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Error Code P0656 is defined as Fuel Level Output 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 Ford, GMC, Holden, Mercedes Benz, Saturn, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.
When Error Code P0656 is stored, this 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 fuel level output circuit.
The fuel level gauge allows the driver to monitor the vehicle fuel level when operating the vehicle. Usually, the PCM receives a signal from the fuel level sensor, which is located in the fuel storage tank. This signal allows the PCM to calculate the fuel mileage and monitor the fuel tank pressure, including fuel temperature. Data is then calculated for the engine delivery strategy, while the fuel level output signal provides to the fuel gauge and instrument panel controller. It can also be output for low fuel lamp and other onboard controllers.
If the PCM detects a problem while monitoring the fuel level output circuit, then Error Code P0656 will be stored, simultaneously activating the Check Engine light.
In many cases, this code may not show drivability issues. Instead, it simply activates the Check Engine light and a non-operating low fuel indicator. Some vehicles, however, will have inaccurate fuel gauges or even inoperable gauges.
Some vehicles may experience drivability issues, which is like caused by accompanied fuel temperature codes, or evaporative emission system codes.
The most common causes of this code are:
- Faulty system circuitry
- Broken, corroded, or loose wiring and electrical connectors
- Fault in the low-level indicator bulb or fuel level sensor
- Defective PCM
How to Check
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 helpful to 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 fails to reset and there are NO drivability symptoms shown, then the vehicle can be operated normally.
If the code resets on the other hand, then continue with the diagnosis. Check all connectors and wirings related to the system. Broken, loose, and unplugged harnesses must be repaired, if not replaced.
If the connectors and wirings appear to be functional, then use a vehicle information source to get the right wiring diagrams, connector pin-out charts, connector face views, and diagnostic flow charts.
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 there’s a fuel level output at the PCM connector, then test the corresponding circuit (as it is presented) at the fuel level gauge pin of the instrument panel connector. If there is still not fuel level output signal, then there’s a good chance you have an open circuit between the fuel gauge in the instrument panel and PCM. Repair or replace the damaged circuit and then retest.
How to Fix
Depending on your diagnosis, common repairs for this code include:
- Repair or replacement of damaged connectors, wirings, and harnesses
- Check the circuitry using Autohex or Tech II scanner that inspects the CAN bus, as this can look for disconnected or loose wiring
- Repair or replace the damaged circuit and then retest
Error Code P0656 doesn’t interfere with the vehicle’s operation. It can, however, result in inaccurate gas level reading, which will persistently cause the PCM to run inefficiently due to flawed data.
The most common misdiagnosis in this code is replacing the engine coolant temperature sensor without a thorough diagnosis, and overlooking the PCM driver. The latter is usually the most common cause of this code.