My steering feels too light rather than too heavy. Is that still this code?

It very likely is, and it is worth saying clearly because it gets dismissed. On a variable-assist system the module firms the steering up as speed rises by actively commanding it to. When the control circuit floats high the module loses that ability and the system defaults to full assist, so the steering stays parking-lot light at motorway speed. The wheel loses its centring feel and the car wanders. That is a genuine handling hazard, not a comfort quirk, and it is exactly what this code predicts.

How can a broken wire and a short to power give the same code?

Because of how the circuit is built. The module controls the actuator by connecting it to ground, not by feeding it power — so "off" means the module lets go and the line rises to supply voltage. A wire that has broken leaves the line high because the module can no longer reach it. A wire touching a live circuit leaves the line high because the module cannot pull it down against the intruding supply. The module sees the same voltage in both cases and reports the same code. Telling them apart is a bench job, not something the code can do for you.

What is the quickest test to narrow this down?

Unplug the steering pump or solenoid and, with the ignition on, connect its control terminal on the harness side to a good ground while watching the circuit state on a scan tool. If the module immediately reports the circuit low, everything from the module to that connector is fine and the fault is in the component. If it still reports high, the fault is in the wiring or in the module's driver. One measurement, half the possibilities gone, and no need to open the loom to get there.

Why is this fault always present, when my previous steering problem came and went?

Because of what causes each. Low-voltage faults are usually resistance, and resistance changes with temperature, vibration and how much current is flowing, which is why they fade in and out. A high reading comes from an open circuit or a short to power, and those are generally binary — the wire is broken or it is not, it is touching the live circuit or it is not. That stability is inconvenient for the driver but helpful for the diagnosis, because the fault will still be there while you are measuring it.

OBD-II trouble code

P0637: Power Steering Control Circuit High

Voltage on the steering control circuit is above its allowed ceiling. The trap in this one is that a broken wire and a short to power look identical to the module, and the symptom is often not heavy steering but steering that is far too light at speed.

Medium severityPowertrainPCM / ElectronicsDrivable short-term

Quick facts

System: Powertrain
Category: PCM / Electronics
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $110 – $2,000
DIY difficulty: Advanced DIY

What does P0637 mean?

Most control outputs in a vehicle are wired the same way, and knowing that is what makes this code diagnosable. The module does not supply the actuator with power; it supplies the path to ground. The actuator sits with battery or reference voltage permanently on one side, and the module completes the circuit by pulling the other side down. When the module wants nothing to happen it simply lets go, and the line floats back up to supply voltage.

That arrangement has a consequence which is the single most important thing to understand about P0637. If the wire between the module and the actuator breaks, the module lets go and the line goes high — exactly as it should. If the wire touches a live circuit, the module tries to pull the line down and cannot, and the line stays high — also high. A broken wire and a short to power produce the same complaint from the module's point of view, and the module has no way of telling you which it is. So the cause list splits cleanly into an open circuit and a short to voltage, and separating them is the whole job.

Fortunately that separation takes one deliberate test rather than a search. Disconnect the actuator and ground its control terminal by hand while watching the circuit state on a scan tool. If the module now sees the line low, the wiring between the module and the connector is intact and the fault lies in the actuator or in whatever feeds it — you were looking at an open inside the component. If the module still reads high with the line grounded at the far end, something between is either broken or is holding the line up, and now the search is genuinely a wiring search. That single measurement removes half the possibilities before the harness is opened.

The symptom pattern is where this code diverges most sharply from its low-voltage sibling, and where it can be misread entirely. On a variable-assist system, letting the line float high often means the assist valve or solenoid defaults to maximum assist rather than to none. The steering is not heavy at all. It is light — pleasantly light in a car park and unnervingly light on a motorway, where the wheel loses its centring feel, the vehicle wanders and small inputs produce more response than the driver intended. Drivers rarely complain about this as a fault because light steering does not feel like a failure, and a workshop primed to expect heavy steering can dismiss the report. Over-assisted steering at speed is a real handling hazard, and on this code it is the expected outcome rather than a surprise.

