Error Code P0655 is defined as Engine Hot Lamp Output Control Circuit Malfunction. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, particularly those made since 1996 up to present. This includes models from, but not limited to Alfa Romeo, Buick, Cadillac, Chevrolet, Dodge, Ford, GMC, Opel, Peugeot, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.
When Error Code P0655 is stored, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) or other related controllers, has determined a discrepancy in the engine hot lamp output control circuit.
Designed to provide drivers with a visual warning lamp when the engine overheats, the engine hot lamp is an extremely important instrument. The PCM gets the signal from the engine coolant temperature sensor, allowing the PCM to monitor the engine coolant temperature whenever the engine is running, or the ignition is in ON position. The PCM uses this data to calculate the engine fuel delivery and ignition timing strategy, and then output the engine hot lamp and instrument panel controller. It can also be output to other controllers. For instance, the engine hot lamp control circuit may be output to the climate control module, so that the air conditioning compressor can be disabled if the engine overheats.
If the PCM determines a problem while monitoring the engine hot lamp output control circuit, then it will activate the Check Engine light and store the Error Code P0655.
This code may trigger both the Check Engine light and engine temperature indicator light. Other common symptoms include:
- Engine drivability issues
- Inoperative or constantly illuminated engine hot lamp
Defective circuitry in the system is the most common cause for this code, which is usually characterized by broken or loose wiring, corroded connectors between PCM and the sensors. Other common causes of this code include:
- Malfunctioning engine over temperature bulb
- Defective PCM
- Faulty engine temperature sensor
How to Check
Other codes such as engine overtemp codes or engine temperature sensor codes must be diagnosed and addressed first before diagnosing this problem.
Before making the diagnosis, make sure the engine is filled with the right amount of coolant and not overheating.
When trying to diagnose this code, refer to your information source or TSB (technical service bulletin); looks for signs and symptoms parallel to the stored code. Search for the year, make, model, and engine of your vehicle. If you are able to find the right TSB, then you may get the best diagnostic information for your problem.
You will need a diagnostic scanner, and digital volt/ohmmeter to diagnose this code accurately.
Next, connect your diagnostic scanner to the vehicle’s diagnostic connector, get all stored trouble codes too. You can write these codes down, including their freeze frame data, as this information can be of good help for you, especially if the code proves to be intermittent.
After recording this information, clear the codes and then take the vehicle for a test drive (if possible), until the code resets or the PCM enters readiness mode.
If the PCM enters readiness mode, that means the problem is intermittent. You may have to wait for the problem to get worse before you can successfully diagnose and repair. If the code resets on the other hand, then continue with the diagnosis.
Check all the related connectors and wirings, including the harnesses. Look for signs of damages. Harnesses that have been unplugged due to damage must be repaired or replaced.
If all the connectors and wirings look good, then use the vehicle information to know the connector pin-out charts, face views, component locators, wiring diagrams, and diagnostic flow charts of the vehicle.
As soon as you have the right information, use the DVOM and an oscilloscope to test the engine RPM at the right pin of the PCM connector. If there is no output signal, then you can suspect a defective PCM or a programming error in the PCM.
If the engine hot lamp output signal is seen at the PCM connector, then test the corresponding circuit (as presented) at the engine hot lamp pin of the instrument panel connector. If the engine hot lamp signal is discovered, then you have an open circuit problem between the hot lamp in the instrument panel and the PCM. Repair or replace the circuit as necessary, and then retest.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Repair or replacement of any damaged connectors, circuits, or wires, and reinstalling them
- Replacement and reprogramming of PCM
If Error Code P0655 appears, and the engine hot lamp is still operational, then you can suspect a programming error or defective PCM.
This code may still allow the vehicle to start, but faulty sensors leads to overheating, which can pose some serious problems.
It’s extremely important to take a closer look at wires and connectors and look for signs of loose connections, as this is a common cause of this code.
The most common misdiagnosis of this code is replacing the engine coolant temperature sensor when the PCM driver is at fault.