Power Steering System Chevrolet Aveo I

Fastener Tightening Specifications

Fastener Tightening Specifications

Power Steering Pump Specifications

Power Steering Pump Specifications

Scheme 1

Scheme 2

Symptoms - Power Steering System

Tools Required

J 44721 Power Steering Analyzer. See Special Tools .

Check the fluid pressure as follows to determine whether the trouble is in the pump or the gear unit.

Test Procedure

1. Check the power steering fluid level and the power steering pump belt tension. Refer to «Checking and Adding Power Steering Fluid»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__checking-and-adding-power-steering-fluid) .
2. Disconnect the high pressure line at the pump. Use a small container to catch any fluid.
3. Connect the hose of the J 44721 to the power steering pressure hose from the power steering pump. See «Special Tools»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__special-tools) .
4. Place the gear selector lever in PARK for automatic transaxle-equipped vehicles or NEUTRAL for manual transaxle-equipped vehicles. Set the parking brake.
5. Open the gage valve fully.
6. Start the engine and let it idle.
7. Turn the steering wheel from lock to lock several times to warm the fluid to operating temperature.
8. Increase the engine speed to 1,500 RPM.

1. Close the gage valve fully, and read the pressure. The pump pressure with the valve closed should be between 7 088-8 619 kPa (1,028-1,250 psi).
2. Immediately open the gage valve fully.
3. Turn the steering wheel all the way to the left and the right. If the pressure is within the specified limits, the problem is not in the pump. Inspect the power steering gear for leaks.

Power Steering Fluid Leaks

Inspect the following

1. Fluid reservoir for overfill
2. Fluid for aeration and overflow
3. Hoses for loose connections
4. Torsion bar, stub shaft, and adjuster seals for leaks
5. Component sealing surfaces for damage

External Leak Check

The purpose of this procedure is to pinpoint the location of the leak. In some cases, the leak can be easily located, but seepage leaks may be harder to find. To locate seepage leaks, use the following method

1. With the engine OFF, wipe dry the complete power steering system.
2. Check the power steering fluid level in the pump's reservoir. Adjust the fluid level, as necessary. Refer to «Checking and Adding Power Steering Fluid»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__checking-and-adding-power-steering-fluid) .
3. Start the engine. Turn the steering wheel counterclockwise and clockwise from stop to stop several times.
4. Find the exact area of the leak and repair it.

Rattle in Rack and Pinion Steering Gear

Rattle in Rack and Pinion Steering Gear

Objectionable Hiss in the Power Steering System

Groan Noise in Steering Pump

Groan Noise in Steering Pump

Steering Vibrates During Low Speed or Static Steering

Steering Vibrates During Low Speed or Static Steering

Removal Procedure

1. Disconnect and remove the power steering gear. Refer to «Power Steering Gear Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
2. Place the power steering gear on a bench next to the vehicle.
3. Disconnect the pressure line at the point where the hose connects to the pipe. Extend this line in order to reach the power steering gear on the bench.
4. Disconnect the return line from the power steering fluid reservoir. Extend this line in order to reach the power steering gear on the bench.
5. Connect the power steering pipes to the power steering gear.
6. Start the engine and allow it to idle for 10 seconds.
7. Check the power steering fluid level.
8. Start the engine and turn the rack and pinion stub shaft to a full turn in each direction. Hold the shaft against each stop for 5 seconds
9. Inspect for possible leak points.

Installation Procedure

1. Stop the engine.
2. Disconnect the power steering pipes from the power steering gear.
3. Remove the extensions and reconnect the pressure and return lines.
4. Install and connect the power steering gear. Refer to «Power Steering Gear Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
5. Start the engine and allow it to idle for 10 seconds.
6. Check the power steering fluid level.

