Parts for your 2019 Toyota Mark x-Oxygen sensor

2019 Toyota Mark X oxygen sensor — what it does and when to replace it

The 2019 Toyota Mark X (GRX130, 4GR‑FSE 2.5‑litre and 2GR‑FSE 3.5‑litre V6) absolutely uses oxygen-sensing hardware. Toyota’s Service Information (TIS) Repair Manual for the Mark X SFI system details both an upstream Air‑Fuel Ratio (A/F) sensor and a downstream Heated Oxygen Sensor (HO2S) on each bank of these engines, and Toyota’s Electronic Parts Catalog for GRX130 lists these sensors as service parts. DENSO’s technical literature on wideband A/F sensors used with 2GR‑series engines backs this up as well. So, yes — the 2019 Mark X is fitted with oxygen/air‑fuel sensors and relies on them for emissions and fuel control.

On this Mark X, the upstream A/F sensor lives in the exhaust manifold and constantly reads how much oxygen is left in the exhaust. The ECU uses that signal to fine‑tune the fuel delivery so the engine runs right on the money — smooth, efficient, and clean. The downstream oxygen sensor sits after the catalytic converter and keeps an eye on the cat’s performance. If the converter isn’t doing its job, that sensor’s pattern gives the game away and will light the check‑engine lamp.

As part of routine servicing, it’s smart to treat the oxygen sensors like the quiet achievers they are. There’s no strict time‑based change interval, but by 160,000–200,000 km many A/F or O2 sensors are tired. If fuel economy drops, the idle goes a bit lumpy, or there’s a whiff of sulphur and a warning lamp, a scan for fault codes and live data should be on the cards.

• Keep an eye out for DTCs related to A/F or O2 sensors (slow response, heater circuit, or catalyst efficiency).
• Inspect connectors and wiring for heat damage or corrosion, fix exhaust leaks upstream of the sensors.
• Avoid silicone sealants and leaded additives — they can poison sensors and catalytic converters.

When replacement is due, genuine or high‑quality DENSO‑equivalent sensors are the go. Let the exhaust go stone cold, pre‑soak threads with penetrant, and use an O2‑sensor socket to avoid twisting the loom. Most new sensors come with the right thread compound pre‑applied, so don’t add extra anti‑seize unless the manufacturer says so. Refit to the correct torque, clear codes, and complete a proper drive cycle so the ECU can relearn trims and set readiness monitors. Done right, the Mark X will reward with crisp throttle response, better economy, and happy emissions testers across Australia and New Zealand.

How many oxygen sensors does a 2019 Toyota Mark X have?

The V6 layout means there are two banks. Typically, each bank gets an upstream wideband A/F sensor and a downstream narrowband O2 sensor — four in total. Exact part numbers can vary by engine (4GR‑FSE vs 2GR‑FSE) and market spec, so confirming by VIN is best practice.

If someone has fitted aftermarket headers or a custom exhaust, sensor positions may have shifted, but the ECU still expects signals from all sensors to keep trims and catalyst monitoring in check.

What are the signs an A/F or O2 sensor is failing on a Mark X?

Common clues include a check‑engine light, rough or hunting idle, increased fuel use, sooty tailpipes, and a sulphur or “rotten egg” smell after a hot run. A scan tool may show stuck or sluggish sensor readings and fuel trims pushed hard positive or negative.

If the downstream sensor mimics the upstream one, the catalytic converter may be tired — or the upstream A/F sensor could be lying. Testing and data analysis beats guesswork every time.

When should these sensors be replaced?

There’s no strict schedule, but many owners see benefits replacing tired upstream A/F sensors around 160,000–200,000 km, especially if economy has drifted. Downstream O2 sensors are usually replaced when fault codes appear or during catalyst work.

Always fix underlying issues (exhaust leaks, oil burning, coolant ingress) first, new sensors won’t stay happy if the root cause remains.