Possible causes

– Intake air heater relay

Tech notes

The most common problem that sets code p0540 is the air intake relay

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0540 GMC Description

The Engine Control Module (ECM) uses an Intake Air Heater (IAH) to warm the incoming air for proper cylinder combustion. The ECM grounds the control coil of the IAH relay to energize the heater during cold operation.