It's a code for your MAF sensor and at the moment I wouldn't worry about it as it's probably more an intermittant connection problem than anything else.
435 is slow O2 sensor so I'd replace the front O2 sensor, clear the codes and see if 121 returns. If anything, apply some dielectric grease to the plug terminals on the MAF meter and wire tie the plug wiring back onto the MAF meter so that the plug wire harness movement is minimalized (was once a campaign for certain years, after '95 I believe).