OBD-II trouble code
P0698: Sensor Reference Voltage "C" Circuit Low
Something is dragging the third five-volt rail down. This rail usually has the fewest sensors on it of the three, which makes the crude method — unplug them one at a time and watch the voltage — the fastest one available rather than the last resort it is elsewhere.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $130 – $2,600
- DIY difficulty
- Advanced DIY
What does P0698 mean?
A regulated five-volt supply sitting below its lower limit means something on the rail is taking more current than the regulator will give, or is providing a path to ground that should not exist. That much is true of any reference rail. What is worth thinking about here is the size of the group, because the third rail is normally the smallest of the three and that changes which diagnostic method is actually worth using.
The standard technique for a shared reference fault on the primary rail is to read the codes that arrived together and work out where the affected sensors overlap. It works there because a dozen sensors go down at once and the intersection is informative. On the third rail there may be only two or three sensors in the whole group, and two data points do not triangulate anything. The code-reading approach that is so efficient on the primary rail gives you almost nothing here.
The inverse is also true, and it is the useful half. The brute-force method — disconnect one sensor, watch whether the rail recovers, reconnect it, move to the next — is dismissed on the primary rail as impractical because there are too many sensors and half of them are buried. With three sensors it is three tests. If the rail springs back to five volts the moment a particular sensor is unplugged, that sensor is the fault, proven, with no interpretation required and no wiring diagram needed beyond knowing which connectors to visit. The technique that is a last resort on the other rails is the first thing to try on this one, and it is worth knowing that inversion before spending an hour on cleverer approaches.
Where the rail is sitting is worth noting rather than just that it is low, because the two cases behave differently. A rail near zero is being held to ground hard, by bare metal contact or by a component that has failed short, and it will read the same in any weather at any temperature. A rail sitting at two or three volts is being loaded through a resistance rather than shorted, and a resistive fault has conditions attached — it may appear only when hot, only when wet, or only after the vehicle has been running long enough for a marginal component to heat up. If the rail measures correctly in the workshop on a code that is definitely stored, that reading is telling you the fault is condition-dependent, not that it has gone away.
One characteristic of this rail's likely membership deserves mentioning. Where the third rail carries exhaust-side sensors, its components live in the harshest thermal environment on the vehicle, and components that fail from heat frequently show it. A sensor whose body is discoloured, whose plastic has distorted, or whose connector shows signs of having been cooked is worth suspecting on sight, and that kind of visible evidence is rarer on the rails whose sensors live in the engine bay or under the floor. It is not a substitute for the disconnection test, but it is a good way to choose which sensor to unplug first.
Common causes
- A sensor on the third rail failed internally and loading the supply, most often one exposed to exhaust heat
- Reference wire shorted to ground against the exhaust, a heat shield, a crossmember or a bracket
- Connector at an exhaust-side sensor damaged by heat, allowing the reference pin to bridge to ground
- Moisture or corrosion in an underbody connector creating a resistive path to ground
- Insulation degraded by prolonged exhaust heat until adjacent conductors touch
- Wiring pinched or trapped during exhaust, particulate filter or reductant system service
- Damage from road debris to harness sections routed beneath the vehicle
- Poor module ground making a healthy rail measure low against a reference that has itself shifted
- Failed internal regulator in the supplying module, the least likely realistic cause
Symptoms
- Check engine light with the vehicle driving normally in most cases
- Two or three exhaust or aftertreatment sensor codes appearing together with this one
- Exhaust pressure or filter differential readings pinned at zero or at an implausible fixed value
- Regeneration cycles not starting, or being aborted shortly after they begin
- Reduced power or a derate once an untended particulate filter has loaded enough to restrict flow
- Fault present at operating temperature and absent when the vehicle is cold, in resistive cases
- Fault appearing or worsening in wet weather where an underbody connector is involved
- Emissions test failure with no complaint the driver would otherwise have reported
Diagnostic steps
- 1.Measure the third rail with the ignition on and note the actual voltage rather than just that it is low. Near zero indicates a hard short to ground; two to three volts indicates something resistive, which has conditions attached and may not be present in the workshop.
- 2.Compare the third rail against the first at the same moment. One correct and one low confirms the fault immediately without needing a published specification.
- 3.Identify which sensors sit on this rail from manufacturer-specific data. The group is normally small, and knowing its exact membership is what makes the next step quick.
- 4.Disconnect the sensors on the rail one at a time, watching the voltage after each. Because the group is small this is a matter of minutes, and a rail that recovers the instant a particular sensor is unplugged identifies the failed component conclusively.
- 5.If the rail recovers with every sensor disconnected but stays low with them all connected, suspect combined load or a marginal regulator rather than a single failed sensor.
- 6.If the rail stays low with every sensor disconnected, the fault is in the harness or the module, and the sensors can be set aside entirely.
- 7.Inspect exhaust-side sensors visually before testing further. Discoloured bodies, distorted plastic and cooked connectors are common here and point at which one to unplug first.
- 8.Check the supplying module's grounds, since a degraded ground shifts the module's own reference and can make a correct rail measure low.
- 9.Where the fault is intermittent, monitor the rail on a scan tool or meter through a heat cycle and a road test rather than trying to catch it stationary.
Repair cost
$130 – $2,600
Diagnosis is $120 to $200, though the small size of this rail's sensor group often makes the actual testing quicker than on the other two. A failed exhaust-side pressure sensor is $150 to $450 fitted. A shorted or heat-damaged wiring repair is $200 to $750 depending on how much of it lies under the vehicle. Module replacement and programming reaches $2,000 in the rare case where the regulator itself has failed. As with the other faults on this rail, the figure that grows while the code goes unattended is the aftertreatment one: a forced regeneration is $150 to $400 and a particulate filter that has loaded past recovery is $1,200 to $2,600.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with wiring harness / circuit repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.