Hard Steering

Hard Steering

Rattle or Knock Noise in Steering Pump

Rattle or Knock Noise in Steering Pump

Swish Noise in Steering Pump

Swish Noise in Steering Pump

Whine Noise in Steering Pump

Whine Noise in Steering Pump

Wander or Poor Steering Stability

Wander or Poor Steering Stability

Erratic Steering When Braking

Erratic Steering When Braking

Foaming, Milky-Appearing Power Steering Fluid

Foaming, Milky-Appearing Power Steering Fluid

Increase in Effort While Turning Steering Wheel

Increase in Effort While Turning Steering Wheel

Poor Return of Steering Wheel

Poor Return of Steering Wheel

Steering Wheel Surges/Jerks While Turning

Steering Wheel Surges/Jerks While Turning

Steering Wheel Kickback

Steering Wheel Kickback

Steering Effort Hard in One or Both Directions

Steering Effort Hard in One or Both Directions

Too Much Play in Steering

Too Much Play in Steering

1. Siphon the power steering fluid from the fluid reservoir.
2. Disconnect the pressure line pipe and the supply line hose from the inlet and outlet connections on the power steering pump.
3. Remove the air cleaner housing, if needed.
4. Disconnect the pressure line and return line hoses from the retaining clips at the bottom and the side of the radiator.
5. Disconnect the pressure line inlet pipe from the steering gear.
6. Disconnect the supply line hose from the power steering fluid reservoir.
7. Remove the power steering pump pressure line and return line.

1. Install the power steering pump pressure line and return line.
2. Connect the supply line hose to the power steering fluid reservoir.
3. Connect the pressure line inlet pipe to the steering gear. Tighten: Tighten the steering gear inlet pipe fitting to 28 N.m (21 lb ft).
4. Connect the pressure line and return line hoses to the retaining clips at the bottom and the side of the radiator.
5. Connect the pressure line pipe and the supply line hose to the inlet and outlet connections on the power steering pump.
6. On dual overhead cam (DOHC) engines, install the air cleaner housing, if removed. Tighten: Tighten the air cleaner housing bolts to 12 N.m (106 lb in). Tighten the power steering pump pressure line union nut to 28 N.m (21 lb ft).
7. Fill the fluid reservoir with power steering fluid.
8. Inspect for leaks. If there are leaks, correct the cause of the leaks and bleed the system. Refer to «Bleeding the Power Steering System»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .

1. Inspect for any damage on the rack gear.
2. Inspect for damage on the pinion gear and valve assembly.
3. Inspect the tie rod and ball joint.

1. Install the pinion lock nut. Tighten: Tighten the pinion lock nut to 30 N.m (22 lb ft).
2. Install the adjust nut and its lock nut. Refer to «Rack and Pinion Gear Rack Bearing Preload Adjustment - Off Vehicle»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
3. Install the tie rod. Tighten: Tighten the tie rod to 100 N.m (75 lb ft).

Adjustment Procedure

Make the rack bearing adjustment with the front wheel raised and the steering wheel centered. Be sure to check the returnability of the steering wheel to center after the adjustment.

1. Loosen the locknut and turn the adjuster plug clockwise until a torque of 10 N.m (88 lb in) is obtained.
2. Back the adjuster plug off by 55-65 degrees. Check the pinion preloaded torque is within the range 0.9-1.7 N.m (8-15 lb in).
3. Tighten the adjuster plug locknut while holding the adjuster plug stationary. Tighten: Tighten the adjuster plug locknut to 70 N.m (52 lb ft).
4. After all the operations with the steering gear, be sure to check the exact straight-ahead position of the steering in each case. Refer to «Straight Ahead Diagnosis»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#manual-rack-pinion-steering-system) in Manual Steering Gear.

Disassembly Procedure

1. Remove the rack and pinion steering assembly from the vehicle. Refer to «Power Steering Gear Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
2. Remove the dust cover from the lower end of the housing.
3. While holding the stub shaft with a wrench, remove the locknut from the pinion.
4. With the gear centered, mark the location of the stub shaft notch on the housing to aid in properly installing the pinion and valve assembly.
5. Remove the upper housing retaining ring.
6. Remove the valve and pinion assembly from the stub shaft housing.
7. Remove the stub shaft dust seal, the stub shaft bearing annulus assembly.

