Check engine light stuck

1. Turn ignition off. Disconnect ECM connector B136 (30-pin connector). Using an ohmmeter, measure resistance between ground and ECM connector B136 (30-pin connector) terminal No. 4. See Fig. 9. If resistance is more than 700 k/ohms, go to step 3. If resistance is less than 700 k/ohms, go to next step.

2. Measure resistance between ECM connector B136 (30-pin connector) terminal No. 4 and ground. If resistance is less than 400 k/ohms, go to step 5. If resistance is more than 400 k/ohms, go to step 6.

3. Turn ignition off. Reconnect ECM connector. Disconnect electrical connector from KS. Using an ohmmeter, check resistance between ground and KS harness terminal No. 2. See Fig. 18. If resistance is more than 700 k/ohms, go to next step. If resistance is less than 700 k/ohms, check for open circuit in harness and bad contact in connectors between KS and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article.

4. Check KS installation bolt for tightness. If bolt is tight, replace KS. If bolt is not tight, tighten bolt and retest.

5. Disconnect KS connector. Measure resistance between KS harness terminal No. 2 and ground. If resistance is 400 k/ohms or less, replace KS sensor. If resistance is more than 400 k/ohms, check for short to ground in wiring between KS and ECM connector. Ensure shield is also in tact for this circuit.

6. Reconnect ECM and KS connectors. Turn ignition on. Using voltmeter connected to ground, backprobe ECM connector B136 (30-pin connector) terminal No. 4. If voltage is 2 volts or less, repair poor contact in ECM connector. If voltage is more than 2 volts, circuit is normal. Check for poor contact at KS or ECM connectors.

Sorry the above is for checking the Knock Sensor (P0325)

Here is the checks for code P0420

DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD

1. Connect scan tool. Turn ignition on. Retrieve DTCs. If DTC P0420 is only DTC set, go to next step. If any other DTCs exist, diagnose those DTCs. Do not diagnose DTC P0420.

2. Turn ignition off. Check for exhaust leaks or air suction caused by loose or missing nuts and bolts, or damage to exhaust system. Check especially between cylinder head and front exhaust pipe, between front exhaust pipe and front catalytic converter, and between front and rear catalytic converters. If damage is found, repair or replace exhaust system. If no damage is found, go to next step.

3. Separate rear catalytic converter from exhaust pipe. If there is damage at rear face of rear catalytic converter, replace front and rear catalytic converters. If there is no damage to rear converter, remove front catalytic converter. If there is damage to front or rear face of front catalytic converter, replace front catalytic converter.

Also FYI - You cannot Clear Codes by removing the Battery Cable. On Older model vehicle you can but on the newer vehicle you need to use a scan tool to clear codes. Once you get the problems fixed you can go by your local parts store and ask them to clear the codes for you.

.