Parts for your 1999 Mitsubishi Pajero-Oxygen sensor

1999 Mitsubishi Pajero Oxygen Sensor: What’s Fitted and What To Do About It

Based on Mitsubishi factory documentation, oxygen sensors are relevant to the 1999 Mitsubishi Pajero—if it’s a petrol model. The Mitsubishi Workshop Manual for V6 petrol engines (6G72 3.0L and 6G74 3.5L, 1997–2000) lists heated oxygen sensors (HO2S) in the engine management and exhaust sections, and the Mitsubishi ASA/CAPS parts catalogue for V20-series Pajero shows upstream (and, in some markets, downstream) O2 sensors to suit these engines. By contrast, the 2.8L 4M40 turbo-diesel engine wiring diagrams and engine manual of the same era do not include an oxygen sensor, reflecting typical diesel control strategies of the time (EGR and fuel metering without closed-loop lambda feedback).

For petrol 1999 Pajero owners, the oxygen sensor does a lot of heavy lifting. Sitting in the exhaust stream, it tells the ECU how much oxygen is left in the burnt gases, so the ECU can tweak fuel delivery to keep the mixture on point. That helps the catalytic converter do its job, trims fuel use on the highway, and keeps the Paj running smoothly under our Aussie and Kiwi conditions.

They’re wear items, though. A tired O2 sensor can cause rough idle, flat spots on take-off, higher fuel consumption, or a check engine light. If the exhaust smells a bit rich or the cat runs hotter than usual, it’s worth a look. On these late-90s Mitsis, many owners opt to replace the upstream sensor somewhere between 100,000 and 160,000 kilometres, especially if fuel economy has drifted or the sensor is original.

Good servicing habits help:

• Inspect the sensor wiring and connector at each major service, heat and road grime are tough on insulation.
• Fix any exhaust leaks before the sensor—false air will mess with readings.
• Use a quality, correct-spec replacement (heated narrowband) and torque it properly, a touch of sensor-safe anti-seize on the threads helps next time.
• Avoid silicone-based sealants upstream of the cat—siloxanes can poison sensors and catalysts.
• After replacement, clear any fault codes and let the ECU relearn trims with a decent mixed drive.

How many sensors you’ve got can vary by market: most V6s run at least one upstream sensor before the cat, some have a downstream sensor to monitor the cat’s efficiency. Diesel 4M40 Pajeros of 1999 generally don’t run an oxygen sensor at all—their fuelling strategy and emissions hardware of that era don’t rely on lambda feedback.

Popular questions

How many oxygen sensors are on a 1999 Mitsubishi Pajero?
Petrol V6 models commonly have one upstream sensor before the catalytic converter, and some markets add a downstream sensor after the cat. The 2.8L 4M40 diesel typically has none. A quick look under the bonnet and along the front section of the exhaust, or checking by VIN, will confirm your setup.

What are the signs my Pajero’s oxygen sensor needs replacing?
Tell-tales include higher-than-normal fuel use, a check engine light, rough idle, hesitation, or a sooty exhaust. If the cat seems unusually hot or there’s a sulphur smell after a run, the sensor may be misreporting and sending the mixture rich. Scan tool data showing a lazy or stuck sensor is another giveaway.

Can an oxygen sensor be cleaned, or should it just be replaced?
Cleaning is rarely effective and can damage the sensing element. If the sensor is slow, contaminated, or tripping codes, replacement with a correct-spec heated narrowband unit is the reliable fix. Make sure to address any root causes like exhaust leaks or oil-burning that might foul a new sensor.