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Forum > Smog Check Repair > Question

1996 Jeep Grand Cherokee 4.0 failed functional test. Check engine light reveals a "21 code - Oxygen

We replaced the catalytic converter lastyear and recently the Check Engine light came in just before the test. It passed emissions, but not functional. Is one of the oxygen sensors triggering this fault code? What is the cost to repair? Could the oxygen sensor on the cat be the problem? Any advice?
Answer:

Code 21 on your Jeep Cherokee indicates that one or more of the following conditions exist. Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature. posted by SmogTips Support

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