A bus is to be retrofitted with R-407C. Technician A says the components of R-407C contain no chlorine. Technician B says R-407C is intended as a replacement for R-22. Who is right?Answer: Both technicians. Source: Wikipedia, R-407C, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
A bus system is being changed from R-22 to a chlorine-free HFC refrigerant. Which lubricant is used with HFC systems?Answer: Polyolester (POE) oil. Source: Wikipedia, Polyolester, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
A bus system is labeled for R-407C. Which three refrigerants are blended to make R-407C?Answer: R-32, R-125 and R-134a. Source: Wikipedia, R-407C, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
Show all 42 questions (39 more)
A/C system, general, continued
A discharge line is checked by touch and temperature during diagnosis. In what state does refrigerant leave the compressor?Answer: A superheated vapor. Source: Wikipedia, Vapor-compression refrigeration, Description. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
A refrigerant label gives the ASHRAE safety group A1. What does class A1 mean?Answer: Lower toxicity and non-flammable. Source: Wikipedia, Refrigerant, Refrigerant safety, environmental management, and reclamation. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
A system charged with a zeotropic blend has leaked vapor for weeks. Why can the makeup of the charge that is left differ from the label?Answer: Liquid and vapor change in composition as the blend boils. Source: Wikipedia, Zeotropic mixture, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
A transit bus air conditioner is charged with R-22. Why were refrigerant uses of R-22 phased out under the Montreal Protocol?Answer: Its ozone depletion potential and high global warming potential. Source: Wikipedia, Chlorodifluoromethane, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
R-407C is a zeotropic blend with a temperature glide. What does temperature glide mean?Answer: The blend changes phase over a range of temperatures, not at one. Source: Wikipedia, Zeotropic mixture, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
Refrigerant is recovered from a bus through hoses with low-loss fittings. What is a low-loss fitting, as 40 CFR 82.152 defines it?Answer: A connecting device that closes when disconnected, to limit refrigerant release. Source: 40 CFR 82.152, Low-loss fitting. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
While liquid refrigerant remains in the evaporator, what sets the highest temperature the refrigerant can reach there?Answer: Its boiling point at the pressure in the evaporator. Source: Wikipedia, Vapor-compression refrigeration, Description. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
Why is liquid refrigerant subcooled before it reaches the thermal expansion valve?Answer: To be sure it is fully liquid when it reaches the valve. Source: Wikipedia, Subcooling, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A1. A/C System and Component Diagnosis, Service and Repair: A/C System, General (10 of 40)
Compressor and clutch 6
A bus has a six-cylinder reciprocating compressor. Besides size and number of cylinders, how is the capacity of a reciprocating compressor varied?Answer: By cylinder unloading. Source: Wikipedia, Compressor, Reciprocating compressors. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
A low pressure cutout switch keeps opening and closing near its setting. What is the deadband of a pressure switch?Answer: A range around the setpoint in which small pressure changes do not move the contacts. Source: Wikipedia, Pressure switch, Construction and types. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
A transit bus compressor is an open type, driven from the engine. What does an open compressor rely on to hold its internal pressure?Answer: Rotary seals around the drive shaft. Source: Wikipedia, Compressor, Hermetically sealed, open, or semi-hermetic. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
How do the two scrolls of a scroll compressor move to compress refrigerant?Answer: One is fixed while the other orbits without rotating. Source: Wikipedia, Scroll compressor, Design. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
When does slippage normally occur in an electromagnetic compressor clutch that is sized properly?Answer: Only during acceleration, as it engages. Source: Wikipedia, Electromagnetic clutch, Cycling. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
Zero superheat is measured at the evaporator outlet. Why is that a risk to the compressor?Answer: Liquid may reach the compressor and damage it. Source: Wikipedia, Thermal expansion valve, Theory of operation. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A2. A/C System and Component Diagnosis, Service and Repair: Compressor and Clutch (6 of 40)
Evaporator, condenser and related parts 6
A high pressure relief valve has released refrigerant. At what point is a relief valve designed to open?Answer: At a predetermined set pressure. Source: Wikipedia, Relief valve, Pressure relief. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
