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2004 Eclipse Check Engine Light with Codes P0106, P0108, P0421 and Low Compression in Cylinder 1?

Model: 04 Eclipse Fault Code: P0106 Posted: 2014-10-19 21:05

Our daughter's 2004 Dodge Eclipse has been causing significant trouble for over a year. The check engine light first appeared more than a year ago, and despite multiple visits to different repair shops, the issue persists. We recently replaced both the Powertrain Control Module (PCM) and the pre-catalyst converter, but the problem remains unresolved. Approximately three months before symptoms began, we installed a rebuilt engine, which initially performed well with no issues. At first, we suspected the fuel cap and replaced it several times. The check engine light would go off temporarily but would reappear later. One garage identified that cylinder #1 has significantly lower compression compared to the other three cylinders during a compression test. The vehicle runs roughly at idle, even after replacing the engine mounts—another issue we previously considered. The engine is now out of warranty. We're wondering if low compression in one cylinder can trigger the codes P0106 (MAP Sensor Circuit Malfunction), P0108 (MAP Sensor Circuit Range/Performance Issue), and P0421 (Catalyst Efficiency Below Threshold). One garage recommends replacing the MAP sensor and catalytic converter, while another believes these codes are secondary symptoms of the underlying compression issue. Any technical insights or guidance would be greatly appreciated. We’re unable to continue spending money on this problem, and the car has now failed its inspection due to the active check engine light.

Related fault codes
P0106P0108P0421
Comments (2)
Anonymous 2014-10-20 09:05

Focus first on verifying the compression issue with a proper compression test and record the readings. Codes P0106 and P0108 indicate circuit-related problems in the MAP sensor—using an OBD2 scanner, monitor the actual MAP voltage values and review freeze frame data to identify whether the sensor is reading both high and low under specific conditions. This inconsistency suggests a faulty signal or underlying mechanical issue. Code P0421 (catalyst efficiency below threshold) should be addressed only after resolving the root causes such as engine performance or sensor issues. If necessary, monitor fuel trim values and oxygen sensor readings using the scanner to assess system response. Reference: https://www.obd-codes.com/p0420

Anonymous 2014-10-21 21:02

We're returning tomorrow to the garage that replaced the pre-converter. I'll request a full compression test and ask them to provide detailed readings. The previous shop actually believed low cylinder compression was the root cause. Their manager suggested contacting the PCM manufacturer to reprogram certain parameters—specifically, adjusting the engine control logic so that when cylinder #1 shows lower compression, it wouldn't trigger a diagnostic code or illuminate the check engine light. Unfortunately, I no longer have my notes from that conversation, and the technician has since left.