1. Install the bushing and a new lower pinion valve seal.
2. Coat all the seals with power steering fluid to ensure proper sealing.
3. When the valve and pinion assembly is fully seated in the housing, be sure the notch in the stub shaft and the mark on the housing line up. If this is not done the vehicle will not pass the straight-ahead check and will have poor steering performance.

1. Siphon the power steering fluid from the fluid reservoir.
2. Disconnect and remove the steering gear cylinder pipes at the rack and pinion housing. Remove the O-ring seals from the housing.

1. Install new O-rings onto the cylinder pipes. Install the steering gear cylinder pipes into the rack and pinion housing. Tighten: Tighten the power steering line fittings at the cylinder end to 28 N.m (21 lb ft). Tighten the power steering line fittings at the valve end to 18 N.m (13 lb ft).
2. Fill the fluid reservoir with power steering fluid.
3. Inspect for leaks. If there are leaks, correct the cause of the leaks and bleed the system. Refer to «Bleeding the Power Steering System»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
4. Lower the vehicle.

There is no provision for overhaul or disassembly of the rack and pinion power steering gear. The power steering gear is served only as an assembly.

1. Remove the rack and pinion steering assembly from the vehicle.
2. Remove the adjuster plug locknut from the adjuster plug, and remove the adjuster plug from the housing with the rack guide spring cap wrench (22 mm).
3. Remove the adjuster spring and rack bearing.

Scheme 3

1. Coat the rack bearing, the adjuster spring, and the adjuster plug with lithium-based grease.
2. With the rack centered, turn the adjuster plug clockwise until a torque of 12 N.m (9 lb ft) is obtained, then back it off by 50-70 degrees. Tighten: Tighten the adjuster plug to 12 N.m (9 lb ft).

1. Turn the wheels all the way to the left and add the power steering fluid to the MIN mark on the fluid level indicator.
2. Start the engine. With the engine running at fast idle, recheck the fluid level. If necessary, add fluid to bring the level up to the MIN mark.
3. Bleed the system by turning the wheels from side to side without reaching the stop at either end. Keep the fluid level at the MIN mark. The air must be eliminated from the fluid before normal steering action can be obtained.
4. Return the wheels to the center position. Continue running the engine for 2-3 minutes.
5. Road test the car to be sure the steering functions normally and is free from noise.
6. Recheck the fluid level as described in steps 1 and 2. Make sure the fluid level is at the MAX mark after the system has stabilized at its normal operating temperature. Add fluid as needed.

Checking and Adding Power Steering Fluid

1. The power steering fluid level is indicated either by marks on a see-through fluid reservoir or by marks on a fluid level indicator on the fluid reservoir cap.
2. If the fluid is warmed up to 66°C (150°F), the fluid level should be between the MAX and MIN marks. Add fluid as needed.
3. If the fluid is cool, 21°C (70°F), the fluid level should be at the MIN mark. Add fluid as needed.

1. Siphon the power steering fluid from the fluid reservoir.
2. Loosen the hose clamps and remove both hoses.
3. Remove the fluid reservoir attaching bolts and remove the fluid reservoir.

1. Attach the fluid reservoir with the power steering fluid reservoir attaching bolts. Tighten: Tighten the power steering fluid reservoir attaching bolts to 7 N.m (62 lb in).
2. Connect both hoses and secure the hose clamps.
3. Fill the fluid reservoir with power steering fluid.
4. Inspect for leaks. If there are leaks, correct the cause of the leaks and bleed the system. Refer to «Bleeding the Power Steering System»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .

1. Loosen the steering pump pulley bolt.
2. Remove the pump drive belt from the pulley by moving the auto-tensioner.
3. Remove the steering pump pulley bolt that connect the pulley to the pump.
4. Remove the pulley from the pump.

