Parts for your 1998 Daihatsu Terios-Oxygen sensor

1998 Daihatsu Terios Oxygen Sensor: What It Does and When to Replace It

Based on the Daihatsu J100-series workshop manual for the HC‑EJ 1.3L EFI engine, the Daihatsu Electronic Parts Catalogue (EPC), and common aftermarket catalogues from Denso and NTK, the 1998 Daihatsu Terios is equipped with a heated oxygen (lambda) sensor in the exhaust system. Most AU/NZ-market Terios models of this era have a primary sensor before the catalytic converter, some markets also include a secondary post‑cat sensor.

The oxygen sensor’s job is simple but crucial: it sniffs the exhaust for leftover oxygen so the engine control unit can trim fuelling on the fly. That closed‑loop correction keeps the Terios running clean, helps fuel economy, and protects the catalytic converter. When the sensor switches quickly and accurately, the engine stays in the sweet spot—neither rich nor lean—across warm idle, cruise, and gentle acceleration.

As part of routine servicing on a 1998 Terios, it’s smart to treat the oxygen sensor as a wear item. While there’s no fixed change interval, many original sensors slow down with age, heat, and fuel quality by around 150,000–200,000 km. A sluggish or contaminated sensor can cause higher fuel use, rough idle, sooty tailpipe, or a check‑engine light. If a scan shows slow switching or related fault codes, replacement is the go rather than trying to clean it.

Handy tips for inspection and replacement:

• With the engine hot, check live data if available—healthy sensors switch rapidly around the stoichiometric point. Cold engines run open‑loop, so test at operating temp.
• Visually inspect the wiring and connector for heat damage or oil ingress. Fix any exhaust leaks before the sensor—false air will throw readings out.
• Use a 22 mm O2‑sensor socket. Penetrating oil on the threads (engine cool) helps. Don’t twist the harness.
• Install the new sensor with the correct thread compound if supplied, and tighten to the workshop‑manual spec—avoid over‑torque.
• Clear codes, reset fuel trims if your scan tool allows, and complete a short drive cycle so the ECU relearns properly.

For AU and NZ driving—short trips, dust, and varied fuel—adding an oxygen‑sensor health check to major services is a low‑cost way to keep the Terios perky and economical.

Popular questions

How many oxygen sensors does a 1998 Daihatsu Terios have?
Most have one upstream (pre‑catalyst) sensor. Some markets added a downstream sensor to monitor catalyst efficiency. A quick look along the exhaust—before and after the cat—will confirm what’s fitted on a specific vehicle.

What are the signs the oxygen sensor is failing?
Tell‑tales include higher fuel use, rough or hunting idle, a sooty exhaust tip, sulphur smell, and a check‑engine light. On a scan tool, you’ll often see slow or flat‑lined sensor activity once warm.

Can the Terios be driven with a faulty oxygen sensor?
It’ll usually run, but the ECU may default to richer mixtures, costing fuel and risking the catalytic converter. Best to diagnose and replace promptly to avoid bigger bills.