Why does this feel like an engine problem rather than a transmission problem?

Because the gear that goes missing is at the top of the range. The third shift solenoid generally enables the upper ratios, so the car still pulls away and shifts normally at low speed and only stops changing up further along. From the driver's seat that reads as an engine turning too fast, sounding strained on the motorway, and drinking fuel. Plenty of these cars are booked in as suspected engine or fuel faults with the transmission never mentioned, which is worth knowing before paying for a diagnosis in the wrong department.

Why did it seem to fix itself overnight?

It did not. When the module detects a short to ground on an output it shuts that output down to protect its own driver, and on most systems it will not try to use it again until the ignition has been cycled. So the next morning the module gets one fresh attempt, the car feels normal for a while, and then the fault returns the moment it tries to use that solenoid again. That pattern is characteristic of a shorted circuit rather than evidence that the problem has gone away.

Is it safe to tow with this code stored?

It is the one thing worth genuinely avoiding. A transmission working without one of its ratios runs at higher engine speed and spends longer with the converter unlocked, both of which generate heat, and adding a trailer plus a long hill to that combination is a realistic way to overheat the fluid. Overheated fluid causes damage the electrical fault would never have caused on its own. Unhitch and deal with the code first; a wiring repair is inexpensive and cooked fluid with clutch material in it is not.

Can I keep driving with P0979?

For short local journeys, yes — the car moves under its own power and will get you to a workshop. What to avoid is exactly the driving this fault punishes: long motorway runs, sustained hills and anything towed. Those are the conditions where the missing ratio turns into heat, and heat is what converts a modest wiring repair into a much larger transmission bill. Keep the trips short, keep the revs down, and get it looked at soon rather than living with it.

OBD-II trouble code

P0979: Shift Solenoid "C" Control Circuit Low

Most people who arrive at this code did not arrive with a transmission complaint. They came in about fuel consumption, or engine noise on the motorway, or a tow that went badly — because losing an upper gear feels like an engine problem, not a gearbox one.

High severityPowertrainTransmission / Shift ControlDrivable short-term

Quick facts

System: Powertrain
Category: Transmission / Shift Control
Severity: High severity
Drivable: Usually safe to drive short-term
Repair cost range: $130 – $1,200
DIY difficulty: Advanced DIY

One of 43 shift solenoid codes

P0979 sits in a grid: one axis is which solenoid the letter refers to, the other is what went wrong with it. Each cell is a genuinely different fault with its own diagnosis — but codes from the same row or column often appear together, and what a pairing means is usually the fastest way in. The hub page lays out all 43 and explains how to read them against each other.

See all shift solenoid codes →

Browse every code in P0900–P0999, or start from the full code library.

What does P0979 mean?

P0979 means the module found the shift solenoid 'C' output pulled to ground when it had not commanded that — a chafed control wire, a shorted coil, or fluid bridging pins inside a connector.

What makes this code worth reading carefully is not the electrical theory, which is the same as any shorted output. It is that the complaint almost never sounds like a transmission fault. Because the third shift solenoid is generally what enables the upper ratios, losing it removes gears from the top of the range rather than the bottom. The car pulls away from a standstill perfectly. It changes up through the low gears perfectly. It simply stops changing up at some point, and from there on the engine turns faster than it should for the road speed.

Drivers describe that as the engine being loud, or revving high, or the car feeling like it is straining on the motorway. The number that actually gets people through the door is fuel consumption: an engine held two or three thousand revolutions per minute above where it should sit at a steady seventy is burning fuel at a rate nobody misses on the second tank. It is common for a car with this code to be presented as a suspected engine or fuel system problem, with the transmission not mentioned at all.

That mismatch has a cost, and it is not just diagnostic time. An engine held at high revolutions on a long run makes more heat, and so does a transmission working without one of its ratios and spending longer with the torque converter unlocked. On a vehicle towing anything, that combination is genuinely capable of overheating the transmission fluid on a single long hill, which does damage the code itself never caused. If a caravan or trailer is involved, the honest advice is to unhitch it and deal with the fault before the next trip rather than after it.

One useful practical detail on short-to-ground faults specifically: when the module detects one it usually disables that output to protect itself, and on many systems it will not attempt to drive it again until the ignition has been cycled. So a car that behaves badly on Monday evening and appears fine on Tuesday morning has not repaired itself overnight. It has simply been given a fresh attempt, and the fault will return as soon as the module tries to use that solenoid again. That pattern is characteristic and should not be read as the problem going away.

As with any shorted circuit, repair the wiring before fitting a part. A new solenoid dropped into a circuit that is still shorted will be latched off within seconds, and the money spent on it buys nothing.

Common causes

Control wire insulation worn through against the transmission case, a bracket or an exhaust component
Shift solenoid 'C' with shorted windings
Fluid or moisture bridging pins to the earthed shell of the case connector
Internal transmission harness chafed against a rotating or moving part
Corroded or displaced terminal contacting the connector body
Harness pinched or trapped during earlier work under the vehicle
Module output driver failed in the on state, holding the circuit low
Insulation that only shorts once the transmission is at full operating temperature
Replacement solenoid of the wrong specification drawing more current than the output allows

Symptoms

Engine revolutions much higher than normal at steady motorway speed
Sharp, obvious drop in fuel economy that the driver notices before anything else
Car pulling away and shifting normally at low speed, then running out of gears
Complaint presented as an engine or fuel problem rather than a transmission one
Transmission or engine temperature climbing on long hills, especially when towing
Normal behaviour after a restart followed by the same fault later in the drive
Gear ratio code from the P0730 family stored alongside this one
Check engine light with no harshness, slipping or noise from the transmission

Diagnostic steps

1.Take the fuel economy and high-revolutions complaint seriously as transmission evidence rather than treating it as an engine symptom, since a lost upper ratio presents exactly this way.
2.Compare engine speed against road speed in each gear on a scan tool to identify which ratio the transmission is failing to reach.
3.With the ignition off and the case connector unplugged, measure resistance from the 'C' control pin to a clean chassis ground; near zero on a pin that should be isolated places the short outside the module.
4.Measure coil resistance across the solenoid pins to separate a shorted winding from a shorted wire.
5.Treat a fault that vanishes after a restart as expected behaviour, not as a repair, because the module latches a shorted output off until the ignition is cycled.
6.Inspect harness runs where they cross the case, brackets and exhaust components for the polished witness mark that precedes a short.
7.Repeat the wiggle test on a fully warm transmission after a sustained run, since heat-softened insulation is common in this location.
8.Check transmission fluid temperature history if the vehicle has been towing, because a lost ratio plus a long hill can overheat fluid independently of the electrical fault.
9.Repair the wiring completely before fitting a solenoid, or the module will latch the new part off within seconds and it cannot be validated.

Repair cost

$130 – $1,200

Diagnosis is $110 to $220, and it is often longer than it should be on this code because the car arrives as a suspected engine fault. A wiring repair is $130 to $400 and is the most common outcome when the problem is caught early. Shift solenoid replacement is $250 to $700 fitted. What can push this higher is heat damage: if the vehicle has been towed or driven long distances with a ratio missing, a fluid and filter service at $150 to $350 is not optional, and overheated fluid that has cooked can take clutch material with it. That part of the bill is avoidable and depends entirely on how many long trips happen before the wiring is fixed.

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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.