Service Required 4WD on the Evo X and Lancer Ralliart: What It Means and What to Do

If you drive an Evo X or a Lancer Ralliart, sooner or later you may see it: a SERVICE REQUIRED warning with the 4WD or S-AWC symbol on the dash display. It's one of the most common messages I get asked about, so here's what it actually means, what causes it and what to do — in the right order, so you don't spend money on the wrong part.

Which cars show this message?

This warning belongs to cars with Mitsubishi's electronically controlled centre differential and its hydraulic pump:

• Lancer Evolution X (2008–2015) — S-AWC: ACD plus AYC on GSR and MR, ACD only on the RS.
• Lancer Ralliart and Lancer Sportback Ralliart — AWC based on the ACD.

The exact wording varies a little between model years and markets — you'll see “SERVICE REQUIRED”, “4WD SERVICE REQUIRED” or an S-AWC warning — but it points to the same system.

Two cases where this article is not about your car:

• Regular Lancer AWC (SE, GT, 2012 onwards, with the CVT). Its four-wheel drive uses an electronically controlled coupling, not an ACD with a hydraulic pump. A 4WD warning on that car needs a different diagnosis, and our pumps don't fit it.
• Evo VII–IX. Older Evos don't have this display message; they report ACD/AYC faults through warning lamps and flash codes.

What happens when the warning comes on?

The control unit switches the ACD and AYC off to protect the system. From the driver's seat:

• The Tarmac / Gravel / Snow modes stop doing anything.
• The car still drives normally in a straight line — you won't feel much on your way to work.
• What you lose is the handling: the centre diff no longer adjusts, and on cars with AYC the rear no longer helps the car turn in. You notice it most in corners, in the wet and on the limit.

It isn't an emergency, but don't treat it as normal either. If the pump is failing, every start makes it work harder and longer, and that can finish off the motor.

What causes “Service Required”?

The most common causes, roughly in the order I see them:

1. The pump can't build pressure. Internal wear, corrosion and tired seals mean the pump runs but never reaches the target pressure. This is behind most cases, and it usually logs code C161E.
2. The electric motor fails. This is a very common one. The pump motor wears out and stops running or can't turn the pump properly — and a motor that has spent years working against a tired hydraulic side wears out faster. On the Evo X this is the known weak point that our Stage 2 rebuild and the restored motor address.
3. Low fluid or air in the circuit — after a leak, after work on the lines, or after a pump swap without a proper bleed.
4. The pressure sensor. A faulty sensor can report low pressure when the pump is fine.
5. The pump relay or wiring — a dead relay, corroded connector or broken ground stops the pump from running at all.
6. The ACD proportioning valve circuit — an electrical fault in the valve that controls the centre diff (code C161C).
7. Cold weather. Mitsubishi notes that at low ambient temperature the control fluid thickens and the warning can appear. If it only happens on cold mornings and clears once the car is warm, that's the likely reason — though a pump that struggles every winter is usually a tired pump.

It's also worth knowing that the pump's operation depends on a whole chain of sensors: the ABS wheel speed sensors, the steering angle sensor and the throttle position sensor. If any one of them fails, the system forcibly switches the ACD/AYC pump off — so a faulty sensor can bring up the same warning while the pump itself is fine.

What should you do first?

Read the fault codes. That single step tells you which of the causes above you're dealing with. You need a scan tool that can talk to the all-wheel-control ECU (AWC-ECU): Mitsubishi's MUT-III, or an aftermarket tool that supports it — a basic engine-code reader usually won't see these codes.

While you're at it, two quick checks you can do yourself:

• Fluid level in the ACD/AYC reservoir. If it's low, find out where it went before topping up.
• Does the pump run? With the ignition on, you should hear it charge for a few seconds and then stop. If it's silent, suspect the relay, fuse or wiring; if it runs on and on, suspect the pump. The full self-test is in Signs of AYC pump failure.

Can you just clear the warning?

You can clear the codes and dismiss the message, but if the fault is real it comes back — usually on the next start or the next time the system asks the pump for pressure. Clearing is useful for one thing: checking whether the fault returns after a repair or a warm-up.

How do you fix it?

• Fluid or air: fix the leak, refill and bleed — see How to bleed the ACD/AYC system.
• Relay, wiring, sensor: replace the faulty part. These are the cheap fixes, which is why you rule them out first.
• Pump not building pressure: the pump needs rebuilding or replacing. A proper rebuild renews the worn internals and seals and is load-tested on the bench before it goes back in — see ACD pump restoration process. On the Evo X the motor is a known weak point, which is what our Stage 2 rebuild addresses.

A new pump from Mitsubishi is expensive and not always available. We restore ACD/AYC pumps, and a failed core can go towards a restored unit through our trade-in programme.

Parts for this job

Pick your car — each link goes straight to the part:

Not sure which part your car needs? Message me with the codes you're seeing and I'll tell you where to start.