1. Replace the pump on the engine if the pump was previously removed. Refer to «Power Steering Pump Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
2. Install the pulley on the pump. Install the steering pump pulley bolt.
3. Install the pump drive belt.
4. Tighten the steering pump pulley bolt to fasten the pulley to the pump. Tighten: Tighten the steering pump pulley bolts to 25 N.m (18 lb ft).
5. Adjust the belt tension. Refer to «Power Steering Pump Drive Belt Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
6. If the pump was removed from the engine, bleed the power steering system. Refer to «Power Steering Pump Drive Belt Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
7. Inspect for leaks. If leaks are found, correct the cause of the leak and bleed the system.

1. Remove the air cleaner housing by removing the housing bolts and loosening the clamp.
2. Loosen the steering pump pulley bolt.
3. Remove the pump drive belt from the pulley by moving the auto-tensioner.
4. Remove the steering pump pulley bolt that connect the pulley to the pump.
5. Remove the pulley from the pump.

1. Replace the pump on the engine if the pump was previously removed. Refer to «Power Steering Pump Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
2. Install the pulley onto the pump. Install the steering pump pulley bolt.
3. Install the pump drive belt.
4. Tighten the steering pump pulley bolt to fasten the pulley to the pump. Tighten: Tighten the steering pump pulley bolt to 25 N.m (18 lb ft).
5. Adjust the belt tension. Refer to «Power Steering Pump Drive Belt Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
6. Install the air cleaner housing with the housing bolts and the clamp. Tighten: Tighten the air cleaner housing bolts to 12 N.m (106 lb in).
7. If the pump was removed from the engine, bleed the power steering system.
8. Inspect for leaks. If leaks are found, correct the cause of the leak and bleed the system.

1. Remove the air cleaner housing by removing the housing bolts and loosening the clamp.
2. Remove the pump drive belt from the pulley by moving the auto-tensioner.
3. Remove the power steering pump pulley bolt and the power steering pump pulley.
4. Drain the power steering fluid by disconnecting the pressure and supply lines from the pump.
5. Remove the pump assembly by removing the steering pump retaining bolts.

1. Install the pump and the steering pump retaining bolts. Tighten: Tighten the steering pump retaining bolts to 25 N.m (18 lb ft).
2. Connect the pressure and supply lines to the power steering pump.
3. Install the power steering pump pulley and the pulley bolt. Tighten: Tighten the power steering pump pulley bolt to 25 N.m (18 lb ft).
4. Install the pump drive belt onto the pulley by moving the auto-tensioner.
5. Install the air cleaner housing with the housing bolts and the clamp. Tighten: Tighten the air cleaner housing bolts to 12 N.m (106 lb in).
6. Refill the pump with new fluid and bleed the air from the system.
7. Inspect for leaks. If leaks are found, correct the cause of the leak and bleed the system.

J 23600-B Belt Tension Gage. See Special Tools .

Checking Belt Tension

1. Using the J 23600-B , check the tension on the pump drive belt. See «Special Tools»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__special-tools) .
2. If the J 23600-B does not indicate the proper tension, adjust the belt. See «Special Tools»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__special-tools) .

Adjusting Belt Tension

1. With the J 23600-B on the pump drive belt, loosen the alternator adjusting bolt and move the alternator until the gage indicates the proper tension. See «Special Tools»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system__special-tools) .
2. Tighten the alternator adjusting bolt. Tighten: Tighten the alternator adjusting bolt to 20 N.m (15 lb ft).

KM-507-C Ball Joint Remover

1. Remove the outer tie rod hex nut and disconnect the outer tie rod from the steering knuckle using the KM-507-C .
2. Loosen the tie rod lock nuts and remove the outer tie rod.

