass/Fail (Combination)**: Loss must not exceed **4 PSI** per minute.
5. **Audit**: Listen for any audible leaks throughout the chassis.

#### Part 2: Low Air Warning Device Test

1. **Fanning**: With the key ON, rapidly apply and release (fan) the service brakes to bleed air.
2. **Verification**: Confirm that the visual warning light and audible buzzer activate **at or above 60 PSI**.

#### Part 3: Spring Brake (Pop-Out) Test

1. **Fanning**: Continue fanning the service brake pedal to further deplete the secondary and primary tanks.
2. **Verification**: Observe the parking brake valve (yellow knob) and tractor protection valve (if equipped).
   
     - **Pass/Fail**: The valves must automatically "pop out" (close) between **20 and 45 PSI**.

### Post-Test Steps (EV Only)

1. **Reactivate**: Perform a full key cycle (Turn Key OFF, then back to ON).
2. **Confirmation**: Verify the HV system engages and the air compressor begins to recover the system pressure.

### Diagnosis and Repair Summary

| **Symptom** | **Potential Cause** |
| --- | --- |
| **Leakage \> 3-4 PSI/Min** | Leaking brake chamber diaphragm, loose fittings, or failed O-rings in the treadle valve. |
| **Warning \< 60 PSI** | Faulty low-pressure sending unit or electrical circuit fault to the dash cluster. |
| **No "Pop-Out" \< 20 PSI** | Failed PP-1 or MV-3 control valve; internal spring failure or debris in the valve body. |
| **EV Compressor stays ON** | Faulty "Air System Test" switch or HVIL (High Voltage Interlock Loop) circuit issue. |

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