Parts for your 1990 Mitsubishi Pajero-Oxygen sensor

1990 Mitsubishi Pajero oxygen sensor — what’s fitted and what’s not

Based on factory and workshop references, whether a 1990 Mitsubishi Pajero uses an oxygen sensor depends on the engine. The petrol 3.0L V6 (6G72) with ECI-Multi fuel injection is fitted with a heated oxygen sensor (HO2S) in the exhaust for closed-loop control. This is documented in the Mitsubishi Pajero/Montero Factory Service Manual (1989–1991, Engine/ECI-Multi sections) and reflected in the Mitsubishi ASA electronic parts catalogue, which lists O2 sensor part numbers for V6 EFI variants. Haynes’ Mitsubishi Montero/Pajero/Dodge Raider manual (1983–1991) also covers O2 sensor testing for fuel-injected models only. By contrast, the 2.6L 4G54 carburetted petrol and the 2.5L 4D56 diesel of the same era typically do not use an O2 sensor in AU/NZ-spec, as noted in Gregory’s Pajero manual No. 505 (1983–1991) and the diesel FSM, which show no exhaust oxygen sensor in those systems.

For Pajeros fitted with an O2 sensor (notably the 3.0 V6 EFI), the sensor’s job is to measure oxygen in the exhaust so the ECU can fine-tune fuelling. That keeps things running sweet as, helping fuel economy, throttle response, and emissions, and preventing the catalytic converter from copping a hiding. Over time, sensors get lazy from heat cycling, leaded fuel residue from the old days, silicone vapours, or just age. When they drift, the ECU can run rich or lean, leading to rough idle, flat spots, higher fuel use and a whiffier tailpipe.

As part of regular servicing, it’s worth checking live O2 activity on a scan tool if available, or performing the voltage swing test described in workshop manuals. Many owners treat O2 sensors as maintenance items at around 100,000–160,000 km. If replacement’s due, go for a quality direct-fit unit for the V6 ECI-Multi, make sure the engine’s cold, crack it with an O2 socket, and torque the new one to spec (typically around 35–40 Nm, check the FSM). Most new sensors arrive with thread compound—don’t add extra sealant. Avoid contaminating the tip with grease or coolant, and sort any exhaust leaks upstream, as they’ll throw readings out.

• Signs it’s time: higher fuel use, intermittent check-engine light, sooty plugs, sluggish performance.
• Good habits: fix vacuum and exhaust leaks, keep the air filter clean, and use quality fuel.

If the Pajero is a carburetted 2.6 petrol or a 2.5 diesel, there’s no O2 sensor to replace. Those engines use mechanical mixture control (carb) or diesel injection without petrol-style closed-loop feedback, so an oxygen sensor wouldn’t add value to their original emissions strategy in 1990.

Popular questions about 1990 Mitsubishi Pajero oxygen sensors

Where is the oxygen sensor on a 1990 Pajero 3.0 V6?
On the V6 ECI-Multi, the primary O2 sensor sits in the exhaust near the engine—typically on the downpipe ahead of the catalytic converter. Access is from underneath with the vehicle safely supported. Look for a small sensor threaded into the pipe with a two to four-wire harness leading to an engine loom connector.

How often should the O2 sensor be replaced?
There’s no strict interval, but many technicians recommend inspection or proactive replacement between 100,000 and 160,000 km, or sooner if there are driveability issues or a stored fault code. If fuel economy drops off or the tailpipe smells rich, testing the sensor is a good first step in a service.

Can a dodgy O2 sensor cause a WOF/rego emissions fail or high fuel use?
Yes—on EFI petrol models, a slow or failed O2 sensor skews fuelling, which can push hydrocarbons up and burn more petrol. Replacing a tired sensor often tidies the tune, smooths idle, and helps pass basic emissions checks where applicable.