Symptom
Only happens when towing or heavily loaded
The car is faultless empty and misbehaves with a caravan behind it or four people and the boot full. Load is the honest test of a great deal that a light throttle never asks for: the engine works near its limits for minutes rather than seconds, the gearbox holds lower gears and gets hotter, the cooling system meets the demand it was specified for, and the rear suspension sits where it rarely sits. A part that is marginal passes every ordinary drive and fails here. Say the weight and the gradient if you can — up a long hill with a trailer is a much more specific test than when loaded.
Also described as: only when towing, with the caravan on, when fully loaded, pulling a trailer, when the car is full, towing up a hill, فقط با بار سنگین, موقع یدک کشیدن, solo con remolque, pasa cargado, al remolcar falla, solo con peso, cuando llevo remolque, solo remolcando, فقط عند السحب, لما تكون محملة, عند جر المقطورة
Some causes mean you should stop driving
19 of the 102 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
Load is the honest test of a great deal that a light throttle never asks for. The engine works near its limits for minutes rather than seconds, the gearbox holds lower gears and gets hotter, the cooling system meets the demand it was actually specified for, and the rear suspension sits where it rarely sits. A part that is marginal passes every ordinary drive and fails here — which makes this complaint a test result rather than a mystery.
Heat is the mechanism behind most of it. Sustained load puts heat into the coolant, the transmission fluid, the turbocharger and the charge air, and anything that removes heat marginally well enough for ordinary driving is exposed. A radiator packed with insects, a viscous fan coupling that has weakened, a transmission cooler that was adequate empty — none of these produces a symptom until the day something is behind the car.
Boost and fuelling are the next population. An engine that meets its boost target on a short overtake may not hold it up a long hill, and a fuel delivery ceiling that never shows in town appears when demand is sustained. Both are measured under the load that causes them, not at idle.
Then there is the half that is about the trailer rather than the car. A vehicle detects a connected trailer and changes its behaviour — extending stability control, altering the lighting strategy, and on many vehicles disabling the parking sensors. A fault that appears only with the trailer plugged in may be in the trailer or its connector, and unplugging it while still loaded separates the two for nothing.
Say the weight and the gradient if you can. Up a long hill with a trailer is a much more specific test than when loaded.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Temperature rises, or a warning appears, only under sustained load | Marginal heat rejection exposed — a blocked radiator core, a weak fan or viscous coupling, or an undersized cooler | Reproduce on a long climb with a logger running. Inspect the front of the radiator stack, which collects insects and leaves invisibly. | Clean the stack, renew the fan or coupling, or fit the auxiliary cooling the vehicle's towing specification calls for. |
| Power fades or limp mode appears part-way up a long hill | Boost or fuelling that cannot be sustained — a charge leak, a sticking vane mechanism, or a delivery ceiling | Log requested against actual boost, and fuel or rail pressure, during the climb. Static testing finds nothing here. | Repair what the log names. This is the same population as ordinary power loss, exposed by duration rather than by throttle. |
| Transmission harshness, slip or an overheating warning while towing | Fluid temperature beyond what the cooler removes — heat that ordinary driving never generates | Read transmission fluid temperature during the tow. Check the cooler and its lines are clear. | Service the fluid, clear or upgrade the cooler. Heat-cycled fluid does not recover when it cools. |
| Fault only with the trailer plugged in | The trailer or its connector rather than the vehicle — the car measures each lamp circuit and reports what it finds | Unplug the trailer while still loaded. A fault that clears has told you which end to work on, for nothing. | Repair the trailer's wiring or clean the socket pins. See the towing warning entry in this vocabulary. |
| Rear suspension or ride height faults only when loaded | The suspension sitting where it rarely sits — a level sensor at the end of its travel, or air suspension unable to hold height | Compare ride height sensor values loaded and unloaded. A sensor reading implausibly at one end of its range names itself. | Renew the sensor or repair the air suspension leak. Check the vehicle's plated towing and payload limits while there. |
| Fault appears loaded and does not clear when unloaded | The load exposed a marginal part and finished it — the pattern is history now | Diagnose conventionally; a permanent fault is far easier than an intermittent. | Repair normally. The towing pattern explains the cause rather than describing the current state. |
-
Temperature rises, or a warning appears, only under sustained load
- Likely cause
- Marginal heat rejection exposed — a blocked radiator core, a weak fan or viscous coupling, or an undersized cooler
- How to confirm it
- Reproduce on a long climb with a logger running. Inspect the front of the radiator stack, which collects insects and leaves invisibly.
