P0651 Code: Sensor Reference Voltage "B" Circuit/Open
The PCM's second 5-volt sensor supply is out of range – usually a pedal, throttle or pressure sensor.
At a glance
Severity: Moderate
- Meaning
- Sensor Reference Voltage "B" Circuit/Open
- System
- ECU & Outputs · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$1,700
- DIY difficulty
- Moderate
Engines with electronic throttle control deliberately split their sensors across two or more 5-volt references, so that one wiring fault can never corrupt both tracks of the accelerator pedal or both throttle position signals at once. P0651 means reference 'B' – the second of these supplies – is outside its expected voltage window. Which sensors sit on reference B depends on the engine: often one pedal track and one throttle track, and on diesels frequently boost, rail pressure or EGR position sensors. Because the redundant partner sensor on reference A still works, the engine usually keeps running but the PCM can no longer cross-check the throttle, so it limits power. As with reference A, a shorted sensor or wire is a far more common cause than a real open inside the PCM.
Can I keep driving? The engine will usually run, but expect limp mode with limited throttle and no highway-speed acceleration. Drive gently to a shop and avoid situations where you need sudden power, such as merging or passing.
How to read P0651
- P Powertrain
- 0 Generic (SAE)
- 6 Computer & outputs
- 51 Specific fault
Symptoms
- Reduced power or limp mode, with the throttle barely responding above a fast idle
- Pedal or throttle correlation codes such as P2138 or P2135 stored together with P0651
- Cruise control and sometimes traction control switched off
- On diesels, low boost or rail pressure codes appearing at the same time
- Fault appears after installing a throttle response controller or after carpet gets wet
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- A sensor on reference B shorted internally – pedal assembly, throttle body, boost or rail pressure sensor
- Reference B wire chafed to ground at the firewall pass-through or along the engine
- Aftermarket throttle response controller plugged in at the pedal connector
- Water in the driver's footwell corroding the pedal connector
- Loose or fretted terminal at the PCM connector
- Failed reference B regulator inside the PCM
Diagnosis step by step
Map the dead sensors in live data
Graph both pedal tracks and both throttle position signals, plus boost and rail pressure on diesels. Sensors fed by reference B will sit at 0 V or pinned at a fixed value while their partners on reference A still move with the pedal. That tells you which circuit you are chasing without pulling a single connector.
Tools: OBD-II scanner
Remove any add-on pedal module
Throttle response controllers and 'pedal boxes' plug in between the pedal and its harness and pass the reference through their own electronics. Unplug the module, reconnect the original connector directly and clear the codes before any other test.
Tools: Screwdriver
Check the pedal area for moisture
Lift the carpet under the accelerator pedal and look for dampness, rust on the pedal bracket or green residue in the connector. Leaking door seals, a blocked cowl drain or a heater core leak can soak the pedal connector.
Tools: Flashlight
Measure reference B at the pedal and throttle
Back-probe the reference B pin with the key on: expect about 4.8–5.2 V. If it is low, unplug the throttle body connector first (the easiest to reach), then the pedal, then any other sensor on the circuit, and watch for the voltage to recover.
Tools: Multimeter
Check terminal fit at the PCM
If the voltage is only lost intermittently, disconnect the PCM plug and test each reference and sensor ground terminal with a matching test pin – it should drag noticeably. Loose, fretted or corroded pins are repaired by replacing the terminal, not by bending it.
Tools: Flashlight, Multimeter
Relearn and verify
After the repair, clear codes and perform the throttle or pedal relearn if the manufacturer requires one. Confirm that all pedal and throttle tracks now move together smoothly across the full pedal travel.
Tools: OBD-II scanner
Where the parts are
- 1MAF sensor
- 2Turbocharger
- 3Throttle body
- 4Intake manifold
- 5Fuel injector
- 6EGR valve
- 7Fuel pump
- 8Thermostat
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Remove aftermarket pedal module and reconnect original plug | Yes | $0–$80 |
| Replace accelerator pedal assembly | Yes | $150–$450 |
| Replace throttle body or pressure sensor on reference B | No | $120–$600 |
| Repair chafed reference wire or PCM terminal | No | $150–$450 |
| Replace PCM with programming | No | $700–$1,700 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Field data
| Chevrolet | 3 | Impala, Cobalt |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0651 lights the check engine light and usually causes limp mode, so the vehicle fails an OBD emissions test. After repair and any required relearn, drive normally until the monitors complete.
Frequently asked questions
Why is the engine in limp mode if the other sensors still work?
The PCM uses two independent signals for pedal and throttle so it can cross-check them. With reference B down, one of each pair is lost and the PCM can't confirm what the driver is asking for, so it caps power as a safety measure.
Could my throttle response controller have caused P0651?
Yes. These plug-in devices sit between the pedal and the harness and handle the pedal's reference and signals. A failing unit or a poor connection can pull reference B down. Unplug it and see whether the code returns before replacing anything.
What's the difference between P0641 and P0651?
They describe the same kind of fault on two different 5-volt supplies. Which sensors are affected, and therefore the symptoms, depends on how the manufacturer split the sensors between reference A and reference B.
Do I need to replace the accelerator pedal?
Only if unplugging the pedal brings reference B back to 5 V or its connector is corroded beyond repair. Replacing the pedal on the strength of the code alone often fails because the short is elsewhere on the circuit.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- NHTSA Complaints API
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy