ECM/PCM Power Relay Sense Circuit Low
An abnormal voltage reading has been detected in the relay control circuit which supplies the PCM with power.
Code Set Parameters
Most vehicles utilize a PCM/ECM relay supply battery voltage (and/or ground) signals to the PCM/ECM. These relays require a battery voltage signal, ground signal, and an ignition switch input signal in order to operate properly. An output signal is also used to supply the PCM with data. If the PCM detects a level of voltage that is not consistent with the manufacturer’s recommended specifications, a code will be stored and a malfunction indicator lamp will be illuminated. Some applications require multiple ignition cycles (with a failure) in order for the malfunction indicator lamp to be illuminated and others will activate the malfunction indicator lamp on the first failure.
Symptoms may range from just the service engine soon lamp and a stored code to a no start/no crank condition. In some instances, the service engine soon lamp will be totally disabled when the PCM relay is defective.
In choosing a single most common cause, I would have to say a faulty PCM relay. Other common causes may include a blown fuse, a bad fusible link, shorted or open circuits, a defective battery, faulty battery cables/cable ends, or a defective PCM.
A rushed diagnosis will often lead to unnecessary PCM replacement. Technicians report that shortcomings in the voltage supply to the PCM relay are more common in late model vehicles than in earlier vehicles equipped with the OBD-II systems.
- Some vehicles use a PCM/ECM relay in order to supply the PCM with power and ground signals
- Others use a system that does not include a relay but instead uses a fused wire that comes directly from the ignition switch or battery junction block
- This type of code will set only in the former
The PCM and ECM are generally the same component
- ECM is short for “engine control module.” Since the engine control module and the transmission control module are frequently integrated into a single module, the computer that controls them is referred to as the “powertrain control module.” Ninety-five percent of late-model vehicles use a powertrain control module (PCM).
Voltage is usually supplied to the PCM using a contact relay
- A contact relay used in automotive applications will normally use between three and five circuits for operation
- The first circuit is constant battery voltage
- This circuit is usually fused and stays “hot” at all times
- The next circuit is a ground circuit and it is also constant
- The third circuit will likely be an ignition power circuit that supplies a second supply of battery voltage
- The contacts inside the relay are closed when the third circuit is activated, sending power to the PCM
- A fourth (and fifth) circuit provides the PCM (and other system controllers) with an input signal that reflects relay voltage
- The PCM and other controllers use the input voltage signal to monitor relay voltage and store a trouble code if the signal is abnormal
Begin your diagnosis with a visual inspection of the battery cables and battery cable ends
- Clean or replace cables and ends as required
- Make sure that the battery is fully charged and then perform a battery load and starting/charging system test
- Compare your findings with manufacturer’s recommendations and replace faulty components as needed
If the battery and starting/charging system are normal, then test system fuses and fusible links
- Replace faulty components as needed and retest the system
- Without a constant supply of battery voltage to the PCM, your vehicle could continue to store trouble codes, stall at idle or under acceleration, or exhibit a no start condition.
Several tools will be instrumental in attempting to successfully diagnose this code
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in trying to perform a successful diagnosis
- Gaining access to access a manufacturer’s wiring schematic will also prove to be necessary to successfully diagnosing this code.
Perform a careful visual inspection of all PCM wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can even be attempted
If the code resets, or if you are dealing with a no start condition, begin by unplugging the PCM relay connector
- Test for voltage and ground signals on the appropriate circuits and compare your findings with the wiring schematic or manufacturer’s specifications
- Probe any circuits that do not coincide with the manufacturer’s specification with the digital volt/ohmmeter (be sure to disconnect the PCM and other related control module connectors before testing individual circuits for continuity and resistance)
- Repair open, disconnected, or shorted circuits as required and retest the system.
If all of the circuits in the relay connector are in line with manufacturer’s specs and the relay fails to provide an output signal to the PCM, suspect a faulty relay
- If the service engine soon lamp fails to illuminate when the key is in the key on engine off position, suspect a faulty PCM relay.
If all system circuits check out, and the PCM relay is providing all necessary power and ground circuits, suspect a faulty PCM
- PCM replacement will require reprograming of one or more control modules