Recently my 94 has been throwing the occasional check engine light. The car will run fine at startup and then after driving for about 20 minutes the check engine light will come on and stay on about 10 min. Both fans come on during this time. After about 10 min it will go away. The drivability of the car doesn’t really change although there may be a slight at idle miss but it’s not really noticeable. The exhaust clearly smells rich when sitting at a light so I’m assuming the car just goes into open loop mode when the light comes on? I ran the codes and the only code was H64 right bank lean O2. I’m not sure if it’s a bad O2 sensor or what. I did get under the car today and unplug the right O2 sensor and the car still seems to run fine (I’m assuming because it’s running in open loop) Any ideas?

 

First off, make sure that the wiring for that O2 sensor isn't melted. The wires for our O2 sensors are routed in a very specific way and hooked to metal brackets because of just how close they are to the exhaust. If they are touching at all, the wires can melt. And if the O2 sensor heater is intermittent, that can throw this code. A failed O2 sensor can likewise throw it. They're cheap, so I'd honestly just toss two new sensors onto the thing and see if that solves the problem. They've probably never been replaced anyway. No need to replace the third sensor behind the cat on the driver's side; it's used for internal OBDII compliance testing, and while it will make the OBDII stuff mad, that code isn't actually used for anything relating to how the car drives, and won't turn on your check engine light. You can safely ignore it.

If replacing the O2 sensors doesn't solve the problem, next comes actual issues with the engine. Failing injector causing a lean condition (usually sets a DTC, but occasionally won't if it's not super out of spec), bad fuel, clogged fuel filter, failing fuel pump, vacuum leaks, exhaust manifold or catalytic converter interfaces leaking, or AIR system failure.