Diagnostic Aids

WARNING: This page is about a different car, the 1992 Geo Prizm. However, it is still accessible from the selected car via links, so may be relevant.

If fuel system pressure becomes high, the ECM can compensate for some increase; however, if fuel pressure becomes too high, Code 26 will set. See BASIC TESTING article in this section. A leaking fuel injector can cause this code to set. See BASIC TESTING article in this section.

A faulty canister purge system can cause Code 26 to set. See I - SYS/COMP TESTS article in this section. A faulty MAP sensor can cause this code to set. Disconnect MAP sensor connector. This allows the ECM to substitute a fixed MAP sensor signal value. If rich condition is gone, a faulty MAP sensor is the cause of rich condition.

An intermittent throttle switch output will cause the system to operate rich due to a false signal of engine acceleration. Inspect O2 sensor for silicone contamination from fuel or improper use of RTV sealant. The sensor may have White-powdery coating, which can cause a high, but false, signal voltage (rich condition). Check for a sticking EGR valve at idle. If Code 26 is intermittent, see TESTS W/O CODES article in this section.

Fig 1: Code 26 Schematic (GSi), Oxygen Sensor Circuit Rich Exhaust Indicated

Fig 2: Code 26 Flow Chart (GSi), Oxygen Sensor Circuit Rich Exhaust Indicated