A thermal expansion valve cannot hold the evaporator at a precise temperature. What does the evaporator temperature vary with?Answer: The evaporating pressure. Source: Wikipedia, Thermal expansion valve, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
Superheat at the evaporator outlet is far above specification. What does excessive superheat indicate?Answer: Too little refrigerant is flowing through the evaporator coil. Source: Wikipedia, Thermal expansion valve, Theory of operation. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
Water collects in an evaporator housing. Where should water from the evaporator go?Answer: Into a drip pan, then away by gravity or a condensate pump. Source: Wikipedia, Vapor-compression refrigeration, Description. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
What happens to the latent heat of the refrigerant in the condenser?Answer: It is released and passed to the surroundings. Source: Wikipedia, Condenser (heat transfer), lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
Where is the sensing bulb of a thermal expansion valve thermally connected?Answer: To the output of the evaporator. Source: Wikipedia, Thermal expansion valve, Theory of operation. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), A3. A/C System and Component Diagnosis, Service and Repair: Evaporator, Condenser, and Related Components (6 of 40)
Heating and engine cooling 9
A coolant test shows the conditioner (inhibitor) level is low. What do the inhibitors in ethylene glycol antifreeze prevent?Answer: Oxidation of the glycol into organic acids. Source: Wikipedia, Antifreeze, Ethylene glycol. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
A driver heater core is found coated inside and flowing poorly. Which additive in automotive ethylene glycol coolants can coat or clog heat exchanger surfaces?Answer: Silicate based rust inhibitors. Source: Wikipedia, Antifreeze, Other industrial uses. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
An operator reports fogged windows. Why is defrost air in many cases passed through the A/C evaporator before the heater core warms it?Answer: To cool it down and dehumidify it. Source: Wikipedia, Defogger, Primary defogger. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
Coolant flow through a driver heater is traced. Where does the hot coolant go after it gives off heat in the heater core?Answer: Back to the engine cooling circuit. Source: Wikipedia, Heater core, How it works. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
The heater and defroster blowers of a bus are centrifugal fans. What is such a fan typically used for?Answer: Pushing or pulling air through ductwork or a heat exchanger. Source: Wikipedia, Centrifugal fan, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
How does an expansion (surge) tank protect a closed coolant system from excess pressure?Answer: Air in the tank compresses as the coolant expands. Source: Wikipedia, Expansion tank, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
Moisture condenses on the inside of the glass when the glass is colder than a certain temperature of the cabin air. What is that temperature called?Answer: The dew point. Source: Wikipedia, Dew point, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
Outlet air stays cold although hot coolant flows through the heater core. Which air-side part can cause that?Answer: A damper door that sends incoming air past the heater core. Source: Wikipedia, Heater core, Control. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
What does the wax element of an engine thermostat do?Answer: Turns heat into mechanical movement as the wax expands on melting. Source: Wikipedia, Wax thermostatic element, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), B. Heating and Engine Cooling Systems Diagnosis, Service and Repair (8 of 40)
Electrical and electronic systems 6
A blower motor has a thermal cutoff in its windings. How does a thermal fuse differ from a thermal switch?Answer: A thermal fuse is for one-time use, and a thermal switch can be reset. Source: Wikipedia, Thermal cutoff, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
A diode is connected across the coil of a compressor clutch. What is this flyback diode for?Answer: Eliminating the voltage spike when coil current is interrupted. Source: Wikipedia, Flyback diode, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
A pressure switch in a compressor control circuit is checked with a meter. On which change of pressure may a pressure switch be designed to make contact?Answer: Either on pressure rise or on pressure fall. Source: Wikipedia, Pressure switch, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
A pressure transducer on the discharge line is tested. What does a pressure sensor that acts as a transducer do?Answer: It generates a signal as a function of the pressure imposed. Source: Wikipedia, Pressure measurement, Instruments. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
An evaporator fan motor is a brushless DC type. What does the work of the brushes and commutator in such a motor?Answer: An electronic controller that switches current to the windings. Source: Wikipedia, Brushless DC electric motor, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
