The 2006 Nissan Altima was offered with two engine sizes -- a 3.5-liter V-6 and 2.5-liter, inline four-cylinder. Like all modern vehicles, both engine types receive combustion ratio feedback from sensors in the exhaust system. These Altimas have air-fuel sensors before the catalytic converters and heated oxygen sensors behind them. The number of sensors depends upon the engine type, but they can all be removed using the same size tool setups.
The six-cylinder engine has two oxygen sensors, one behind each catalytic converter. Four-cylinder engines have just one behind their sole catalytic converter. Additionally, both engines have air/fuel ratio sensors on each exhaust manifold just downstream of the convergence points.
Ideally, you would use an oxygen sensor wrench or socket specialty tool. They have slots machined into the side which align with the lead and avoid the need to splice wires. The socket size is either 7/8-inch or 22 mm with a 3/8-inch-drive. The 22 mm socket has a better grip as it is about a hundredth of an inch tighter -- 0.866-inch versus 0.875-inch. If the special tool is not available or a non-OEM sensor is being installed and thus may need to be spliced regardless, a deep-well 22 mm or 7/8-inch socket can be used.
