Error Code P0542 is defined as Intake Air Heater A Circuit High, meaning failure in the #1 Air Intake Heater Relay Circuit has been detected, a fault likely caused by poor wiring or faulty intake air relay.


Error Code P0542 is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present, and with air intake heater, such as Ford (Powerstroke), Chevrolet GMC (Duramax), Dodge, Honda, Nissan, etc. Though generic, specification on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

Error Code P0542 is one of a number of possible codes associated with a malfunction in the intake air heater circuit, which is an essential component of diesel engines that assist the starting process.

The job of the Intake Air Heater “A” is to operate the component that provides warm air that facilitates diesel engine starting and idling at various temperatures. This usually includes a heater element, temperature sensor, relays, and blower(s). There are also routed air ducts to route the warm air to the intake and electrical connections and wiring control for the operation of these components.

When the code P0542 is set, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicles makes) has received an abnormally high signal coming the from air intake heater relay, suggesting a circuit low condition. This means the voltage is higher than its normal range.

Other related codes include:

Common Symptoms

As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other common and noticeable symptoms include:

  • Hard starting condition or engine may not start at all
  • Engine stalls
  • Rough idle at a cold temperature
  • Cranking time takes longer than normal

There are some cases where the vehicle may not show discernible symptoms for this code.

Possible Causes

Defective intake air heater relay is the most common cause for this code. Other possible causes include:

  • Loose wiring connection
  • Failed intake air heater relay
  • Open circuit between the air heater relay and PCM
  • Burn out heater elements
  • Corroded, open, or shorted connectors or wirings
  • Defective PCM (rare)

How to Check

As with most error codes, it is important to check the TSB (Technical Service Bulletin) of your specific vehicle including its year, model, and powertrain, to save time and money in the right diagnosis and repairs.

Intake Air Heating Circuit may not automatically work if the ambient air temperature or engine temperature is more than the limit set by the manufacturer. The circuit should activate automatically if commanded “ON” by the scanner, or if power is applied manually.

Basic Diagnosis

  • Check whether the heater element is turning “ON”

Note: Make sure you don’t touch the element or heat shield

  • Check if the blower motor is turning “ON”
  • Check the circuit connections and wiring for obvious defects
  • Check electrical connections for signs of damages, such as corrosion and security issues
  • Check the condition of the air ducts for signs of damage and defects

Mechanics will use OBD-II scanner to verify the code from the PCM. In many cases, the codes can simply be cleared. Then the engine must be restarted to meet operating parameters for the intake air heater system to determine if the code comes back.

If the code comes back, then mechanics will proceed with checking the signal voltage at the intake air heater at cold start key cycle. If the signal circuit doesn’t power up, that means there’s a faulty in the connection. They will then check the wiring to the relay and looks for signs of breaks and shorting.

Next, the relay will be tested to verify if it switches voltage properly. If not, then it must be replaced.

If the PCM relay tests correctly, the mechanic will proceed to verify the signal circuit for high resistance by tracing the connection from the relay to the PCM. In some rare cases, an internal short in the PCM prevents the intake heater relay from powering up. In this case, it must be replaced.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Replacement of faulty intake air heater
  • Repair of shorted, open, or corroded wiring or connectors
  • Replacement of burnt out heater elements
  • Replacement of bad intake air heater blower
  • Replacement of faulty intake air temperature sensor
  • Reconnection of disconnected intake air temperature sensor
  • PCM replacement (in rare cases)

The severity of this condition varies from moderate to serious, which is primarily based on a specific malfunction.