The Check Engine Light (CEL) sometimes turns on in the van.

I have a cheap OBD-II reader in my vehicle kit — along with other essentials like a jump pack and air pump. When the CEL lights up, I plug it in and read the code. Calm decisions follow.

It’s rarely dramatic. Last year: EVAP major leak. Yesterday: erratic O2 sensor readings before the catalytic converter.

There are two O2 sensors. The upstream sensor tunes air-fuel ratio and directly affects drivability. The downstream sensor sits after the catalytic converter and mainly monitors emissions performance. The failing sensor is the latter.

Haynes notes that during cold start the system runs in open loop. The O2 sensors aren’t used until operating temperature is reached and closed loop control engages. If O2 readings become unreliable, the ECU reverts back to open loop.

The van runs fine. And if it didn’t I got legs and a bike.

A CEL is an nudge, not a verdict. Don’t panic. Read the engine code.

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