1. Install the outer tie rod with the lock nuts. Tighten: Tighten the lock nuts to 54 N.m (40 lb ft).
2. Install the outer tie rod with the hex nut. Tighten: Tighten the hex nut to 45 N.m (33 lb ft).

1. Siphon the power steering fluid from the fluid reservoir.
2. Disconnect the return line pipe from the outlet connection at the steering gear.
3. Disconnect the fluid reservoir hose from the fluid reservoir.
4. Remove the fluid reservoir hose.

1. Connect the fluid reservoir hose at the fluid reservoir.
2. Connect the fluid reservoir pipe to the outlet connection at the steering gear. Tighten: Tighten the steering gear outlet pipe fittings to 28 N.m (21 lb ft).
3. Fill the fluid reservoir with power steering fluid.
4. Inspect for leaks. If there are leaks, correct the cause of the leaks and bleed the system. Refer to «Bleeding the Power Steering System»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .

KM-507-C Ball Joint Remover

1. Disconnect the negative battery cable.
2. Position the tires straight ahead by turning the steering wheel.
3. Remove the intermediate shaft.
4. Remove the front tires.
5. Drain the power steering fluid from the rack and pinion.
6. Disconnect the steering gear inlet and outlet pipe fittings.
7. Remove the outer tie rod hex nuts. Refer to «Rack and Pinion Outer Tie Rod End Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
8. Remove the ball joint hex nuts and disconnect the stabilizer shaft from the knuckle by removing the stabilizer shaft link assembly. Refer to «Stabilizer Shaft Link Replacement»(/chevrolet/aveo/i-2003-2008/remont/suspension-front/#front-suspension-system) in Front Suspension.
9. Remove the cross member by removing the nuts and bolts to the underbody.
10. Remove the rack and pinion assembly by disconnecting the steering gear retaining bracket nuts.

1. Install the rack and pinion assembly by connecting the steering gear retaining bracket nuts. Tighten: Tighten the steering gear retaining bracket nuts to 50 N.m (37 lb ft).
2. Install the cross member by tightening the nuts and bolts to the underbody. Tighten: Tighten the cross member by tightening the nuts and bolts to the underbody to 150 N.m (111 lb ft).
3. Install the ball joint hex nuts and connect the stabilizer shaft to the knuckle by tightening the bolt with stabilizer shaft link assembly. Refer to «Stabilizer Shaft Link Replacement»(/chevrolet/aveo/i-2003-2008/remont/suspension-front/#front-suspension-system) in Front Suspension. Tighten: Tighten the ball joint hex nuts to knuckle to 45 N.m (33 lb ft). Tighten the bolts of the stabilizer shaft to the knuckle to 45 N.m (33 lb ft).
4. Install the outer tie rod hex nuts. Refer to «Rack and Pinion Outer Tie Rod End Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) . Tighten: Tighten the outer tie rod hex nuts to 45 N.m (33 lb ft).
5. Connect the steering gear inlet and outlet pipe fittings. Tighten: Tighten the nuts of the steering gear inlet and outlet pipe fittings to 22 N.m (16 lb ft).
6. Install the front tires. Refer to «Tire and Wheel Removal and Installation»(/chevrolet/aveo/i-2003-2008/remont/wheel-tire-system/#tires-and-wheels) in Tires and Wheels.
7. Refill the power steering fluid.
8. Install the intermediate shaft.
9. Install the steering wheel and column.
10. Connect the negative battery cable.

1. Raise and suitably support the vehicle.
2. Center the steering wheel.
3. Remove the power steering gear. Refer to «Power Steering Gear Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
4. Loosen the locknut and the adjuster plug clockwise until a torque of 12 N.m (9 lb ft) is obtained, then loosen it by 50-70 degrees.
5. Tighten the locknut on the adjuster plug while holding the adjuster plug stationary.
6. Install the power steering gear. Refer to «Power Steering Gear Replacement»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
7. Be sure to check the return ability of the steering wheel to the center position after adjustment. Tighten: Tighten the adjuster plug to 12 N.m (9 lb ft). Tighten the adjuster plug lock nut to 30 N.m (22 lb ft).