- What fixes it
- Clean the stack, renew the fan or coupling, or fit the auxiliary cooling the vehicle's towing specification calls for.
-
Power fades or limp mode appears part-way up a long hill
- Likely cause
- Boost or fuelling that cannot be sustained — a charge leak, a sticking vane mechanism, or a delivery ceiling
- How to confirm it
- Log requested against actual boost, and fuel or rail pressure, during the climb. Static testing finds nothing here.
- What fixes it
- Repair what the log names. This is the same population as ordinary power loss, exposed by duration rather than by throttle.
-
Transmission harshness, slip or an overheating warning while towing
- Likely cause
- Fluid temperature beyond what the cooler removes — heat that ordinary driving never generates
- How to confirm it
- Read transmission fluid temperature during the tow. Check the cooler and its lines are clear.
- What fixes it
- Service the fluid, clear or upgrade the cooler. Heat-cycled fluid does not recover when it cools.
-
Fault only with the trailer plugged in
- Likely cause
- The trailer or its connector rather than the vehicle — the car measures each lamp circuit and reports what it finds
- How to confirm it
- Unplug the trailer while still loaded. A fault that clears has told you which end to work on, for nothing.
- What fixes it
- Repair the trailer's wiring or clean the socket pins. See the towing warning entry in this vocabulary.
-
Rear suspension or ride height faults only when loaded
- Likely cause
- The suspension sitting where it rarely sits — a level sensor at the end of its travel, or air suspension unable to hold height
- How to confirm it
- Compare ride height sensor values loaded and unloaded. A sensor reading implausibly at one end of its range names itself.
- What fixes it
- Renew the sensor or repair the air suspension leak. Check the vehicle's plated towing and payload limits while there.
-
Fault appears loaded and does not clear when unloaded
- Likely cause
- The load exposed a marginal part and finished it — the pattern is history now
- How to confirm it
- Diagnose conventionally; a permanent fault is far easier than an intermittent.
- What fixes it
- Repair normally. The towing pattern explains the cause rather than describing the current state.
Codes that produce this symptom (102)
Listed with the codes that most typically present this way first — the likeliest place to start.
- C0072 Brake Temperature Too High Typical of this code High
- P0168 Fuel Temperature Too High Typical of this code Moderate
- P0218 Transmission Fluid Temperature Sensor A Over Temperature Condition Typical of this code Stop driving High
- P0481 Cooling Fan 2 Control Circuit Typical of this code Moderate
- P0693 Cooling Fan 2 Control Circuit Low Typical of this code Moderate
- P0694 Cooling Fan 2 Control Circuit High Typical of this code Moderate
- P0701 Transmission Control System Range/Performance Typical of this code High
- P0730 Incorrect Gear Ratio Typical of this code High
- P0867 Transmission Fluid Pressure Typical of this code High
- P0897 Transmission Fluid Deteriorated Typical of this code Moderate
- P0944 Hydraulic Pressure Unit Loss of Pressure Typical of this code Stop driving High
- P0A2F Drive Motor A Over Temperature Typical of this code High
- P0A35 Drive Motor B Over Temperature Typical of this code High
- P0A44 Drive Motor A Position Sensor Circuit Overspeed Typical of this code High
- P0A4A Drive Motor B Position Sensor Circuit Overspeed Typical of this code High
- P0A50 Generator Position Sensor Circuit Overspeed Typical of this code High
- P0CBC Drive Motor A Coolant Temperature Sensor Circuit Typical of this code High
- P0E23 Drive Motor Inverter Temperature Sensor G Circuit Typical of this code High
- P0E24 Drive Motor Inverter Temperature Sensor G Circuit Range/Performance Typical of this code High
- P0E25 Drive Motor Inverter Temperature Sensor G Circuit Low Typical of this code High
- P0E26 Drive Motor Inverter Temperature Sensor G Circuit High Typical of this code High
- P0E27 Drive Motor Inverter Temperature Sensor G Circuit Intermittent/Erratic Typical of this code High
- P0E28 Drive Motor Inverter Temperature Sensor H Circuit Typical of this code High
- P0E29 Drive Motor Inverter Temperature Sensor H Circuit Range/Performance Typical of this code High
- P0E2A Drive Motor Inverter Temperature Sensor H Circuit Low Typical of this code High
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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