OBD-II trouble code
P0593: Cruise Control Multi-Function Input "B" Circuit High
The "B" input has floated above every recognised command band. On a pull-up circuit that is the signature of an open — a disconnected plug, a broken conductor, or a corroded pin — and it is the one code in this family that regularly turns out to be a connector nobody plugged back in.
Quick facts
- System
- Powertrain
- Category
- Cruise Control
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $420
- DIY difficulty
- Intermediate DIY
What does P0593 mean?
P0593 is the open-circuit end of the multiplexed switch input, and it is the most mechanically boring member of the family — which is exactly why it is worth reading before spending money.
The module holds the input up through an internal pull-up resistor and waits for the switch ladder to pull it down. Every legal command is a defined amount of pull-down. Remove the pull-down entirely and the wire has nothing left to oppose the pull-up, so it climbs to reference voltage and parks there. That reading sits above the topmost command band with a margin, which is why calibrations treat it as unambiguous. Where a range/performance fault represents a value the module cannot classify, this one represents a value the module can classify perfectly well: it means nothing is connected.
That has an immediate consequence for how the fault feels from the driver's seat. There is no partial behaviour, no button that works some of the time, no weather dependence. The circuit is either connected or it is not, so cruise control simply does not respond at all, and the code returns the instant it is cleared. A complaint of intermittent cruise with P0593 stored usually means the open is intermittent — a pin backing out of a connector body, or a conductor broken but still touching — rather than that the code is behaving oddly.
The population caveat is the most valuable thing on this page. P0593 shows up disproportionately after somebody has been behind the steering wheel. Airbag recall work, clockspring replacement, steering wheel swaps, column switch service, aftermarket audio or remote-start installs that borrow a wheel-control connector — all of them involve unplugging the switch loom, and all of them present an opportunity to leave a plug half-seated or a locking tab unlatched. A connector that clicks but is not fully home will pass a visual inspection and still hold the circuit open. If this code arrived within a few days of any work in that area, checking the plugs is not a formality; it is the most likely fix on the list.
Where no recent work explains it, the failure is corrosion or fracture. Terminals behind the wheel sit in a cabin that breathes moist air every day, and oxide on a low-current signal pin builds resistance long before it looks bad — there is no current here to burn through a film the way a power circuit would. A conductor fractured inside intact insulation is the other classic, and it is nearly invisible; the wire looks perfect and reads open only when flexed. A resistor in the switch ladder that has failed open produces the same reading from inside the switch, which is the one cause that a connector inspection will never find.
A useful confirmation, and one that does not apply to the low code: with the key on, briefly jumper the switch signal wire to a known good ground at the switch connector. If the module's live reading drops, the wiring from that connector back to the module is intact and the open is on the wheel side — the switch itself or the path through the steering wheel. If the reading stays high, the break is between the connector and the module.
Common causes
- Switch connector left partly seated after airbag, clockspring or steering wheel service
- Terminal backed out of the connector body so the pin no longer contacts
- Conductor fractured inside intact insulation, reading open only when flexed
- Corroded or oxidised signal pin, which a low-current circuit cannot burn through
- Resistor in the switch ladder failed open
- Broken trace in the clockspring ribbon carrying the switch signal
- Damaged locking tab allowing a connector to vibrate loose over time
- Aftermarket accessory installation that tapped and then abandoned a wheel-control circuit
- Open in the switch circuit's ground path, which lifts the reading the same way a signal open does
Symptoms
- Check engine light on with P0593 stored
- Cruise control completely unresponsive, with no button doing anything
- Live data showing the "B" circuit sitting at reference voltage regardless of which button is pressed
- Code returning immediately every time it is cleared
- Fault appearing within days of airbag recall, clockspring or steering wheel work
- Fault appearing after an aftermarket alarm, remote start or stereo installation
- Other controls sharing the same connector also dead
- Cruise working only when the wheel is held at a particular angle, if the open is intermittent
Diagnostic steps
- 1.Establish first whether anything has been done behind the steering wheel recently. A half-seated connector left after airbag or column work is the single highest-yield cause of this code.
- 2.Unlatch and firmly reseat every connector in the switch circuit rather than only looking at them. A plug that appears home can still be one click short of contact.
- 3.Read the live "B" circuit voltage while an assistant presses each button. A value pinned at reference voltage through every press confirms the circuit is open rather than mis-scaled.
- 4.With the key on, briefly jumper the signal wire to a known good ground at the switch connector. A live reading that drops proves the harness back to the module is intact and puts the open on the wheel side.
- 5.Measure resistance across each button directly at the switch connector. An infinite reading on one button with the others correct is a failed resistor inside the switch.
- 6.Inspect terminals for green oxide, spread contacts, or a pin sitting lower than its neighbours in the connector body.
- 7.Flex the loom section by section while watching live data. A conductor broken inside good insulation looks perfect and only reads open under movement.
- 8.Check the switch circuit ground as well as the signal. An open ground raises the reported voltage in exactly the same way and is easy to overlook.
- 9.Disable the supplemental restraint system according to the manufacturer's procedure and wait the specified period before disturbing anything behind the wheel.
Repair cost
$0 – $420
A surprising share of these cost nothing but time, because reseating a connector left half-home after other work is the fix. Repairing a corroded terminal or a broken conductor is typically $80-$280 depending on access and how much trim has to come out. A steering wheel control switch with an open resistor is $30-$200 in parts plus 0.5-1.5 hours. If the break is in the clockspring ribbon, the part is $60-$350 with 1-2 hours of labour. Because the connector check is free and the parts are not, insist on a live-data confirmation before anything is ordered.
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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 intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.