1. Remove the tie rod.
2. Remove the rack bearing. Refer to «Valve and Pinion Assembly Disassemble»(/chevrolet/aveo/i-2003-2008/remont/manual-power-steering/#power-steering-system) .
3. Remove the pinion shaft ball bearing.
4. Remove the sir clip from the rack gear housing.
5. Remove the rack gear and stopper by pulling them.
6. Remove the rack gear stopper.

Note. Refer to Clutch Hydraulic System Lubricant Notice in Cautions and Notices.

Note. Refer to Air in the Power Steering System Notice in Cautions and Notices.

Note. Refer to Power Steering Hose Disconnected Notice in Cautions and Notices.

The power steering system consists of 3 components: the power steering pump, the power steering fluid reservoir, and the power steering rack and pinion gear. The power steering pump is a vane-type pump providing hydraulic pressure for the system and is powered by the engine. It draws on the power steering fluid reservoir, which in turn is connected to the power steering gear. A pressure-relief valve inside the flow control valve limits the pump pressure. The power steering rack and pinion gear has a rotary control valve which directs hydraulic fluid coming from the power steering pump to one side or the other side of the rack piston. The integral rack piston is attached to the rack. The rack piston converts hydraulic pressure to a linear force which moves the rack to the left or the right. The force is then transmitted through the inner and the outer tie rods to the steering knuckles, which turn the wheels.

Power Steering Pump Description

The pump has a remote fluid reservoir. A pressure-relief valve inside the flow control valve limits pump pressure.

Seal Replacement

Lip seals, which seal the rotating shafts, are used on the driveshaft of the pump. This type of seal requires special treatment. When leakage occurs in this area, always replace the seal after inspecting and thoroughly cleaning the sealing surfaces. If the corrosion in the lip seal contact zone is slight, clean the surface of the shaft with a crocus cloth. Replace the shaft only if the leakage cannot be stopped by first smoothing the shaft with the crocus cloth.

Power Steering Gear Description

Note. Refer to Steering Gear Preload Adjustment Notice in Cautions and Notices.

Note. Refer to Power Steering Hose Disconnected Notice in Cautions and Notices.

The power rack and pinion steering system has a rotary control valve which directs the hydraulic fluid coming from the hydraulic pump to one side or the other side of the rack piston. The integral rack piston is attached to the rack. The rack piston converts hydraulic pressure to a linear force which moves the rack left or right. The force is then transmitted through the inner and the outer tie rods to the steering knuckles, which turn the wheels.

If hydraulic assist is not available, manual control is maintained. However, under these conditions, more steering effort is required. The movement of the steering wheel is transferred to the pinion. The movement of the pinion is then transferred through the pinion teeth, which mesh with the teeth on the rack, causing the rack to move.

A vane-type pump provides hydraulic pressure for the system.

The boot and rack guide, the rack bearings, and the valve and pinion assembly are no longer serviceable on this vehicle. They must be replaced as whole units.

Special Tools

Special Tools Illustration Tool Number/ Description J-23600-B Belt Tension Gage J-44721 (KM-354-B) Power Steering Analyzer KM-507-C Ball Joint Remover

Scheme 4

Scheme 5

See also:
• Steering Wheel in the Full Turn Position Notice
• Power Steering Hose Disconnected Notice
• Fastener Notice
• Straight Ahead Diagnosis
• Safety Glasses Caution
• Air in the Power Steering System Notice
• Belt Dressing Notice
• Battery Disconnect Caution
• Stabilizer Shaft Link Replacement
• Tire and Wheel Removal and Installation
• Vehicle Lifting and Jacking Notice
• Clutch Hydraulic System Lubricant Notice
• Steering Gear Preload Adjustment Notice
• Special Tools
• Checking and Adding Power Steering Fluid
• Power Steering Gear Replacement