The other characteristic of a high fault is that it tends to be stable. Resistance moves with heat and vibration, so low-side faults fade in and out; opens and shorts to power generally either exist or do not. A steering complaint that behaves identically on every drive, in every temperature, from the moment the key is turned, fits this code far better than a complaint that comes and goes — and that consistency is useful, because it means the fault will still be there while you are testing it, which is not always true elsewhere in this family.

Common causes

Open circuit in the control wire between the module and the steering pump, solenoid or assist unit
Control wire shorted to a battery-voltage or ignition-switched circuit through chafed insulation
Failed low-side driver inside the engine controller or steering module, unable to pull the circuit down
Disconnected, damaged or partially unlatched connector at the steering pump or solenoid
Open winding inside a variable-assist solenoid or pump control coil
Corrosion having eaten through a terminal, producing an open at the connector rather than in the wire
Moisture bridging adjacent pins in a connector where one pin carries supply voltage
Incorrect wiring from a previous repair, accessory installation or aftermarket steering conversion

Symptoms

Steering that feels too light and vague at motorway speed, with poor self-centring
Assist that stays at maximum regardless of road speed, on variable-assist systems
Complete absence of assist from the moment the engine starts, on systems that default to off
A fault that behaves identically every drive rather than coming and going
Electro-hydraulic pump not running at all, or running continuously at one speed
Power steering warning lamp on constantly rather than intermittently
Vehicle wandering or feeling nervous at speed while behaving normally at low speed
Check engine light on with a steering complaint the driver may not have thought worth mentioning

Diagnostic steps

1.Confirm which state the system defaults to on this vehicle — maximum assist or no assist — before interpreting the driver's description. A complaint of steering that is too light is a completely valid symptom of this code and is easy to dismiss if you are expecting the opposite.
2.Remember that this code cannot distinguish an open circuit from a short to voltage. Both leave the line high. Plan the testing around separating those two rather than around finding a single fault type.
3.Disconnect the steering pump, solenoid or assist unit and inspect both halves of the connector for corroded, pushed-back or burnt terminals before any meter work.
4.With the actuator disconnected, ground its control terminal at the harness side while watching the circuit state in live data. Reading low proves the wiring back to the module is sound and moves the fault into the component; still reading high proves the fault is in the wiring or in the module driver.
5.Measure resistance across the solenoid or pump control coil and compare against specification. An open coil is a common and simple cause of a high reading, and it is quicker to check than any harness test.
6.If the wiring is implicated, check for a short to voltage before assuming an open. Disconnect both ends of the control wire and measure it to ground with the ignition on — any voltage present on a wire disconnected at both ends means it is being fed from somewhere it should not be.
7.Note whether the high reading is present with the ignition on and the engine off, only with the engine running, or all the time including with everything off. That narrows a short to an ignition-switched, run-fed or permanently live branch and turns a whole-harness search into a single-branch one.
8.Inspect any recent work, accessory wiring or previous splices in the area. Mis-wired repairs are disproportionately represented in shorts to voltage compared with any other fault type.
9.Treat module driver failure as the conclusion rather than the starting assumption. It is real, but it is rarer than an open coil or a damaged wire and it is the most expensive thing on the list.

Repair cost

$110 – $2,000

Diagnosis is $100 to $180, and it usually resolves quickly on this code because the fault is stable rather than intermittent. An open or shorted section of harness repaired properly is $150 to $600 depending on how buried the run is. A variable-assist solenoid or pump control coil with an open winding is $200 to $500 fitted. An electro-hydraulic pump assembly is $600 to $1,500. A failed driver inside the engine controller or steering module puts the job at $900 to $2,000 including programming, and on vehicles with electric assist the steering angle sensor generally has to be recalibrated afterwards, which needs a capable scan tool rather than a road test.

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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.