The condenser fan motors of a bus are AC motors on a variable-frequency drive. How does the drive control motor speed?Answer: By varying the frequency of the electricity supplied. Source: Wikipedia, Variable-frequency drive, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C1. Operating Systems and Related Controls: Electrical/Electronic Systems (6 of 40)
Electronic temperature control 4
A coolant valve actuator reports its position through a potentiometer. What is a potentiometer?Answer: A three-terminal resistor with a moving contact, forming an adjustable voltage divider. Source: Wikipedia, Potentiometer, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C2. Operating Systems and Related Controls: Electronic Temperature Control System (4 of 40)
An air door is moved by a stepper motor. How does a stepper motor respond to its input pulses?Answer: Each pulse turns the shaft through a fixed angle. Source: Wikipedia, Stepper motor, Mechanism. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C2. Operating Systems and Related Controls: Electronic Temperature Control System (4 of 40)
One HVAC control module transmits a frame on a CAN network. Which devices on the network receive that frame?Answer: All of them, the transmitting device included. Source: Wikipedia, CAN bus, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C2. Operating Systems and Related Controls: Electronic Temperature Control System (4 of 40)
Two HVAC control modules on a CAN network transmit at the same moment. What happens?Answer: The highest priority device continues and the others back off. Source: Wikipedia, CAN bus, lead section. Outline: ASE Transit Bus Study Guide (2026), Heating, Ventilation and Air Conditioning (Test H7), C2. Operating Systems and Related Controls: Electronic Temperature Control System (4 of 40)
How this list was compiled: Our own questions, written to the content areas of the Heating, Ventilation, and Air Conditioning (Test H7) test specifications and task list in the ASE Transit Bus Study Guide (copyright 2026, https://ase.com/wp-content/uploads/ASE_TransitBus_Study_Guide.pdf, read 2026-10-14): A A/C system and component diagnosis, service and repair 22 of 40 scored questions (A/C system, general 10; compressor and clutch 6; evaporator, condenser and related components 6), B heating and engine cooling systems 8, C operating systems and related controls 10 (electrical/electronic systems 6, electronic temperature control system 4). The bank holds 11, 6, 6, 9, 6 and 4. Every fact was read on 2026-10-14 in the public text the question names: 40 CFR 82.152 at ecfr.gov, and Wikipedia articles on vapor-compression refrigeration, zeotropic mixtures, R-407C, chlorodifluoromethane (R-22), subcooling, the thermal expansion valve, refrigerants, polyolester oil, compressors, the scroll compressor, the electromagnetic clutch, pressure switches, condensers, relief valves, defoggers, the dew point, expansion tanks, antifreeze, the heater core, the wax thermostatic element, centrifugal fans, brushless DC motors, variable-frequency drives, pressure measurement, flyback diodes, thermal cutoffs, the CAN bus, potentiometers and stepper motors. The sample questions printed in the study guide were not used. Licence: Questions, choices and explanations are original work of this site. No question or example is copied from a source.
How it works
This ASE H7 study guide sets out the content areas of the transit bus HVAC test and practises each with questions of our own. Unofficial: the National Institute for Automotive Service Excellence (ASE) has no part in it.
Read the full guide
The H7 content areas
ASE's Transit Bus study guide gives A/C system and component diagnosis 22 of the 40 scored questions: the system in general 10, compressor and clutch 6, evaporator, condenser and related components 6. Heating and engine cooling have 8. Operating systems and controls have 10: electrical and electronic systems 6, electronic temperature control 4.
What is particular to a bus
The H7 task list names the refrigerants R22, R134a and R407c, compressor loaders and unloaders, evaporator pressure regulators, booster pumps, AC and DC fan motor drivers, and the Controlled Area Network.
Where the H7 answers come from
Almost every answer rests on a Wikipedia article, named and linked under its question. One rests on EPA's refrigerant rule, 40 CFR 82.152.
ASE's guide notes that H7 does not meet EPA's Section 609 requirement for refrigerant work. That is a separate program.
Questions and answers
How many questions are on the ASE H7 test?
ASE's 2026 Transit Bus study guide lists 50 for H7, 40 scored and 10 unscored, with 75 minutes allowed. Recertification is 20 questions in 45 minutes.
Where is the official ASE H7 test study guide?
ASE publishes it free: the Transit Bus study guide covers the H series, with the H7 specifications, task list and ten sample questions. This page adds practice on the same content areas. None of ASE's sample questions is used.
Is H7 required for Master Transit Bus Technician?
Yes. ASE's study guide marks H3 through H8, H7 among them, as required, along with H1 or H2, to be recognized as an ASE Certified Master Transit Bus Technician.